flatcc
0.6.1 Bugfix and maintenance release
Ubuntu, macOS e Windows: Windows: semanalmente:
O analisador JSON pode alterar a interface para analisar vetores da União em uma versão futura que exige que a geração de código corresponda às versões da biblioteca.
flatcc não possui dependências externas, exceto para ferramentas de compilação e compilador e a biblioteca de tempo de execução C. Com compilações simultâneas de ninja, um pequeno projeto de cliente pode construir FLATCC com bibliotecas, gerar código de esquema, vincular o projeto e executar um caso de teste em poucos segundos, produzir binários entre 15k e 60k, leia pequenos tampões em 30ns, construir peças de impressão em cerca de 600 e uma maior executiva também manipulam 10ns, o JSON, JSON, JSONSing, ou mais, a mais de um tipo de fábrica de impressão.
Este projeto constrói o FLATCC, um compilador que gera código de buffers planos para C, dado um arquivo de esquema FlatBuffer. Esta introdução também cria um projeto de teste separado com o exemplo tradicional de monstros, aqui em uma versão C.
Por enquanto, suponha que um sistema Unix, embora isso não seja um requisito geral - consulte também a construção. Você precisará de git, cmake, bash, um compilador C e o sistema de construção ninja ou fazer.
git clone https://github.com/dvidelabs/flatcc.git
cd flatcc
# scripts/initbuild.sh ninja
scripts/initbuild.sh make
scripts/setup.sh -a ../mymonster
ls bin
ls lib
cd ../mymonster
ls src
scripts/build.sh
ls generated
scripts/initbuild.sh é opcional e escolhe o back -end de construção, que padroniza para o ninja.
O script de configuração cria planiccc usando o cmake e cria um diretório de projeto de teste com o exemplo de monstro e um script de construção que é apenas um pequeno script de shell. Os cabeçalhos e bibliotecas estão simbolicamente ligados ao projeto de teste. Você não precisa de cmake para construir seus próprios projetos assim que o FLATCC for compilado.
Para criar outro projeto de teste chamado Foobar, ligue para scripts/setup.sh -s -x ../foobar . Isso evitará a reconstrução do projeto FLATCC do zero.
Nota: Veja Changelog. Existem mudanças de quebra de pequenas pequenas, à medida que as inconsistências da API são descobertas. A menos que seja declarado claramente, as mudanças de quebra não afetarão a biblioteca de tempo de execução compilada, apenas os arquivos do cabeçalho. Em caso de problemas, verifique se a ferramenta flatcc é a mesma versão que o caminho include/flatcc .
O projeto inclui:
flatcc executável para C e uma biblioteca correspondente libflatcc.a . O compilador gera arquivos de cabeçalho C ou um esquema binário de plakbuffers.libflatccrt.a para construir e verificar os plakingbuffers de C. Generated Builder Headers dependem desta biblioteca. Também pode ser útil para outras interfaces de idiomas. A biblioteca mantém um estado de pilha para facilitar a criação de buffers a partir de um analisador ou similar.flatcc/portable apenas para compiladores não compilantes não C11 e pequenos ajudantes para todos os compiladores, incluindo manuseio Endian e impressão e análise numérica.Veja também:
Relatórios de bugs
Google Flatbuffers
Construir instruções
Investir rápido
Referência da interface do construtor
Benchmarks
O compilador flatcc é implementado como uma ferramenta independente, em vez de estender o compilador flatc do Googles para ter uma implementação de biblioteca C portátil pura do compilador de esquema, projetado para falhar graciosamente em informações abusivas em processos de longa execução. Acredita -se também que uma versão C possa ajudar a fornecer o esquema analisar para outras interfaces de idiomas que encontram interface com C mais fácil que o C ++. A equipe Flatbuffers do Googles FPL Lab tem sido muito útil para fornecer feedback e responder a muitas perguntas para ajudar a garantir a melhor compatibilidade possível. Observe o nome flatcc (compilador FlatBuffers C) vs Googles flatc .
O formato JSON é compatível com a ferramenta Googles flatc . A ferramenta flatc converte JSON na linha de comando usando um esquema e um buffer como entrada. flatcc gera código específico do esquema para ler e escrever JSON no tempo de execução. Embora a abordagem flatcc seja provavelmente muito mais rápida e também mais fácil de implantar, a abordagem flatc provavelmente é mais conveniente ao trabalhar manualmente com o JSON, como editar cenas de jogos. Ambas as ferramentas têm seu lugar.
NOTA: As plataformas Big-Endian são suportadas apenas na versão 0.4.0.
Ele está sendo considerado adicionar suporte ao sistema de construção de meson, mas seria bom com algum feedback sobre isso via edição #56
Se possível, forneça um esquema reprodutível e um arquivo de origem curto com um programa principal o retorno 1 por erro e 0 no sucesso e um pequeno script de construção. De preferência, gerar um hexdump e chamar o verificador de buffer para garantir que a entrada seja válida e vincule -se à biblioteca de debug flatccrt_d .
Consulte também Debugando um buffer e ReadFile.h útil para ler um buffer existente para verificação.
Exemplo:
Amostras/BugReport
eclectic.fbs:
namespace Eclectic ;
enum Fruit : byte { Banana = -1 , Orange = 42 }
table FooBar {
meal : Fruit = Banana ;
density : long ( deprecated );
say : string ;
height : short ;
}
file_identifier "NOOB" ;
root_type FooBar ;myissue.c:
/* Minimal test with all headers generated into a single file. */
#include "build/myissue_generated.h"
#include "flatcc/support/hexdump.h"
int main ( int argc , char * argv [])
{
int ret ;
void * buf ;
size_t size ;
flatcc_builder_t builder , * B ;
( void ) argc ;
( void ) argv ;
B = & builder ;
flatcc_builder_init ( B );
Eclectic_FooBar_start_as_root ( B );
Eclectic_FooBar_say_create_str ( B , "hello" );
Eclectic_FooBar_meal_add ( B , Eclectic_Fruit_Orange );
Eclectic_FooBar_height_add ( B , -8000 );
Eclectic_FooBar_end_as_root ( B );
buf = flatcc_builder_get_direct_buffer ( B , & size );
#if defined( PROVOKE_ERROR ) || 0
/* Provoke error for testing. */
(( char * ) buf )[ 0 ] = 42 ;
#endif
ret = Eclectic_FooBar_verify_as_root ( buf , size );
if ( ret ) {
hexdump ( "Eclectic.FooBar buffer for myissue" , buf , size , stdout );
printf ( "could not verify Electic.FooBar table, got %sn" , flatcc_verify_error_string ( ret ));
}
flatcc_builder_clear ( B );
return ret ;
}build.sh:
#! /bin/sh
cd $( dirname $0 )
FLATBUFFERS_DIR=../..
NAME=myissue
SCHEMA=eclectic.fbs
OUT=build
FLATCC_EXE= $FLATBUFFERS_DIR /bin/flatcc
FLATCC_INCLUDE= $FLATBUFFERS_DIR /include
FLATCC_LIB= $FLATBUFFERS_DIR /lib
mkdir -p $OUT
$FLATCC_EXE --outfile $OUT / ${NAME} _generated.h -a $SCHEMA || exit 1
cc -I $FLATCC_INCLUDE -g -o $OUT / $NAME $NAME .c -L $FLATCC_LIB -lflatccrt_d || exit 1
echo " running $OUT / $NAME "
if $OUT / $NAME ; then
echo " success "
else
echo " failed "
exit 1
fi A versão 0.6.2 (em desenvolvimento) é principalmente uma liberação de correção de bugs, consulte Changelog para obter detalhes. Um bug de longa data foi corrigido onde os objetos criados antes de uma chamada para _create_as_root não seriam alinhados adequadamente, e a extremidade do buffer agora também é acolchoada para o maior objeto observado dentro do buffer. Observe que, para o Clang Debug Builds, -fsanitize = indefinido foi adicionado e isso pode exigir código -fonte dependente para usar também esse sinalizador para evitar que a falta de símbolos do vinculador. O recurso pode ser desativado em cmakelists.txt.
A versão 0.6.1 contém principalmente correções de bugs e inúmeras contribuições da comunidade para lidar com casos de borda da plataforma. Além disso, os avisos do GCC Pendântico são desativados, confiando em Clang, pois o GCC é muito agressivo, quebra com frequência e trabalha contra a portabilidade. Um caso de teste C ++ existente garante que o código C também funcione com compiladores C ++ comuns, mas pode quebrar alguns ambientes; portanto, agora existe um sinalizador para desativar esse teste sem desativar todos os testes. Foi adicionado suporte a valores escalares opcionais no formato FlatBuffer. Também há suporte aprimorado para abstrair a alocação de memória em várias plataformas. <table>_identifier foi depreciado a favor <table>_file_identifier no código gerado devido ao identifier levando facilmente a nomear conflitos. file_extension A constante no código gerado agora está sem ponto prefixado (.).
A versão 0.6.0 apresenta um atributo "primário" a ser usado em conjunto com um atributo -chave para escolher a chave padrão para encontrar e classificar. Se o primário estiver ausente, a chave com o menor ID se tornará primária. Tabelas e vetores agora podem ser classificados recursivamente nas teclas primárias. Quebra: anteriormente o primeiro listado, não o ID mais baixo, seria a chave primária. Também introduz matrizes escalares de comprimento fixo em campos de estrutura (elementos de estrutura e enum não são suportados). As estruturas suportam campos de matriz de comprimento fixo, incluindo matrizes de char. As estruturas vazias nunca foram totalmente funcionadas e não são mais suportadas, elas também não são mais suportadas pelo FLATC. NOTA: Atualmente, as matrizes de char não fazem parte do compilador do Googles Flatc - as matrizes INT8 podem ser usadas. Quebra: estruturas vazias não são mais suportadas - elas também não são válidas no compilador do Googles FLATC. Veja Changelog para alterações adicionais. Depreciado: cast_to/from de Functions em flatcc_accessors.h será removido em favor de read/write_from/to porque a interface de elenco quebra a conversão de flutuação em algumas plataformas incomuns. Isso não deve afetar o uso normal, mas permanece válido nesta versão.
A liberação 0.5.3 inclui várias correções de insetos (consulte Changelog) e uma quebra, mas provavelmente a baixa mudança de impacto: quebra: 0.5.3 Alterações comportamentais do construtor criam chamadas para que os argumentos sejam sempre ordenados pelo ID do campo quando os atributos de ID estão sendo usados, por exemplo, MyGame_Example_Monster_create() em monster_test.fbs (#81). Corrige o comportamento indefinido ao classificar tabelas por um campo de chave numérico.
A versão 0.5.2 apresenta o sufixo opcional _get nos métodos do leitor. Usando apenas os métodos flatcc -g _get são válidos. Isso remove o nome em potencial descreve alguns nomes de campo. 0.5.2 também apresenta a tão aguardada operação de clone para tabelas e vetores. Foi adicionado um SmokEtest C ++ para reduzir o número de erros de atribuição de ponteiro de vazios que continuavam se esgueirando. A biblioteca de tempo de execução agora precisa de um arquivo extra refmap.c .
A liberação 0.5.1 corrige um buffer invadido na impressora json e melhora as bibliotecas portáteis <stdalign.h> compatibilidade com C ++ e a biblioteca padrão newlib incorporado. A impressão e a análise JSON foram tornados mais consistentes para ajudar a analisar e imprimir tabelas que não a raiz do esquema, como visto no driver de teste em test_json.c. O arquivo monster_test.fbs foi reorganizado para manter a tabela de monstros mais consistente com a versão FLATC do Googles e uma inconsistência menor de namespace de esquema foi resolvida como resultado. Referências explícitas a cabeçalhos portáteis foram afastadas da fonte gerada. Os guardas C ++ externo "C" adicionados em torno de cabeçalhos gerados. 0.5.1 também limpou a interface da união de baixo nível para que os termos {tipo, valor} sejam usados de forma consistente {type, membro} e {tipos, membros}.
be Branch.Não há planos para fazer atualizações frequentes quando o projeto se tornar estável, mas a entrada da comunidade sempre será bem -vinda e incluída em lançamentos, quando relevante, especialmente no que diz respeito aos testes em diferentes plataformas de destino.
Esta lista é um tanto desatualizada, as versões mais recentes do compilador são adicionadas e algumas antigas são removidas quando as plataformas de IC não são mais suportadas, mas em grande parte os alvos suportados permanecem inalterados. O MSVC 2010 pode ser preterido no futuro.
O CI-More Branch testa compiladores adicionais:
C11/C ++ 11 é a referência que deve sempre funcionar.
A opção GCC --pedantic Compiler não é suportada a partir do GCC-8+ porque força alterações de código não portáveis e porque tende a quebrar a base de código com cada nova versão do GCC.
O MSVC 2017 nem sempre é testado porque o ambiente de IC não suportará o MSVC 2010.
As versões mais antigas/não padronizadas dos compiladores de C ++ causam problemas porque static_assert e alignas se comportam de maneiras estranhas em que não estão ausentes nem funcionando totalmente como esperado. Muitas vezes, existem soluções alternativas, mas é mais confiável usar -std=c++11 ou -std=c++14 .
A biblioteca portavelmente não suporta o GCC C ++ Pre 4.7 porque a biblioteca portátil não funciona em torno das limitações de C ++ em stdalign.h e stdint.h antes do GCC 4.7. Isso pode ser corrigido, mas não é uma prioridade.
Algumas versões do compilador de teste anteriormente podem ter sido aposentadas à medida que o ambiente de CI é atualizado. Veja .travis.yml e appveyor.yml na ramificação ci-more para a configuração atual.
A amostra de monstro não funciona com o MSVC 2010 porque usa intencionalmente o código do estilo C99 para seguir melhor a versão C ++.
A opção Build FLATCC_TEST pode ser usada para desativar todos os testes que podem fazer com que o FLATCC seja compilado em plataformas que, de outra forma, são problemáticas. A opção BULD FLATCC_CXX_TEST pode ser desativada especificamente para testes C ++ (um arquivo C ++ simples que inclui código C gerado).
Não há razão para que outros compiladores ou mais antigos não possam ser suportados, mas pode exigir algum trabalho na configuração de compilação e possivelmente atualizações na biblioteca portátil. O exposto acima é simplesmente o que foi testado e configurado.
A camada de portabilidade possui alguns recursos que geralmente são importantes para coisas como o manuseio da Endian e outros para fornecer compatibilidade com recursos C11 opcionais e ausentes. Juntos, isso deve apoiar a maioria dos compiladores C, mas depende do feedback da comunidade para maturidade.
O tamanho necessário do tempo de execução inclui os arquivos pode ser reduzido significativamente usando -std = c11 e evitando o JSON (que precisa de muito suporte numérico de análise) e, removendo include/flatcc/reflection que está presente para apoiar o manuseio de arquivos de esquema include/flatcc/support e pode ser gerado a partir da reflection/reflection.fbs . O conjunto exato dos arquivos necessários pode mudar de liberação para liberação e realmente não importa em relação ao tamanho do código compilado.
A prioridade tem sido projetar uma interface de construtor C fácil de usar, que é razoavelmente rápido, adequado para servidores e dispositivos incorporados, mas com usabilidade sobre o desempenho absoluto - ainda assim a taxa de saída de buffer pequena é medida em milhões por segundo e leitura de 10 a 100 milhões de buffers por segundo a partir de uma estimativa aproximada. Ler Flatbuffers é mais do que uma ordem de magnitude mais rápida do que construí -los.
Para buffers de 100 MB com 1000 monstros, nomes de monstros dinamicamente estendidos, vetor de monstros e vetor de inventário, a largura de banda atinge cerca de 2,2 GB/se 45ms/buffer na CPU do núcleo i7 de 2,2 GHz. Isso inclui a leitura e a validação de todos os dados. A leitura de apenas alguns campos -chave aumenta a largura de banda para 2,7 GB/se 37ms/op. Para buffers de 10 MB, a largura de banda pode ser maior, mas eventualmente os buffers menores serão atingidos por chamadas no alto e, portanto, descemos para 300 MB/s a cerca de 150ns/OP que codificam pequenos buffers. Esses números são apenas uma diretriz aproximada - eles obviamente dependem de hardware, compilador e dados codificados. As medições estão excluindo uma etapa de aquecimento inicial.
Os analisadores JSON gerados são aproximadamente 4 vezes mais lentos do que a construção de um planilha diretamente em C ou C ++, ou cerca de 2200ns vs 600ns para uma mensagem JSON de 700 bytes. O JSON Parsing é, portanto, aproximadamente duas ordens de magnitude mais rápida do que ler o buffer de protocolo equivalente, conforme relatado na página Benchmarks do Google Flatbuffers. A compressão LZ4 estimou o dobro do tempo geral de processamento da análise JSON. A impressão JSON é mais rápida que a análise, mas não muito significativamente. O JSON se comprime para aproximadamente metade do tamanho de peças de fogo comprimidas em buffers grandes, mas comprime pior em pequenos tampões (sem mencionar quando não se comprimem).
Deve -se notar que o desempenho de leitura de Blatbuffer exclui a verificação que os analisadores JSON e os buffers de protocolo incluem inerentemente por sua natureza. A verificação não foi comparada, mas presumivelmente adicionaria menos de 50% de leitura lida, a menos que apenas uma fração de um buffer grande seja lida.
Veja também Benchmarks.
O código C Client pode evitar quase qualquer tipo de alocação para criar buffers como uma pilha de construtores, fornece uma arena extensível antes de cometer objetos - por exemplo, anexando strings ou vetores aos poucos. A pilha é ignorada principalmente quando um objeto completo pode ser construído diretamente, como um vetor a partir de matriz inteiro em plataformas Little Endian.
A interface do leitor deve ser muito rápida, como está com menos espaço para melhorias no desempenho. Também é muito mais simples que o construtor.
A usabilidade também foi priorizada sobre o menor código -fonte gerado possível e o tempo de compilação. Não deve afetar muito o tamanho compilado.
A saída binária compilada deve ser razoavelmente pequena para tudo, exceto os microcontroladores mais restritivos. Um arquivo de teste de origem de monstro de 33k (além dos cabeçalhos gerados e da biblioteca do construtor) resulta em um arquivo executável binário otimizado de menos de 50 mil, incluindo sobrecarga para instruções PrintF e outra lógica de suporte, ou um arquivo de objeto de 30k, excluindo a biblioteca do construtor.
Os binários somente de leitura são menores, mas não necessariamente muito menores do que os construtores, considerando que fazem menos trabalho: o teste de compatibilidade lê um arquivo monsterdata_test.golden binário pré-gerado e o arquivo monstro e verifica que todo o conteúdo é o esperado. Isso resulta em um executável binário otimizado de 13K ou um arquivo de objeto 6K. A fonte para esta verificação é 5k, excluindo arquivos de cabeçalho. Os leitores não precisam vincular -se a uma biblioteca.
Os analisadores JSON incham o binário C compilado em comparação com o uso puro de buffer de planície porque eles embrulham a árvore de decisão do analisador. Um analisador JSON para Monster.fbs pode adicionar configurações de otimização de 100k +/- ao binário executável.
O código gerado para a construção de planícies e para analisar e imprimir planos de planos, todos precisam de acesso para include/flatcc . O leitor não confia em nenhuma biblioteca, mas todos os outros arquivos gerados dependem da biblioteca libflatccrt.a Runtime. Observe que libflatcc.a é necessário apenas se o próprio compilador FLATCC for necessário como uma biblioteca.
O leitor e o construtor dependem de arquivos de cabeçalho comuns de leitor e construtor gerados. Esse arquivo comum possibilita alterar o espaço de nome global e redefinir os tipos básicos ( uoffset_t etc.). No futuro, isso pode passar para o código da biblioteca e usar macros para essas abstrações e, eventualmente, ter um conjunto de arquivos predefinidos para tipos além do deslocamento não assinado padrão de 32 bits ( uoffset_t ). A biblioteca de tempo de execução é específica para um conjunto de definições de tipo.
Consulte o monster_test.c e os arquivos gerados para obter orientações detalhadas sobre uso. O esquema de monstros usado neste projeto é uma ligeira adaptação ao original para testar alguns casos de borda adicionais.
Para a construção de plakbuffers, um arquivo de cabeçalho do construtor separado é gerado por esquema. Requer um arquivo flatbuffers_common_builder.h também gerado pelo compilador e uma pequena biblioteca de tempo de execução libflatccrt.a . É por causa desse requisito que o código gerado por leitor e construtor seja mantido separado. Os usos típicos podem ser vistos no arquivo monster_test.c. O construtor permite um empurrão repetido de conteúdo para um vetor ou uma string enquanto uma tabela contendo está sendo atualizada, o que simplifica a análise de formatos externos. Também é possível construir buffers aninhados em linha - a princípio, isso pode parecer excessivo, mas é útil ao envolver uma união de buffers em uma interface de rede e garante o alinhamento adequado de todos os níveis de buffer.
Para verificar os plakbuffers, um myschema_verifier.h é gerado. Depende da biblioteca de tempo de execução e do cabeçalho do leitor.
Os analisadores e impressoras JSON geram um arquivo por arquivo de esquema e o esquema incluído terá seus próprios analisadores e impressoras que incluir analisadores e impressoras dependerão, de maneira semelhante à maneira como os construtores funcionam.
Nota de nível baixo: o construtor gera todos os VTables no final do buffer em vez de ad-hoc na frente de cada tabela, mas, de outra forma, faz a mesma desduplicação de VTables. Isso torna possível agrupar VTables em cache quente ou garantir que todos os VTables estejam disponíveis ao transmitir parcialmente um buffer. Esse comportamento pode ser desativado por um sinalizador de tempo de execução.
Como alguns casos de uso podem incluir dispositivos incorporados muito restritos, a Biblioteca Builder pode ser personalizada com um objeto de alocador e um objeto emissor de buffer. O emissor separado garante que um buffer possa ser construído sem exigir que um buffer completo esteja presente na memória de uma só vez, se desejado.
A Biblioteca Builder Typendless está documentada em FLATCC_BUILDER.H e FLATCC_EMITTER.H enquanto a API de construtora digitada gerada para C é documentada na referência da interface do construtor.
Ocasionalmente, é levantada uma preocupação com a natureza densa das macros usadas no código gerado. Essas macros dificultam a compreensão de quais funções estão realmente disponíveis. A referência da interface do construtor tenta documentar as operações de maneira geral. Para obter informações mais detalhadas, os protótipos de função gerados podem ser extraídos com o script scripts/flatcc-doc.sh .
Alguns também estão preocupados com as macros sendo "inseguras". As macros não são inseguras quando usadas com FLATCC porque geram funções embutidas estáticas ou estáticas. Isso acionará erros de tempo de compilação se usados incorretamente na mesma extensão que fariam no código C direto.
A expansão comprime a saída gerada por mais de um fator 10, garantindo que o código sob controle de origem não exploda e possibilite a comparação de versões do código gerado de maneira significativa e veja se corresponde ao esquema pretendido. As macros também são importantes para lidar com abstrações de plataforma através dos cabeçalhos portáteis.
Ainda assim, é possível ver a saída gerada, embora não seja suportada diretamente pelo sistema de construção. Como exemplo, include/flatcc/reflection contém arquivos de cabeçalho pré-gerados para o esquema de reflexão. Para ver a saída expandida usando a cadeia de ferramentas do compilador clang , execute:
clang -E -DNDEBUG -I include
include/flatcc/reflection/reflection_reader.h |
clang-format
Outros comandos semelhantes provavelmente estão disponíveis em plataformas que não suportam CLANG.
Observe que o compilador otimizará quase todo o código gerado e usará apenas a lógica realmente referenciada pelo código do usuário final porque as funções são em linha estática ou estática. As peças restantes geralmente incluem eficientemente no código do aplicativo, resultando em um tamanho de código binário razoavelmente pequeno.
Mais detalhes podem ser encontrados em #88
A expansão do código gerado pode ser usado para obter documentação para um tipo de objeto específico.
O script a seguir automatiza este processo:
scripts/flatcc-doc.sh <schema-file> <name-prefix> [<outdir>]
Escrevendo protótipos de função para <outdir>/<name-prefix>.doc .
Observe que o script requer o compilador CLANG e a ferramenta de formato de clang, mas o script provavelmente também pode ser adaptado para outras cadeias de ferramentas.
O princípio por trás do script pode ser ilustrado usando o esquema de reflexão como exemplo, onde a documentação da tabela de objeto é extraída:
bin/flatcc reflection/reflection.fbs -a --json --stdout |
clang - -E -DNDEBUG -I include |
clang-format -style="WebKit" |
grep "^static.* reflection_Object_w*(" |
cut -f 1 -d '{' |
grep -v deprecated |
grep -v ");" |
sed 's/__tmp//g' |
sed 's/)/);/g'
O estilo webkit de formato de clang garante que os parâmetros e o tipo de retorno sejam todos colocados na mesma linha. Grep extrai os cabeçalhos da função e os corpos de função das tiras de corte que começam na mesma linha. Tiras sed __tmp sufixo de nomes de parâmetros usados para evitar conflitos de nomes de macro. Tiras de grep ); para remover declarações avançadas redundantes e sed, então adiciona; Para tornar cada linha um protótipo C válido.
O exposto acima não é garantido que sempre funcione conforme a saída pode mudar, mas deve percorrer um longo caminho.
Um pequeno extrato da saída, a partir de Flatcc-V0.5.2
static inline size_t reflection_Object_vec_len(reflection_Object_vec_t vec);
static inline reflection_Object_table_t reflection_Object_vec_at(reflection_Object_vec_t vec, size_t i);
static inline reflection_Object_table_t reflection_Object_as_root_with_identifier(const void* buffer, const char* fid);
static inline reflection_Object_table_t reflection_Object_as_root_with_type_hash(const void* buffer, flatbuffers_thash_t thash);
static inline reflection_Object_table_t reflection_Object_as_root(const void* buffer);
static inline reflection_Object_table_t reflection_Object_as_typed_root(const void* buffer);
static inline flatbuffers_string_t reflection_Object_name_get(reflection_Object_table_t t);
static inline flatbuffers_string_t reflection_Object_name(reflection_Object_table_t t);
static inline int reflection_Object_name_is_present(reflection_Object_table_t t);
static inline size_t reflection_Object_vec_scan_by_name(reflection_Object_vec_t vec, const char* s);
static inline size_t reflection_Object_vec_scan_n_by_name(reflection_Object_vec_t vec, const char* s, int n);
...
Exemplos são fornecidos no script a seguir usando o esquema de reflexão e monstro:
scripts/reflection-doc-example.sh
scripts/monster-doc-example.sh
O exemplo do monstro Doc liga essencialmente:
scripts/flatcc-doc.sh samples/monster/monster.fbs MyGame_Sample_Monster_
resultando no arquivo MyGame_Sample_Monster_.doc :
static inline size_t MyGame_Sample_Monster_vec_len(MyGame_Sample_Monster_vec_t vec);
static inline MyGame_Sample_Monster_table_t MyGame_Sample_Monster_vec_at(MyGame_Sample_Monster_vec_t vec, size_t i);
static inline MyGame_Sample_Monster_table_t MyGame_Sample_Monster_as_root_with_identifier(const void* buffer, const char* fid);
static inline MyGame_Sample_Monster_table_t MyGame_Sample_Monster_as_root_with_type_hash(const void* buffer, flatbuffers_thash_t thash);
static inline MyGame_Sample_Monster_table_t MyGame_Sample_Monster_as_root(const void* buffer);
static inline MyGame_Sample_Monster_table_t MyGame_Sample_Monster_as_typed_root(const void* buffer);
static inline MyGame_Sample_Vec3_struct_t MyGame_Sample_Monster_pos_get(MyGame_Sample_Monster_table_t t);
static inline MyGame_Sample_Vec3_struct_t MyGame_Sample_Monster_pos(MyGame_Sample_Monster_table_t t);
static inline int MyGame_Sample_Monster_pos_is_present(MyGame_Sample_Monster_table_t t);
static inline int16_t MyGame_Sample_Monster_mana_get(MyGame_Sample_Monster_table_t t);
static inline int16_t MyGame_Sample_Monster_mana(MyGame_Sample_Monster_table_t t);
static inline const int16_t* MyGame_Sample_Monster_mana_get_ptr(MyGame_Sample_Monster_table_t t);
static inline int MyGame_Sample_Monster_mana_is_present(MyGame_Sample_Monster_table_t t);
static inline size_t MyGame_Sample_Monster_vec_scan_by_mana(MyGame_Sample_Monster_vec_t vec, int16_t key);
static inline size_t MyGame_Sample_Monster_vec_scan_ex_by_mana(MyGame_Sample_Monster_vec_t vec, size_t begin, size_t end, int16_t key);
...
Os tipos nativos de Flatbuffer também podem ser extraídos, por exemplo, operações de string:
scripts/flatcc-doc.sh samples/monster/monster.fbs flatbuffers_string_
resultando em flatbuffers_string_.doc :
static inline size_t flatbuffers_string_len(flatbuffers_string_t s);
static inline size_t flatbuffers_string_vec_len(flatbuffers_string_vec_t vec);
static inline flatbuffers_string_t flatbuffers_string_vec_at(flatbuffers_string_vec_t vec, size_t i);
static inline flatbuffers_string_t flatbuffers_string_cast_from_generic(const flatbuffers_generic_t p);
static inline flatbuffers_string_t flatbuffers_string_cast_from_union(const flatbuffers_union_t u);
static inline size_t flatbuffers_string_vec_find(flatbuffers_string_vec_t vec, const char* s);
static inline size_t flatbuffers_string_vec_find_n(flatbuffers_string_vec_t vec, const char* s, size_t n);
static inline size_t flatbuffers_string_vec_scan(flatbuffers_string_vec_t vec, const char* s);
static inline size_t flatbuffers_string_vec_scan_n(flatbuffers_string_vec_t vec, const char* s, size_t n);
static inline size_t flatbuffers_string_vec_scan_ex(flatbuffers_string_vec_t vec, size_t begin, size_t end, const char* s);
...
Consulte o flatcc -h para obter detalhes.
Uma versão on -line listada aqui: flatcc -help.md, mas use flatcc -h para obter uma referência atualizada.
O compilador pode gerar um único arquivo de cabeçalho ou cabeçalhos para todos os esquemas incluídos e um arquivo comum e com ou sem suporte para leitura (padrão) e Writing (-W) Flatbuffers. A opção mais simples é usar (-a) para todos e incluir o arquivo myschema_builder.h .
O (-a) ou (-v) também gera um arquivo verificador.
Certifique -se de que flatcc sob a pasta include esteja visível nos compiladores C inclua o caminho ao compilar construtores de buffer planície.
O Path Path flatcc (-i) assume que todos os arquivos de esquema com o mesmo nome de base (Insentimento do Case) são idênticos e incluirão apenas o primeiro. Todos os arquivos gerados usam o nome de entrada e pousarão no diretório de trabalho ou no caminho definido por (-o).
Os arquivos podem ser gerados para stdout usando (--stdout). C Cabeçalhos serão ordenados e concatenados, mas são idênticos à saída de arquivo separada. Cada uma declaração incluída é protegida para que isso não leve à falta de arquivos incluem arquivos.
O código gerado, especialmente com todos combinados com -Stdout, pode parecer grande, mas apenas as peças realmente usadas ocuparão o espaço do executável final ou do arquivo de objeto. Compiladores modernos embutidos e incluem apenas partes necessárias da Biblioteca Builder estaticamente vinculada.
A impressora e o analisador JSON podem ser gerados usando a bandeira --json ou --json-PRINTER ou JSON-PARSER se apenas um deles for necessário. Existem alguns sinalizadores de tempo de compilação da biblioteca de tempo de execução que podem otimizar a impressão de enumes simbólicas, mas elas também podem ser desativadas em tempo de execução.
Certifique -se de vincular -se ao libflatccrt (RT para tempo de execução) e não libflatcc (o compilador de esquema); caso contrário, o construtor não estará disponível. Também certifique -se de ter a 'inclusão' da raiz do projeto FLATCC no caminho de inclusão.
Por padrão, o FLLCC espera um file_identifier no buffer ao ler ou verificar um buffer.
Um buffer pode ter um identificador de 4 bytes inesperado no deslocamento 4, ou o identificador pode estar ausente.
Nem todas as interfaces de idioma suportam identificadores de arquivo em buffers e, se o fizerem, podem não fazê -lo em uma versão mais antiga. Os usuários relataram problemas com as interfaces Python e Lua, mas isso é facilmente resolvido.
Verifique o valor de retorno do verificador:
int ret;
char *s;
ret = MyTable_verify_as_root(buf, size);
if (ret) {
s = flatcc_verify_error_string(ret);
printf("buffer failed: %sn", s);
}
Para verificar um buffer sem identificador ou para ignorar um identificador diferente, use a versão _with_identifier do verificador com um identificador nulo:
char *identifier = 0;
MyTable_verify_as_root_with_identifier(buf, size, identifier);
Para ler um uso de buffer:
MyTable_as_root_with_identifier(buf, 0);
E para construir um buffer sem um uso de identificador:
MyTable_start_as_root_with_identifier(builder, 0);
...
MyTable_end_as_root_with_identifier(builder, 0);
Várias outras chamadas as_root possuem uma versão as_root_with_identifier , incluindo a impressão json.
Depois de construir a flatcc tool , os binários estão localizados nos diretórios da bin e lib sob a árvore de origem flatcc .
Você pode pular diretamente para o exemplo de monstro que segue o tutorial do Googles Flatbuffers, ou pode ler ao longo do Guia do Investirt Rick abaixo. Se você seguir o tutorial de monstros, convém clonar e criar o FLATCC e copiar a fonte para um diretório de projeto separado da seguinte maneira:
git clone https://github.com/dvidelabs/flatcc.git
flatcc/scripts/setup.sh -a mymonster
cd mymonster
scripts/build.sh
build/mymonster
scripts/setup.sh como um link mínimo da biblioteca e ferramenta em um diretório personalizado, aqui mymonster . Com (-a), ele também adiciona um script de construção simples, copia o exemplo e atualiza .gitignore -consulte scripts/setup.sh -h . A configuração também pode criar o FLATCC, mas você ainda precisa garantir que o ambiente de construção seja configurado para o seu sistema.
Para escrever seus próprios arquivos de esquema, siga a documentação principal do projeto FlatBuffers sobre os arquivos de esquema de gravação.
A referência da interface do construtor pode ser útil depois de estudar a amostra de monstros e o início do rápido.
Ao procurar exemplos avançados, como classificar vetores e encontrar elementos por uma chave, você deve encontrá -los no projeto test/monster_test .
O seguinte Guia Quickstart é uma ampla simplificação do projeto test/monster_test - observe que o esquema é um pouco diferente do tutorial. O foco está na estrutura específica de C, em vez dos conceitos gerais de plankbuffers.
Você ainda pode usar a ferramenta de configuração para criar um projeto vazio e acompanhar, mas não há suposições sobre isso no texto abaixo.
Aqui, fornecemos um exemplo rápido de acesso somente leitura ao monstro Flatbuffer - é um extrato adaptado do arquivo monster_test.c.
Primeiro, compilamos o esquema somente leitura com o cabeçalho de suporte comum (-C) e adicionamos a recursão porque monster_test.fbs inclui outros arquivos.
flatcc -cr --reader test/monster_test/monster_test.fbs
Por simplicidade, assumimos que você crie um projeto de exemplo na pasta raiz do projeto, mas na Praxis você deseja alterar alguns caminhos, por exemplo:
mkdir -p build/example
flatcc -cr --reader -o build/example test/monster_test/monster_test.fbs
cd build/example
Nós conseguimos:
flatbuffers_common_reader.h
include_test1_reader.h
include_test2_reader.h
monster_test_reader.h
(Há também as samples/monster/monster.fbs , mas você não receberá arquivos de esquema incluídos).
Os namespaces podem ser longos, por isso, opcionalmente, usamos uma macro para gerenciar isso.
#include "monster_test_reader.h"
#undef ns
#define ns(x) FLATBUFFERS_WRAP_NAMESPACE(MyGame_Example, x)
int verify_monster(void *buffer)
{
ns(Monster_table_t) monster;
/* This is a read-only reference to a flatbuffer encoded struct. */
ns(Vec3_struct_t) vec;
flatbuffers_string_t name;
size_t offset;
if (!(monster = ns(Monster_as_root(buffer)))) {
printf("Monster not availablen");
return -1;
}
if (ns(Monster_hp(monster)) != 80) {
printf("Health points are not as expectedn");
return -1;
}
if (!(vec = ns(Monster_pos(monster)))) {
printf("Position is absentn");
return -1;
}
/* -3.2f is actually -3.20000005 and not -3.2 due to representation loss. */
if (ns(Vec3_z(vec)) != -3.2f) {
printf("Position failing on z coordinaten");
return -1;
}
/* Verify force_align relative to buffer start. */
offset = (char *)vec - (char *)buffer;
if (offset & 15) {
printf("Force align of Vec3 struct not correctn");
return -1;
}
/*
* If we retrieved the buffer using `flatcc_builder_finalize_aligned_buffer` or
* `flatcc_builder_get_direct_buffer` the struct should also
* be aligned without subtracting the buffer.
*/
if (vec & 15) {
printf("warning: buffer not aligned in memoryn");
}
/* ... */
return 0;
}
/* main() {...} */
Supondo que nosso arquivo acima seja monster_example.c a seguir, algumas maneiras de compilar o projeto para somente leitura - a compilação com a biblioteca de tempo de execução é mostrada posteriormente.
cc -I include monster_example.c -o monster_example
cc -std=c11 -I include monster_example.c -o monster_example
cc -D FLATCC_PORTABLE -I include monster_example.c -o monster_example
O caminho de inclusão ou o caminho de origem provavelmente é diferente. Alguns arquivos include/flatcc/portable sempre são usados, mas o sinalizador -D FLATCC_PORTABLE inclui arquivos adicionais para suportar compiladores sem recursos C11.
NOTA: Em algumas plataformas CLANG/GCC, pode ser necessário usar -std = gnu99 ou -std = gnu11 se o vinculador não puder encontrar posix_memalign , consulte também comentários em paligned_alloc.h.
Aqui, fornecemos um exemplo muito limitado de como criar um buffer - apenas alguns campos são atualizados. Por favor, consulte o monster_test.c e o diretório do documento para obter mais informações.
Primeiro, devemos gerar os arquivos:
flatcc -a monster_test.fbs
Isso produz:
flatbuffers_common_reader.h
flatbuffers_common_builder.h
include_test1_reader.h
include_test1_builder.h
include_test1_verifier.h
include_test2_reader.h
include_test2_builder.h
include_test2_verifier.h
monster_test_reader.h
monster_test_builder.h
monster_test_verifier.h
NOTA: Na verdade, não faríamos a geração leitura mostrada anteriormente, a menos que pretendemos apenas ler os buffers - a geração do construtor sempre gera o ACCES de leitura também.
Ao incluir "monster_test_builder.h" todos os outros arquivos estão incluídos automaticamente. O compilador C precisa da diretiva -I include para acessar flatcc/flatcc_builder.h , flatcc/flatcc_verifier.h e outros arquivos, dependendo das especificidades, assumindo que a raiz do projeto seja o diretório atual.
Os verificadores não são necessários e apenas criados porque escolhemos preguiçosamente a opção -a.
O construtor deve ser inicializado primeiro para configurar o ambiente de tempo de execução de que precisamos para criar buffers com eficiência - o construtor depende de um objeto emissor para construir o buffer real - aqui usamos implicitamente o padrão. Depois de termos isso, podemos considerar o construtor um identificador e focar na API gerada por planadores até finalizar o buffer (ou seja, acesse o resultado). Para usos não triviais, é recomendável fornecer um emissor personalizado e, por exemplo, emitir páginas sobre a rede assim que eles concluírem, em vez de mesclar todas as páginas em um único buffer usando flatcc_builder_finalize_buffer , ou o apartamento simplista flatcc_builder_get_direct_buffer , que retorna, se o tampão é muito grande. Consulte também Comentários de documentação em flatcc_builder.h e flatcc_emitter.h. See also flatc_builder_finalize_aligned_buffer in builder.h and the Builder Interface Reference when malloc aligned buffers are insufficent.
#include "monster_test_builder.h"
/* See [monster_test.c] for more advanced examples. */
void build_monster(flatcc_builder_t *B)
{
ns(Vec3_t *vec);
/* Here we use a table, but structs can also be roots. */
ns(Monster_start_as_root(B));
ns(Monster_hp_add(B, 80));
/* The vec struct is zero-initalized. */
vec = ns(Monster_pos_start(B));
/* Native endian. */
vec->x = 1, vec->y = 2, vec->z = -3.2f;
/* _end call converts to protocol endian format - for LE it is a nop. */
ns(Monster_pos_end(B));
/* Name is required, or we get an assertion in debug builds. */
ns(Monster_name_create_str(B, "MyMonster"));
ns(Monster_end_as_root(B));
}
#include "flatcc/support/hexdump.h"
int main(int argc, char *argv[])
{
flatcc_builder_t builder;
void *buffer;
size_t size;
flatcc_builder_init(&builder);
build_monster(&builder);
/* We could also use `flatcc_builder_finalize_buffer` and free the buffer later. */
buffer = flatcc_builder_get_direct_buffer(&builder, &size);
assert(buffer);
verify_monster(buffer);
/* Visualize what we got ... */
hexdump("monster example", buffer, size, stdout);
/*
* Here we can call `flatcc_builder_reset(&builder) if
* we wish to build more buffers before deallocating
* internal memory with `flatcc_builder_clear`.
*/
flatcc_builder_clear(&builder);
return 0;
}
Compile the example project:
cc -std=c11 -I include monster_example.c lib/libflatccrt.a -o monster_example
Note that the runtime library is required for building buffers, but not for reading them. If it is incovenient to distribute the runtime library for a given target, source files may be used instead. Each feature has its own source file, so not all runtime files are needed for building a buffer:
cc -std=c11 -I include monster_example.c
src/runtime/emitter.c src/runtime/builder.c
-o monster_example
Other features such as the verifier and the JSON printer and parser would each need a different file in src/runtime. Which file should be obvious from the filenames except that JSON parsing also requires the builder and emitter source files.
A buffer can be verified to ensure it does not contain any ranges that point outside the the given buffer size, that all data structures are aligned according to the flatbuffer principles, that strings are zero terminated, and that required fields are present.
In the builder example above, we can apply a verifier to the output:
#include "monster_test_builder.h"
#include "monster_test_verifier.h"
int ret;
...
... finalize
if ((ret = ns(Monster_verify_as_root_with_identifier(buffer, size,
"MONS")))) {
printf("Monster buffer is invalid: %sn",
flatcc_verify_error_string(ret));
}
The readfile.h utility may also be helpful in reading an existing buffer for verification.
Flatbuffers can optionally leave out the identifier, here "MONS". Use a null pointer as identifier argument to ignore any existing identifiers and allow for missing identifiers.
Nested flatbuffers are always verified with a null identifier, but it may be checked later when accessing the buffer.
The verifier does NOT verify that two datastructures are not overlapping. Sometimes this is indeed valid, such as a DAG (directed acyclic graph) where for example two string references refer to the same string in the buffer. In other cases an attacker may maliciously construct overlapping datastructures such that in-place updates may cause subsequent invalid buffers. Therefore an untrusted buffer should never be updated in-place without first rewriting it to a new buffer.
The CMake build system has build option to enable assertions in the verifier. This will break debug builds and not usually what is desired, but it can be very useful when debugging why a buffer is invalid. Traces can also be enabled so table offset and field id can be reported.
See also include/flatcc/flatcc_verifier.h .
When verifying buffers returned directly from the builder, it may be necessary to use the flatcc_builder_finalize_aligned_buffer to ensure proper alignment and use aligned_free to free the buffer (or as of v0.5.0 also flatcc_builder_aligned_free ), see also the Builder Interface Reference. Buffers may also be copied into aligned memory via mmap or using the portable layers paligned_alloc.h feature which is available when including generated headers. test/flatc_compat/flatc_compat.c is an example of how this can be done. For the majority of use cases, standard allocation would be sufficient, but for example standard 32-bit Windows only allocates on an 8-byte boundary and can break the monster schema because it has 16-byte aligned fields.
NOTE: as of August 2024 it has been discovered that C++ writer code has been aligning empty vectors to the size field only, even if elements require greater alignment like the double type which requires 8. This would cause the FlatCC verifier to (correctly) reject these vectors because it would result in an invalid C pointer type on some architectures. However, because this has been in effect for over 10 years, the consensus is to have verifiers tolerate this behaviour even if C++ will eventually fix this issue. The FlatCC verifier has been updated to accept such buffers by default with an optional compile time flag to enforce the strict behaviour as well ( FLATCC_ENFORCE_ALIGNED_EMPTY_VECTORS ). In principle the misaligned vectors can potentially lead to undefined behaviour in agressively optimized C compilers. As of now it appears to be safe to read such buffers on common platforms and it is preferable to avoid additional runtime reader overhead to deal with this. For more, see FlatCC #287, Google Flatbuffers #8374, FlatCC #289.
If unfortunate, it is possible to have a read accessor method conflict with other generated methods and typenames. Usually a small change in the schema will resolve this issue.
As of flatcc 0.5.2 read accors are generated with and without a _get suffix so it is also possible to use Monster_pos_get(monster) instead of Monster_pos(monster) . When calling flatcc with option -g the read accesors will only be generated with _get suffix. This avoids potential name conflicts. An example of a conflict is a field name like pos_add when there is also a pos field because the builder interface generates the add suffix. Using the -g option avoids this problem, but it is preferable to choose another name such as added_pos when the schema can be modified.
The -g option only changes the content of the flatbuffers_common_reader.h file, so it is technically possible to use different versions of this file if they are not mixed.
If an external code generator depends on flatcc output, it should use the _get suffix because it will work with and without the -g option, but only as of version 0.5.2 or later. For human readable code it is probaly simpler to stick to the orignal naming convention without the _get suffix.
Even with the above, it is still possible to have a conflict with the union type field. If a union field is named foo , an additional field is automatically - this field is named foo_type and holds, unsurprisingly, the type of the union.
Namespaces can also cause conflicts. If a schema has the namespace Foo.Bar and table named MyTable with a field name hello, then a read accessor will be named: Foo_Bar_MyTable_hello_get . It is also possible to have a table named Bar_MyTable because _ are allowed in FlatBuffers schema names, but in this case we have name conflict in the generated the C code. FlatCC does not attempt to avoid such conflicts so such schema are considered invalid.
Notably several users have experienced conflicts with a table or struct field named 'identifier' because <table-name>_identifier has been defined to be the file identifier to be used when creating a buffer with that table (or struct) as root. As of 0.6.1, the name is <table-name>_file_identifier to reduce the risk of conflicts. The old form is deprecated but still generated for tables without a field named 'identifier' for backwards compatibility. Mostly this macro is used for higher level functions such as mytable_create_as_root which need to know what identifier to use.
When reading a FlatBuffer does not provide the expected results, the first line of defense is to ensure that the code being tested is linked against flatccrt_d , the debug build of the runtime library. This will raise an assertion if calls to the builder are not properly balanced or if required fields are not being set.
To dig further into a buffer, call the buffer verifier and see if the buffer is actually valid with respect to the expected buffer type.
Strings and tables will be returned as null pointers when their corresponding field is not set in the buffer. User code should test for this but it might also be helpful to temporarily or permanently set the required attribute in the schema. The builder will then detect missing fields when cerating buffers and the verifier can will detect their absence in an existing buffer.
If the verifier rejects a buffer, the error can be printed (see Verifying a Buffer), but it will not say exactly where the problem was found. To go further, the verifier can be made to assert where the problem is encountered so the buffer content can be analyzed. This is enabled with:
-DFLATCC_DEBUG_VERIFY=1
Note that this will break test cases where a buffer is expected to fail verification.
To dump detailed contents of a valid buffer, or the valid contents up to the point of failure, use:
-DFLATCC_TRACE_VERIFY=1
Both of these options can be set as CMake options, or in the flatcc_rtconfig.h file.
When reporting bugs, output from the above might also prove helpful.
The JSON parser and printer can also be used to create and display buffers. The parser will use the builder API correctly or issue a syntax error or an error on required field missing. This can rule out some uncertainty about using the api correctly. The test_json.c file and test_json_parser.c have test functions that can be adapted for custom tests.
For advanced debugging the hexdump.h file can be used to dump the buffer contents. It is used in test_json.c and also in monster_test.c. See also FlatBuffers Binary Format.
As of April 2022, Googles flatc tool has implemented an --annotate feature. This provides an annotated hex dump given a binary buffer and a schema. The output can be used to troubleshoot and rule out or confirm suspected encoding bugs in the buffer at hand. The eclectic example in the FlatBuffers Binary Format document contains a hand written annotated example which inspired the --annotate feature, but it is not the exact same output format. Note also that flatc generated buffers tend to have vtables before the table it is referenced by, while flatcc normally packs all vtables at the end of the buffer for better padding and cache efficiency.
See also flatc --annotate.
Note: There is experimental support for text editor that supports clangd language server or similar. You can edit CMakeList.txt to generate build/Debug/compile_comands.json , at least when using clang as a compiler, and copy or symlink it from root. Or come with a better suggestion. There are .gitignore entries for compile_flags.txt and compile_commands.json in project root.
There are two ways to identify the content of a FlatBuffer. The first is to use file identifiers which are defined in the schema. The second is to use type identifiers which are calculated hashes based on each tables name prefixed with its namespace, if any. In either case the identifier is stored at offset 4 in binary FlatBuffers, when present. Type identifiers are not to be confused with union types.
The FlatBuffers schema language has the optional file_identifier declaration which accepts a 4 characer ASCII string. It is intended to be human readable. When absent, the buffer potentially becomes 4 bytes shorter (depending on padding).
The file_identifier is intended to match the root_type schema declaration, but this does not take into account that it is convenient to create FlatBuffers for other types as well. flatcc makes no special destinction for the root_type while Googles flatc JSON parser uses it to determine the JSON root object type.
As a consequence, the file identifier is ambigous. Included schema may have separate file_identifier declarations. To at least make sure each type is associated with its own schemas file_identifier , a symbol is defined for each type. If the schema has such identifier, it will be defined as the null identifier.
The generated code defines the identifiers for a given table:
#ifndef MyGame_Example_Monster_file_identifier
#define MyGame_Example_Monster_file_identifier "MONS"
#endif
The user can now override the identifier for a given type, for example:
#define MyGame_Example_Vec3_file_identifier "VEC3"
#include "monster_test_builder.h"
...
MyGame_Example_Vec3_create_as_root(B, ...);
The create_as_root method uses the identifier for the type in question, and so does other _as_root methods.
The file_extension is handled in a similar manner:
#ifndef MyGame_Example_Monster_file_extension
#define MyGame_Example_Monster_file_extension "mon"
#endif
To better deal with the ambigouties of file identifiers, type identifiers have been introduced as an alternative 4 byte buffer identifier. The hash is standardized on FNV-1a for interoperability.
The type identifier use a type hash which maps a fully qualified type name into a 4 byte hash. The type hash is a 32-bit native value and the type identifier is a 4 character little endian encoded string of the same value.
In this example the type hash is derived from the string "MyGame.Example.Monster" and is the same for all FlatBuffer code generators that supports type hashes.
The value 0 is used to indicate that one does not care about the identifier in the buffer.
...
MyGame_Example_Monster_create_as_typed_root(B, ...);
buffer = flatcc_builder_get_direct_buffer(B);
MyGame_Example_Monster_verify_as_typed_root(buffer, size);
// read back
monster = MyGame_Example_Monster_as_typed_root(buffer);
switch (flatbuffers_get_type_hash(buffer)) {
case MyGame_Example_Monster_type_hash:
...
}
...
if (flatbuffers_get_type_hash(buffer) ==
flatbuffers_type_hash_from_name("Some.Old.Buffer")) {
printf("Buffer is the old version, not supported.n");
}
More API calls are available to naturally extend the existing API. See monster_test.c for more.
The type identifiers are defined like:
#define MyGame_Example_Monster_type_hash ((flatbuffers_thash_t)0x330ef481)
#define MyGame_Example_Monster_type_identifier "x81xf4x0ex33"
The type_identifier can be used anywhere the original 4 character file identifier would be used, but a buffer must choose which system, if any, to use. This will not affect the file_extension .
NOTE: The generated _type_identifier strings should not normally be used when an identifier string is expected in the generated API because it may contain null bytes which will be zero padded after the first null before comparison. Use the API calls that take a type hash instead. The type_identifier can be used in low level flatcc_builder.h calls because it handles identifiers as a fixed byte array and handles type hashes and strings the same.
NOTE: it is possible to compile the flatcc runtime to encode buffers in big endian format rather than the standard little endian format regardless of the host platforms endianness. If this is done, the identifier field in the buffer is always byte swapped regardless of the identifier method chosen. The API calls make this transparent, so "MONS" will be stored as "SNOM" but should still be verified as "MONS" in API calls. This safeguards against mixing little- and big-endian buffers. Likewise, type hashes are always tested in native (host) endian format.
The flatcc/flatcc_identifier.h file contains an implementation of the FNV-1a hash used. The hash was chosen for simplicity, availability, and collision resistance. For better distribution, and for internal use only, a dispersion function is also provided, mostly to discourage use of alternative hashes in transmission since the type hash is normally good enough as is.
Note: there is a potential for collisions in the type hash values because the hash is only 4 bytes.
JSON support files are generated with flatcc --json .
This section is not a tutorial on JSON printing and parsing, it merely covers some non-obvious aspects. The best source to get started quickly is the test file:
test/json_test/json_test.c
For detailed usage, please refer to:
test/json_test/test_json_printer.c
test/json_test/test_json_parser.c
test/json_test/json_test.c
test/benchmark/benchflatccjson
See also JSON parsing section in the Googles FlatBuffers schema documentation.
By using the flatbuffer schema it is possible to generate schema specific JSON printers and parsers. This differs for better and worse from Googles flatc tool which takes a binary schema as input and processes JSON input and output. Here that parser and printer only rely on the flatcc runtime library, is faster (probably significantly so), but requires recompilition when new JSON formats are to be supported - this is not as bad as it sounds - it would for example not be difficult to create a Docker container to process a specific schema in a web server context.
The parser always takes a text buffer as input and produces output according to how the builder object is initialized. The printer has different init functions: one for printing to a file pointer, including stdout, one for printing to a fixed length external buffer, and one for printing to a dynamically growing buffer. The dynamic buffer may be reused between prints via the reset function. See flatcc_json_parser.h for details.
The parser will accept unquoted names (not strings) and trailing commas, ie non-strict JSON and also allows for hex x03 in strings. Strict mode must be enabled by a compile time flag. In addition the parser schema specific symbolic enum values that can optionally be unquoted where a numeric value is expected:
color: Green
color: Color.Green
color: MyGame.Example.Color.Green
color: 2
The symbolic values do not have to be quoted (unless required by runtime or compile time configuration), but can be while numeric values cannot be quoted. If no namespace is provided, like color: Green , the symbol must match the receiving enum type. Any scalar value may receive a symbolic value either in a relative namespace like hp: Color.Green , or an absolute namespace like hp: MyGame.Example.Color.Green , but not hp: Green (since hp in the monster example schema) is not an enum type with a Green value). A namespace is relative to the namespace of the receiving object.
It is also possible to have multiple values, but these always have to be quoted in order to be compatible with Googles flatc tool for Flatbuffers 1.1:
color: "Green Red"
Unquoted multi-valued enums can be enabled at compile time but this is deprecated because it is incompatible with both Googles flatc JSON and also with other possible future extensions: color: Green Red
These value-valued expressions were originally intended for enums that have the bit flag attribute defined (which Color does have), but this is tricky to process, so therefore any symblic value can be listed in a sequence with or without namespace as appropriate. Because this further causes problems with signed symbols the exact definition is that all symbols are first coerced to the target type (or fail), then added to the target type if not the first this results in:
color: "Green Blue Red Blue"
color: 19
Because Green is 2, Red is 1, Blue is 8 and repeated.
NOTE : Duplicate values should be considered implemention dependent as it cannot be guaranteed that all flatbuffer JSON parsers will handle this the same. It may also be that this implementation will change in the future, for example to use bitwise or when all members and target are of bit flag type.
It is not valid to specify an empty set like:
color: ""
because it might be understood as 0 or the default value, and it does not unquote very well.
The printer will by default print valid json without any spaces and everything quoted. Use the non-strict formatting option (see headers and test examples) to produce pretty printing. It is possibly to disable symbolic enum values using the noenum option.
Only enums will print symbolic values are there is no history of any parsed symbolic values at all. Furthermore, symbolic values are only printed if the stored value maps cleanly to one value, or in the case of bit-flags, cleanly to multiple values. For exmaple if parsing color: Green Red it will print as "color":"Red Green" by default, while color: Green Blue Red Blue will print as color:19 .
Both printer and parser are limited to roughly 100 table nesting levels and an additional 100 nested struct depths. This can be changed by configuration flags but must fit in the runtime stack since the operation is recursive descent. Exceedning the limits will result in an error.
Numeric values are coerced to the receiving type. Integer types will fail if the assignment does not fit the target while floating point values may loose precision silently. Integer types never accepts floating point values. Strings only accept strings.
Nested flatbuffers may either by arrays of byte sized integers, or a table or a struct of the target type. See test cases for details.
The parser will by default fail on unknown fields, but these can also be skipped silently with a runtime option.
Unions are difficult to parse. A union is two json fields: a table as usual, and an enum to indicate the type which has the same name with a _type suffix and accepts a numeric or symbolic type code:
{
name: "Container Monster",
test_type: Monster,
test: { name: "Contained Monster" }
}
based on the schema is defined in monster_test.fbs.
Because other json processors may sort fields, it is possible to receive the type field after the test field. The parser does not store temporary datastructures. It constructs a flatbuffer directly. This is not possible when the type is late. This is handled by parsing the field as a skipped field on a first pass, followed by a typed back-tracking second pass once the type is known (only the table is parsed twice, but for nested unions this can still expand). Needless to say this slows down parsing. It is an error to provide only the table field or the type field alone, except if the type is NONE or 0 in which case the table is not allowed to be present.
Union vectors are supported as of v0.5.0. A union vector is represented as two vectors, one with a vector of tables and one with a vector of types, similar to ordinary unions. It is more efficient to place the type vector first because it avoids backtracking. Because a union of type NONE cannot be represented by absence of table field when dealing with vectors of unions, a table must have the value null if its type is NONE in the corresponding type vector. In other cases a table should be absent, and not null.
Here is an example of JSON containing Monster root table with a union vector field named manyany which is a vector of Any unions in the monster_test.fbs schema:
{
"name": "Monster",
"manyany_type": [ "Monster", "NONE" ],
"manyany": [{"name": "Joe"}, null]
}
As of v0.5.0 it is possible to encode and decode a vector of type [uint8] (aka [ubyte] ) as a base64 encoded string or a base64url encoded string as documented in RFC 4648. Any other type, notably the string type, do not handle base64 encoding.
Limiting the support to [uint8] avoids introducing binary data into strings and also avoids dealing with sign and endian encoding of binary data of other types. Furthermore, array encoding of values larger than 8 bits are not necessarily less efficient than base64.
Base64 padding is always printed and is optional when parsed. Spaces, linebreaks, JSON string escape character '', or any other character not in the base64(url) alphabet are rejected as a parse error.
The schema must add the attribute (base64) or (base64url) to the field holding the vector, for example:
table Monster {
name: string;
sprite: [uint8] (base64);
token: [uint8] (base64url);
}
If more complex data needs to be encoded as base64 such as vectors of structs, this can be done via nested FlatBuffers which are also of type [uint8] .
Note that for some use cases it might be desireable to read binary data as base64 into memory aligned to more than 8 bits. This is not currently possible, but it is recognized that a (force_align: n) attribute on [ubyte] vectors could be useful, but it can also be handled via nested flatbuffers which also align data.
Fixed length arrays introduced in 0.6.0 allow for structs containing arrays of fixed length scalars, structs and chars. Arrays are parsed like vectors for of similar type but are zero padded if shorter than expected and fails if longer than expected. The flag reject_array_underflow will error if an array is shorter than expected instead of zero padding. The flag skip_array_overflow will allow overlong arrays and simply drop extra elements.
Char arrays are parsed like strings and zero padded if short than expected, but they are not zero terminated. A string like "hello" will exactly fit into a field of type [char:5] . Trailing zero characters are not printed, but embedded zero characters are. This allows for loss-less roundtrips without having to zero pad strings. Note that other arrays are always printed in full. If the flag skip_array_overflow is set, a string might be truncated in the middle of a multi-byte character. This is not checked nor enforced by the verifier.
Both the printer and the parser have the ability to accept runtime flags that modifies their behavior. Please refer to header file comments for documentation and test cases for examples. Notably it is possible to print unquoted symbols and to ignore unknown fields when parsing instead of generating an error.
Note that deprecated fields are considered unknown fields during parsing so in order to process JSON from an old schema version with deprecated fields present, unknown symbols must be skipped.
As of v0.5.1 test_json.c demonstrates how a single parser driver can be used to parse different table types without changes to the driver or to the schema.
For example, the following layout can be used to configure a generic parser or printer.
struct json_scope {
const char *identifier;
flatcc_json_parser_table_f *parser;
flatcc_json_printer_table_f *printer;
flatcc_table_verifier_f *verifier;
};
static const struct json_scope Monster = {
/* The is the schema global file identifier. */
ns(Monster_identifier),
ns(Monster_parse_json_table),
ns(Monster_print_json_table),
ns(Monster_verify_table)
};
The Monster scope can now be used by a driver or replaced with a new scope as needed:
/* Abbreviated ... */
struct json_scope = Monster;
flatcc_json_parser_table_as_root(B, &parser_ctx, json, strlen(json), parse_flags,
scope->identifier, scope->parser);
/* Printing and verifying works roughly the same. */
The generated table MyGame_Example_Monster_parse_json_as_root is a thin convenience wrapper roughly implementing the above.
The generated monster_test_parse_json is a higher level convenience wrapper named of the schema file itself, not any specific table. It parses the root_type configured in the schema. This is how the test_json.c test driver operated prior to v0.5.1 but it made it hard to test parsing and printing distinct table types.
Note that verification is not really needed for JSON parsing because a generated JSON parser is supposed to build buffers that always verify (except for binary encoded nested buffers), but it is useful for testing.
Note that json parsing and printing is very fast reaching 500MB/s for printing and about 300 MB/s for parsing. Floating point parsing can signficantly skew these numbers. The integer and floating point parsing and printing are handled via support functions in the portable library. In addition the floating point include/flatcc/portable/grisu3_* library is used unless explicitly disable by a compile time flag. Disabling grisu3 will revert to sprintf and strtod . Grisu3 will fall back to strtod and grisu3 in some rare special cases. Due to the reliance on strtod and because strtod cannot efficiently handle non-zero-terminated buffers, it is recommended to zero terminate buffers. Alternatively, grisu3 can be compiled with a flag that allows errors in conversion. These errors are very small and still correct, but may break some checksums. Allowing for these errors can significantly improve parsing speed and moves the benchmark from below half a million parses to above half a million parses per second on 700 byte json string, on a 2.2 GHz core-i7.
While unquoted strings may sound more efficient due to the compact size, it is actually slower to process. Furthermore, large flatbuffer generated JSON files may compress by a factor 8 using gzip or a factor 4 using LZ4 so this is probably the better place to optimize. For small buffers it may be more efficient to compress flatbuffer binaries, but for large files, json may actually compress significantly better due to the absence of pointers in the format.
SSE 4.2 has been experimentally added, but it the gains are limited because it works best when parsing space, and the space parsing is already fast without SSE 4.2 and because one might just leave out the spaces if in a hurry. For parsing strings, trivial use of SSE 4.2 string scanning doesn't work well becasuse all the escape codes below ASCII 32 must be detected rather than just searching for and " . That is not to say there are not gains, they just don't seem worthwhile.
The parser is heavily optimized for 64-bit because it implements an 8-byte wide trie directly in code. It might work well for 32-bit compilers too, but this hasn't been tested. The large trie does put some strain on compile time. Optimizing beyond -O2 leads to too large binaries which offsets any speed gains.
Attributes included in the schema are viewed in a global namespace and each include file adds to this namespace so a schema file can use included attributes without namespace prefixes.
Each included schema will also add types to a global scope until it sees a namespace declaration. An included schema does not inherit the namespace of an including file or an earlier included file, so all schema files starts in the global scope. An included file can, however, see other types previously defined in the global scope. Because include statements always appear first in a schema, this can only be earlier included files, not types from a containing schema.
The generated output for any included schema is indendent of how it was included, but it might not compile without the earlier included files being present and included first. By including the toplevel myschema.h or myschema_builder.h all these dependencies are handled correctly.
Note: libflatcc.a can only parse a single schema when the schema is given as a memory buffer, but can handle the above when given a filename. It is possible to concatenate schema files, but a namespace; declaration must be inserted as a separator to revert to global namespace at the start of each included file. This can lead to subtle errors because if one parent schema includes two child schema a.fbs and b.fbs , then b.fbs should not be able to see anything in a.fbs even if they share namespaces. This would rarely be a problem in praxis, but it means that schema compilation from memory buffers cannot authoratively validate a schema. The reason the schema must be isolated is that otherwise code generation for a given schema could change with how it is being used leading to very strange errors in user code.
If a field is required such as Monster.name, the table end call will assert in debug mode and create incorrect tables in non-debug builds. The assertion may not be easy to decipher as it happens in library code and it will not tell which field is missing.
When reading the name, debug mode will again assert and non-debug builds will return a default value.
Writing the same field twice will also trigger an assertion in debug builds.
Buffers can be used for high speed communication by using the ability to create buffers with structs as root. In addition the default emitter supports flatcc_emitter_direct_buffer for small buffers so no extra copy step is required to get a linear buffer in memory. Preliminary measurements suggests there is a limit to how fast this can go (about 6-7 mill. buffers/sec) because the builder object must be reset between buffers which involves zeroing allocated buffers. Small tables with a simple vector achieve roughly half that speed. For really high speed a dedicated builder for structs would be needed. See also monster_test.c.
All types stored in a buffer has a type suffix such as Monster_table_t or Vec3_struct_t (and namespace prefix which we leave out here). These types are read-only pointers into endian encoded data. Enum types are just constants easily grasped from the generated code. Tables are dense so they are never accessed directly.
Enums support schema evolution meaning that more names can be added to the enumeration in a future schema version. As of v0.5.0 the function _is_known_value can be used ot check if an enum value is known to the current schema version.
Structs have a dual purpose because they are also valid types in native format, yet the native reprsention has a slightly different purpose. Thus the convention is that a const pointer to a struct encoded in a flatbuffer has the type Vec3_struct_t where as a writeable pointer to a native struct has the type Vec3_t * or struct Vec3 * .
All types have a _vec_t suffix which is a const pointer to the underlying type. For example Monster_table_t has the vector type Monster_vec_t . There is also a non-const variant with suffix _mutable_vec_t which is rarely used. However, it is possible to sort vectors in-place in a buffer, and for this to work, the vector must be cast to mutable first. A vector (or string) type points to the element with index 0 in the buffer, just after the length field, and it may be cast to a native type for direct access with attention to endian encoding. (Note that table_t types do point to the header field unlike vectors.) These types are all for the reader interface. Corresponding types with a _ref_t suffix such as _vec_ref_t are used during the construction of buffers.
Native scalar types are mapped from the FlatBuffers schema type names such as ubyte to uint8_t and so forth. These types also have vector types provided in the common namespace (default flatbuffers_ ) so a [ubyte] vector has type flatbuffers_uint8_vec_t which is defined as const uint8_t * .
The FlatBuffers boolean type is strictly 8 bits wide so we cannot use or emulate <stdbool.h> where sizeof(bool) is implementation dependent. Therefore flatbuffers_bool_t is defined as uint8_t and used to represent FlatBuffers boolean values and the constants of same type: flatbuffers_true = 1 and flatbuffers_false = 0 . Even so, pstdbool.h is available in the include/flatcc/portable directory if bool , true , and false are desired in user code and <stdbool.h> is unavailable.
flatbuffers_string_t is const char * but imply the returned pointer has a length prefix just before the pointer. flatbuffers_string_vec_t is a vector of strings. The flatbufers_string_t type guarantees that a length field is present using flatbuffers_string_len(s) and that the string is zero terminated. It also suggests that it is in utf-8 format according to the FlatBuffers specification, but not checks are done and the flatbuffers_create_string(B, s, n) call explicitly allows for storing embedded null characters and other binary data.
All vector types have operations defined as the typename with _vec_t replaced by _vec_at and _vec_len . For example flatbuffers_uint8_vec_at(inv, 1) or Monster_vec_len(inv) . The length or _vec_len will be 0 if the vector is missing whereas _vec_at will assert in debug or behave undefined in release builds following out of bounds access. This also applies to related string operations.
The FlatBuffers schema uses the following scalar types: ubyte , byte , ushort , short, uint , int , ulong , and long to represent unsigned and signed integer types of length 8, 16, 32, and 64 respectively. The schema syntax has been updated to also support the type aliases uint8 , int8 , uint16 , int16 , uint32 , int32 , uint64 , int64 to represent the same basic types. Likewise, the schema uses the types float and double to represent IEEE-754 binary32 and binary64 floating point formats where the updated syntax also supports the type aliases float32 and float64 .
The C interface uses the standard C types such as uint8 and double to represent scalar types and this is unaffected by the schema type name used, so the schema vector type [float64] is represented as flatbuffers_double_vec_t the same as [double] would be.
Note that the C standard does not guarantee that the C types float and double are represented by the IEEE-754 binary32 single precision format and the binary64 double precision format respectively, although they usually are. If this is not the case FlatCC cannot work correctly with FlatBuffers floating point values. (If someone really has this problem, it would be possible to fix).
Unions are represented with a two table fields, one with a table field and one with a type field. See separate section on Unions. As of flatcc v0.5.0 union vectors are also supported.
A union represents one of several possible tables. A table with a union field such as Monster.equipped in the samples schema will have two accessors: MyGame_Sample_Monster_equipped(t) of type flatbuffers_generic_t and MyGame_Sample_Monster_equipped_type(t) of type MyGame_Sample_Equipment_union_type_t . A generic type is is just a const void pointer that can be assigned to the expected table type, struct type, or string type. The enumeration has a type code for member of the union and also MyGame_Sample_Equipment_NONE which has the value 0.
The union interface were changed in 0.5.0 and 0.5.1 to use a consistent { type, value } naming convention for both unions and union vectors in all interfaces and to support unions and union vectors of multiple types.
A union can be accessed by its field name, like Monster MyGame_Sample_Monster_equipped(t) and its type is given by MyGame_Sample_Monster_type(t) , or a flatbuffers_union_t struct can be returned with MyGame_Sample_monster_union(t) with the fields { type, value }. A union vector is accessed in the same way but { type, value } represents a type vector and a vector of the given type, eg a vector Monster tables or a vector of strings.
There is a test in monster_test.c covering union vectors and a separate test focusing on mixed type unions that also has union vectors.
Googles monster_test.fbs schema has the union (details left out):
namespace MyGame.Example2;
table Monster{}
namespace MyGame.Example;
table Monster{}
union Any { Monster, MyGame.Example2.Monster }
where the two Monster tables are defined in separate namespaces.
flatcc rejects this schema due to a name conflict because it uses the basename of a union type, here Monster to generate the union member names which are also used in JSON parsing. This can be resolved by adding an explicit name such as Monster2 to resolve the conflict:
union Any { Monster, Monster2: MyGame.Example2.Monster }
This syntax is accepted by both flatc and flatcc .
Both versions will implement the same union with the same type codes in the binary format but generated code will differ in how the types are referred to.
In JSON the monster type values are now identified by MyGame.Example.Any.Monster , or just Monster , when assigning the first monster type to an Any union field, and MyGame.Example.Any.Monster2 , or just Monster2 when assigning the second monster type. C uses the usual enum namespace prefixed symbols like MyGame_Example_Any_Monster2 .
Fixed Length Arrays is a late feature to the FlatBuffers format introduced in flatc and flatcc mid 2019. Currently only scalars arrays are supported, and only as struct fields. To use fixed length arrays as a table field wrap it in a struct first. It would make sense to support struct elements and enum elements, but that has not been implemented. Char arrays are more controversial due to verification and zero termination and are also not supported. Arrays are aligned to the size of the first field and are equivalent to repeating elements within the struct.
The schema syntax is:
struct MyStruct {
my_array : [float:10];
}
See test_fixed_array in monster_test.c for an example of how to work with these arrays.
Flatcc opts to allow arbitrary length fixed length arrays but limit the entire struct to 2^16-1 bytes. Tables cannot hold larger structs, and the C language does not guarantee support for larger structs. Other implementations might have different limits on maximum array size. Arrays of 0 length are not permitted.
Optional scalar table fields were introduced to FlatBuffers mid 2020 in order to better handle null values also for scalar data types, as is common in SQL databases. Before describing optional values, first understand how ordinary scalar values work in FlatBuffers:
Imagine a FlatBuffer table with a mana field from the monster sample schema. Ordinarily a scalar table field has implicit default value of 0 like mana : uint8; , or an explicit default value specified in the schema like mana : uint8 = 100; . When a value is absent from a table field, the default value is returned, and when a value is added during buffer construction, it will not actually be stored if the value matches the default value, unless the force_add option is used to write a value even if it matches the default value. Likewise the is_present method can be used to test if a field was actually stored in the buffer when reading it.
When a table has many fields, most of which just hold default settings, signficant space can be saved using default values, but it also means that an absent value does not indicate null. Field absence is essentially just a data compression technique, not a semantic change to the data. However, it is possible to use force_add and is_present to interpret values as null when not present, except that this is not a standardized technique. Optional fields represents a standardized way to achieve this.
Scalar fields can be marked as optional by assigning null as a default value. For example, some objects might not have a meaningful mana value, so it could be represented as lifeforce : uint8 = null . Now the lifeforce field has become an optional field. In the FlatCC implementation this means that the field is written, it will always be written also if the value is 0 or any other representable value. It also means that the force_add method is not available for the field because force_add is essentially always in effect for the field. On the read side, optional scalar fields behave exactly is ordinary scalar fields that have not specified a default value, that is, if the field is absent, 0 will be returned and is_present will return false. Instead optional scalar fields get a new accessor method with the suffix _option() which returns a struct with two fiels: { is_null, value } where _option().is_null == !is_present() and _option().value is the same value is the _get() method, which will be 0 if is_null is true. The option struct is named after the type similar to unions, for example flatbuffers_uint8_option_t or MyGame_Example_Color_option_t , and the option accessor method also works similar to unions. Note that _get() will also return 0 for optional enum values that are null (ie absent), even if the enum value does not have an enumerated element with the value 0. Normally enums without a 0 element is not allowed in the schema unless a default value is specified, but in this case it is null, and _get() needs some value to return in this case.
By keeping the original accessors, read logic can be made simpler and faster when it is not important whether a value is null or 0 and at the same time the option value can be returned and stored.
Note that struct fields cannot be optional. Also note that, non-scalar table fields are not declared optional because these types can already represent null via a null pointer or a NONE union type.
JSON parsing and printing change behavior for scalar fields by treating absent fields differently according the optional semantics. For example parsing a missing field will not store a default value even if the parser is executed with a flag to force default values to be stored and the printer will not print absent optional fields even if otherwise flagged to print default values. Currenlty the JSON printers and parsers do not print or parse JSON null and can only represent null be absence of a field.
For an example of reading and writing, as well as printing and parsing JSON, optional scalar fields, please refer to optional_scalars_test.fbs and optional_scalars_test.c.
The pendian_detect.h` file detects endianness for popular compilers and provides a runtime fallback detection for others. In most cases even the runtime detection will be optimized out at compile time in release builds.
The FLATBUFFERS_LITTLEENDIAN flag is respected for compatibility with Googles flatc compiler, but it is recommended to avoid its use and work with the mostly standard flags defined and/or used in pendian_detect.h , or to provide for additional compiler support.
As of flatcc 0.4.0 there is support for flatbuffers running natively on big endian hosts. This has been tested on IBM AIX. However, always run tests against the system of interest - the release process does not cover automated tests on any BE platform.
As of flatcc 0.4.0 there is also support for compiling the flatbuffers runtime library with flatbuffers encoded in big endian format regardless of the host platforms endianness. Longer term this should probably be placed in a separate library with separate name prefixes or suffixes, but it is usable as is. Redefine FLATBUFFERS_PROTOCOL_IS_LE/BE accordingly in flatcc_types.h. This is already done in the be branch. This branch is not maintained but the master branch can be merged into it as needed.
Note that standard flatbuffers are always encoded in little endian but in situations where all buffer producers and consumers are big endian, the non standard big endian encoding may be faster, depending on intrinsic byteswap support. As a curiosity, the load_test actually runs faster with big endian buffers on a little endian MacOS platform for reasons only the optimizer will know, but read performance of small buffers drop to 40% while writing buffers generally drops to 80-90% performance. For platforms without compiler intrinsics for byteswapping, this can be much worse.
Flatbuffers encoded in big endian will have the optional file identifier byteswapped. The interface should make this transparent, but details are still being worked out. For example, a buffer should always verify the monster buffer has the identifier "MONS", but internally the buffer will store the identifier as "SNOM" on big endian encoded buffers.
Because buffers can be encode in two ways, flatcc uses the term native endianness and protocol endianess. _pe is a suffix used in various low level API calls to convert between native and protocol endianness without caring about whether host or buffer is little or big endian.
If it is necessary to write application code that behaves differently if the native encoding differs from protocol encoding, use flatbuffers_is_pe_native() . This is a function, not a define, but for all practical purposes it will have same efficience while also supporting runtime endian detection where necessary.
The flatbuffer environment only supports reading either big or little endian for the time being. To test which is supported, use the define FLATBUFFERS_PROTOCOL_IS_LE or FLATBUFFERS_PROTOCOL_IS_BE . They are defines as 1 and 0 respectively.
The builder API often returns a reference or a pointer where null is considered an error or at least a missing object default. However, some operations do not have a meaningful object or value to return. These follow the convention of 0 for success and non-zero for failure. Also, if anything fails, it is not safe to proceed with building a buffer. However, to avoid overheads, there is no hand holding here. On the upside, failures only happen with incorrect use or allocation failure and since the allocator can be customized, it is possible to provide a central error state there or to guarantee no failure will happen depending on use case, assuming the API is otherwise used correctly. By not checking error codes, this logic also optimizes out for better performance.
The builder API does not support sorting due to the complexity of customizable emitters, but the reader API does support sorting so a buffer can be sorted at a later stage. This requires casting a vector to mutable and calling the sort method available for fields with keys.
The sort uses heap sort and can sort a vector in-place without using external memory or recursion. Due to the lack of external memory, the sort is not stable. The corresponding find operation returns the lowest index of any matching key, or flatbuffers_not_found .
When configured in config.h (the default), the flatcc compiler allows multiple keyed fields unlike Googles flatc compiler. This works transparently by providing <table_name>_vec_sort_by_<field_name> and <table_name>_vec_find_by_<field_name> methods for all keyed fields. The first field maps to <table_name>_vec_sort and <table_name>_vec_find . Obviously the chosen find method must match the chosen sort method. The find operation is O(logN).
As of v0.6.0 the default key used for find and and sort without the by_name suffix is the field with the smaller id instead of the first listed in the schema which is often but not always the same thing.
v0.6.0 also introduces the primary_key attribute that can be used instead of the key attribute on at most one field. The two attributes are mutually exclusive. This can be used if a key field with a higher id should be the default key. There is no difference when only one field has a key or primary_key attribute, so in that case choose key for compatiblity. Googles flatc compiler does not recognize the primary_key attribute.
As of v0.6.0 a 'sorted' attribute has been introduced together with the sort operations <table_name>_sort and <union_name>_sort . If a table or a union, directly or indirectly, contains a vector with the 'sorted' attribute, then the sort operation is made available. The sort will recursively visit all children with vectors marked sorted. The sort operatoin will use the default (primary) key. A table or union must first be cast to mutable, for example ns(Monster_sort((ns(Monster_mutable_table_t))monster) . The actual vector sort operations are the same as before, they are just called automatically. The sorted attribute can only be set on vectors that are not unions. The vector can be of scalar, string, struct, or table type. sorted is only valid for a struct or table vector if the struct or table has a field with a key or primary_key attribute. NOTE: A FlatBuffer can reference the same object multiple times. The sort operation will be repeated if this is the case. Sometimes that is OK, but if it is a concern, remove the sorted attribute and sort the vector manually. Note that sharing can also happen via a shared containing object. The sort operations are generated in _reader.h files and only for objects directly or indirectly affected by the sorted attribute. Unions have a new mutable case operator for use with sorting unions: ns(Any_sort(ns(Any_mutable_cast)(my_any_union)) . Usually unions will be sorted via a containing table which performs this cast automatically. See also test_recursive_sort in monster_test.c.
As of v0.4.1 <table_name>_vec_scan_by_<field_name> and the default <table_name>_vec_scan are also provided, similar to find , but as a linear search that does not require the vector to be sorted. This is especially useful for searching by a secondary key (multiple keys is a non-standard flatcc feature). _scan_ex searches a sub-range [a, b) where b is an exclusive index. b = flatbuffers_end == flatbuffers_not_found == (size_t)-1 may be used when searching from a position to the end, and b can also conveniently be the result of a previous search.
rscan searches in the opposite direction starting from the last element. rscan_ex accepts the same range arguments as scan_ex . If a >= b or a >= len the range is considered empty and flatbuffers_not_found is returned. [r]scan[_ex]_n[_by_name] is for length terminated string keys. See monster_test.c for examples.
Note that find requires key attribute in the schema. scan is also available on keyed fields. By default flatcc will also enable scan by any other field but this can be disabled by a compile time flag.
Basic types such as uint8_vec also have search operations.
See also Builder Interface Reference and monster_test.c.
The FlatBuffers format does not fully distinguish between default values and missing or null values but it is possible to force values to be written to the buffer. This is discussed further in the Builder Interface Reference. For SQL data roundtrips this may be more important that having compact data.
The _is_present suffix on table access methods can be used to detect if value is present in a vtable, for example Monster_hp_present . Unions return true of the type field is present, even if it holds the value None.
The add methods have corresponding force_add methods for scalar and enum values to force storing the value even if it is default and thus making it detectable by is_present .
The portable library is placed under include/flatcc/portable and is required by flatcc, but isn't strictly part of the flatcc project. It is intended as an independent light-weight header-only library to deal with compiler and platform variations. It is placed under the flatcc include path to simplify flatcc runtime distribution and to avoid name and versioning conflicts if used by other projects.
The license of portable is different from flatcc . It is mostly MIT or Apache depending on the original source of the various parts.
A larger set of portable files is included if FLATCC_PORTABLE is defined by the user when building.
cc -D FLATCC_PORTABLE -I include monster_test.c -o monster_test
Otherwise a targeted subset is included by flatcc_flatbuffers.h in order to deal with non-standard behavior of some C11 compilers.
pwarnings.h is also always included so compiler specific warnings can be disabled where necessary.
The portable library includes the essential parts of the grisu3 library found in external/grisu3 , but excludes the test cases. The JSON printer and parser relies on fast portable numeric print and parse operations based mostly on grisu3.
If a specific platform has been tested, it would be good with feedback and possibly patches to the portability layer so these can be made available to other users.
Note: if a test fails, see Strict Aliasing for a possible resolution.
To initialize and run the build (see required build tools below):
scripts/build.sh
The bin and lib folders will be created with debug and release build products.
The build depends on CMake . By default the Ninja build tool is also required, but alternatively make can be used.
Optionally switch to a different build tool by choosing one of:
scripts/initbuild.sh make
scripts/initbuild.sh make-concurrent
scripts/initbuild.sh ninja
where ninja is the default and make-concurrent is make with the -j flag.
To enforce a 32-bit build on a 64-bit machine the following configuration can be used:
scripts/initbuild.sh make-32bit
which uses make and provides the -m32 flag to the compiler. A custom build configuration X can be added by adding a scripts/build.cfg.X file.
scripts/initbuild.sh cleans the build if a specific build configuration is given as argument. Without arguments it only ensures that CMake is initialized and is therefore fast to run on subsequent calls. This is used by all test scripts.
To install build tools on OS-X, and build:
brew update
brew install cmake ninja
git clone https://github.com/dvidelabs/flatcc.git
cd flatcc
scripts/build.sh
To install build tools on Ubuntu, and build:
sudo apt-get update
sudo apt-get install cmake ninja-build
git clone https://github.com/dvidelabs/flatcc.git
cd flatcc
scripts/build.sh
To install build tools on Centos, and build:
sudo yum group install "Development Tools"
sudo yum install cmake
git clone https://github.com/dvidelabs/flatcc.git
cd flatcc
scripts/initbuild.sh make # there is no ninja build tool
scripts/build.sh
OS-X also has a HomeBrew package:
brew update
brew install flatcc
or for the bleeding edge:
brew update
brew install flatcc --HEAD
Install CMake, MSVC, and git (tested with MSVC 14 2015).
In PowerShell:
git clone https://github.com/dvidelabs/flatcc.git
cd flatcc
mkdir buildMSVC
cd buildMSVC
cmake -G "Visual Studio 14 2015" ....
Optionally also build from the command line (in buildMSVC):
cmake --build . --target --config Debug
cmake --build . --target --config Release
In Visual Studio:
open flatccbuildMSVCFlatCC.sln
build solution
choose Release build configuration menu
rebuild solution
Note that flatccCMakeList.txt sets the -DFLATCC_PORTABLE flag and that includeflatccportablepwarnings.h disable certain warnings for warning level -W3.
Docker image:
Users have been reporting some degree of success using cross compiles from Linux x86 host to embedded ARM Linux devices.
For this to work, FLATCC_TEST option should be disabled in part because cross-compilation cannot run the cross-compiled flatcc tool, and in part because there appears to be some issues with CMake custom build steps needed when building test and sample projects.
2024-03-08: WARNING: -O2 -mcpu=cortex-m7 targets using the arm-none-eabi 13.2.Rel1 toolchain can result in uninitialized stack access when not compiled with -fno-strict-aliasing -mcpu=cortex-m0 and -mcpu=cortex-m1 appears to be unaffected. See also issue #274. 2024-10-03: Fix available on flatcc master branch when you read this. See also CHANGELOG comments for release 0.6.2.
The option FLATCC_RTONLY will disable tests and only build the runtime library.
The following is not well tested, but may be a starting point:
mkdir -p build/xbuild
cd build/xbuild
cmake ../.. -DBUILD_SHARED_LIBS=on -DFLATCC_RTONLY=on
-DCMAKE_BUILD_TYPE=Release
Overall, it may be simpler to create a separate Makefile and just compile the few src/runtime/*.c into a library and distribute the headers as for other platforms, unless flatcc is also required for the target. Or to simply include the runtime source and header files in the user project.
Note that no tests will be built nor run with FLATCC_RTONLY enabled. It is highly recommended to at least run the tests/monster_test project on a new platform.
Some target systems will not work with Posix malloc , realloc , free and C11 aligned_alloc . Or they might, but more allocation control is desired. The best approach is to use flatcc_builder_custom_init to provide a custom allocator and emitter object, but for simpler case or while piloting a new platform flatcc_alloc.h can be used to override runtime allocation functions. Carefully read the comments in this file if doing so. There is a test case implementing a new emitter, and a custom allocator can be copied from the one embedded in the builder library source.
On systems where the default POSIX assert call is unavailable, or when a different assert behaviour is desirable, it is possible to override the default behaviour in runtime part of flatcc library via logic defined in flatcc_assert.h.
By default Posix assert is beeing used. It can be changed by preprocessor definition:
-DFLATCC_ASSERT=own_assert
but it will not override assertions used in the portable library, notably the Grisu3 fast numerical conversion library used with JSON parsing.
Runtime assertions can be disabled using:
-DFLATCC_NO_ASSERT
This will also disable Grisu3 assertions. See flatcc_assert.h for details.
The <assert.h> file will in all cases remain a dependency for C11 style static assertions. Static assertions are needed to ensure the generated structs have the correct physical layout on all compilers. The portable library has a generic static assert implementation for older compilers.
By default libraries are built statically.
Occasionally there are requests #42 for also building shared libraries. It is not clear how to build both static and shared libraries at the same time without choosing some unconvential naming scheme that might affect install targets unexpectedly.
CMake supports building shared libraries out of the box using the standard library name using the following option:
CMAKE ... -DBUILD_SHARED_LIBS=ON ...
See also CMake Gold: Static + shared.
The Flatcc build files should take care of strict aliasing issues on common platforms, but it is not a solved problem, so here is some background information.
In most cases this is a non-issue with the current flatcc code base, but that does not help in the cases where it is an issue.
Compilers have become increasingly aggressive with applying, and defaulting to, strict aliasing rules.
FlatCC does not guarantee that strict aliasing rules are followed, but the code base is updated as issues are detected. If a test fails or segfaults the first thing to check is -fno-strict-aliasing , or the platform equivalent, or to disable pointer casts, as discussed below.
Strict aliasing means that a cast like p2 = *(T *)p1 is not valid because the compiler thinks that p2 does not depend on data pointed to by p1. In most cases compilers are sensible enough to handle this, but not always. It can, and will, lead to reading from uninitialized memory or segfaults. There are two ways around this, one is to use unions to convert from integer to float, which is valid in C, but not in C++, and the other is to use memcpy for small constant sizes, which is guaranteed safe, but can be slow if not optimized, and it is not always optimized. (Not strictly memcpy but access via cast to char * or other "narrow" type).
FlatCC manages this in flatcc_accessors.h which forwards to platform dependent code in pmemaccess.h. Note that is applies to the runtime code base only. For compile time the only issue should be hash tables and these should also be safe.
FlatCC either uses optimized memcpy or non-compliant pointer casts depending on the platform. Essentially, buffer memory is first copied, or pointer cast, into an unsigned integer of a given size. This integer is then endian converted into another unsigned integer. Then that integer is converted into a final integer type or floating point type using union casts. This generally optimizes out to very few assembly instructions, but when it does not, code size and execution time can grow significantly.
It has been observed that targets both default to strict aliasing with -O2 optimization, and at the same to uses a function call for memcpy(dest, src, sizeof(uint32_t)) , but where __builtin_memcpy does optimize well, hence requiring detection of a fast memcpy operation.
This is a game between being reasonably performant and compliant.
-DPORTABLE_MEM_PTR_ACCESS=0 will force the runtime code to not use pointer casts but it can potentially generate suboptimal code and can be set 1 if the compiler and build configuration is known to not have issues with strict aliasing. It is set to 1 for most x86/64 targets since this has been working for a long time in FlatCC builds and tests, while memcpy might not work efficient.
Install targes may be built with:
mkdir -p build/install
cd build/install
cmake ../.. -DBUILD_SHARED_LIBS=on -DFLATCC_RTONLY=on
-DCMAKE_BUILD_TYPE=Release -DFLATCC_INSTALL=on
make install
However, this is not well tested and should be seen as a starting point. The normal scripts/build.sh places files in bin and lib of the source tree.
By default lib files a built into the lib subdirectory of the project. This can be changed, for example like -DFLATCC_INSTALL_LIB=lib64 .
To distribute the compiled binaries the following files are required:
Compilador:
bin/flatcc (command line interface to schema compiler)
lib/libflatcc.a (optional, for linking with schema compiler)
include/flatcc/flatcc.h (optional, header and doc for libflatcc.a)
Runtime:
include/flatcc/** (runtime header files)
include/flatcc/reflection (optional)
include/flatcc/support (optional, only used for test and samples)
lib/libflatccrt.a (runtime library)
In addition the runtime library source files may be used instead of libflatccrt.a . This may be handy when packaging the runtime library along with schema specific generated files for a foreign target that is not binary compatible with the host system:
src/runtime/*.c
The build products from MSVC are placed in the bin and lib subdirectories:
flatccbinDebugflatcc.exe
flatcclibDebugflatcc_d.lib
flatcclibDebugflatccrt_d.lib
flatccbinReleaseflatcc.exe
flatcclibReleaseflatcc.lib
flatcclibReleaseflatccrt.lib
Runtime includeflatcc directory is distributed like other platforms.
Correr
scripts/test.sh [--no-clean]
NOTE: The test script will clean everything in the build directy before initializing CMake with the chosen or default build configuration, then build Debug and Release builds, and run tests for both.
The script must end with TEST PASSED , or it didn't pass.
To make sure everything works, also run the benchmarks:
scripts/benchmark.sh
In Visual Studio the test can be run as follows: first build the main project, the right click the RUN_TESTS target and chose build. See the output window for test results.
It is also possible to run tests from the command line after the project has been built:
cd buildMSVC
ctest
Note that the monster example is disabled for MSVC 2010.
Be aware that tests copy and generate certain files which are not automatically cleaned by Visual Studio. Close the solution and wipe the MSVC directory, and start over to get a guaranteed clean build.
Please also observe that the file .gitattributes is used to prevent certain files from getting CRLF line endings. Using another source control systems might break tests, notably test/flatc_compat/monsterdata_test.golden .
Note: Benchmarks have not been ported to Windows.
The configuration
config/config.h
drives the permitted syntax and semantics of the schema compiler and code generator. These generally default to be compatible with Googles flatc compiler. It also sets things like permitted nesting depth of structs and tables.
The runtime library has a separate configuration file
include/flatcc/flatcc_rtconfig.h
This file can modify certain aspects of JSON parsing and printing such as disabling the Grisu3 library or requiring that all names in JSON are quoted.
For most users, it should not be relevant to modify these configuration settings. If changes are required, they can be given in the build system - it is not necessary to edit the config files, for example to disable trailing comma in the JSON parser:
cc -DFLATCC_JSON_PARSE_ALLOW_TRAILING_COMMA=0 ...
The compiler library libflatcc.a can compile schemas provided in a memory buffer or as a filename. When given as a buffer, the schema cannot contain include statements - these will cause a compile error.
When given a filename the behavior is similar to the commandline flatcc interface, but with more options - see flatcc.h and config/config.h .
libflatcc.a supports functions named flatcc_... . reflection... may also be available which are simple the C generated interface for the binary schema. The builder library is also included. These last two interfaces are only present because the library supports binary schema generation.
The standalone runtime library libflatccrt.a is a collection of the src/runtime/*.c files. This supports the generated C headers for various features. It is also possible to distribute and compile with the source files directly. For debugging, it is useful to use the libflatccrt_d.a version because it catches a lot of incorrect API use in assertions.
The runtime library may also be used by other languages. See comments in flatcc_builder.h. JSON parsing is on example of an alternative use of the builder library so it may help to inspect the generated JSON parser source and runtime source.
Mostly for implementers: FlatBuffers Binary Format
See Security Considerations.
FlatCC coding style is largely similar to the WebKit Style, with the following notable exceptions:
<stdint.h> types are made available.if (err) return -1; .0 ./* A comment. */true and false keywords are not used (pre C99).snake_case is used over camelCase .#pragma once because it is non-standard and not always reliable in filesystems with ambigious paths.config.h inclusion might be handled differently in that flatbuffers.h includes the config file.unsigned is not used without int for historical reasons. Generally a type like uint32_t is preferred.TODO: instead of FIXME: in comments for historical reasons.All the main source code in compiler and runtime aim to be C11 compatible and uses many C11 constructs. This is made possible through the included portable library such that older compilers can also function. Therefore any platform specific adaptations will be provided by updating the portable library rather than introducing compile time flags in the main source code.
See Benchmarks
