flatcc
0.6.1 Bugfix and maintenance release
ubuntu、macos、windows：windows：weekly：
JSONパーサーは、ライブラリバージョンを一致させるためにコード生成を必要とする将来のリリースで、組合ベクターを解析するためのインターフェイスを変更する場合があります。
flatccは、ビルドおよびコンパイラツール、Cランタイムライブラリを除き、外部の依存関係はありません。同時のNinjaビルドを使用すると、小さなクライアントプロジェクトは、ライブラリでFlatCCを構築し、スキーマコードを生成し、数秒でテストケースを実行し、15Kから60Kの間のバイナリを生成し、30nsで小さなバッファーを読み取り、約600nsでフラットバッファーを構築し、より大きな実行可能なJSON Parsing op of the Pishのjson parsing open option option json parsingを処理できます。
このプロジェクトは、FlatBufferスキーマファイルを考慮して、CのFlatBuffersコードを生成するコンパイラであるFlatCCを構築します。このはじめには、ここCバージョンの従来のモンスターの例を備えた個別のテストプロジェクトも作成します。
今のところ、それは一般的な要件ではありませんが、UNIXのようなシステムを想定しています。建物も参照してください。 git、cmake、bash、cコンパイラ、忍者ビルドシステム、またはmakeのいずれかが必要です。
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オプションであり、ビルドバックエンドを選択します。
セットアップスクリプトは、Cmakeを使用してFlatCCを構築し、Monsterの例を使用したテストプロジェクトディレクトリと、単なるシェルスクリプトであるビルドスクリプトを作成します。ヘッダーとライブラリは、テストプロジェクトに象徴的にリンクされています。 FlatCCがコンパイルされたら、独自のプロジェクトを構築するためにCmakeは必要ありません。
Foobarという名前の別のテストプロジェクトを作成するには、 scripts/setup.sh -s -x ../foobarを呼び出します。これにより、FlatCCプロジェクトの再構築をゼロから再構築できなくなります。
注：changelogを参照してください。 APIの矛盾が発見されると、時折の軽微な変化があります。明確に述べられていない限り、変更を破ることはコンパイルされたランタイムライブラリのみに影響しません。ヘッダーファイルのみになります。問題が発生した場合、 flatccツールがinclude/flatccパスと同じバージョンであることを確認してください。
プロジェクトには以下が含まれます：
flatcc Flatbuffersスキーマコンパイラと、対応するライブラリlibflatcc.a 。コンパイラは、Cヘッダーファイルまたはバイナリフラットバッファスキーマを生成します。libflatccrt.a 、このライブラリに依存します。また、他の言語インターフェイスにも役立ちます。ライブラリは、パーサーなどからバッファを簡単に構築できるようにスタック状態を維持しています。flatcc/portableヘッダーのみのライブラリ、およびEndianハンドリングと数値印刷や解析など、すべてのコンパイラ用の小型ヘルパー。参照：
バグの報告
Googleフラットバッファー
指示を作成します
クイックスタート
ビルダーインターフェイスリファレンス
ベンチマーク
flatccコンパイラは、Googles flatcコンパイラを拡張する代わりに、スタンドアロンツールとして実装され、長期走行プロセスで虐待的な入力で丁寧に失敗するように設計されたスキーマコンパイラの純粋なポータブルCライブラリ実装を実装します。また、Cバージョンは、C ++よりもCとのインターフェースが容易になる他の言語インターフェイスへのスキーマ解析を提供するのに役立つと考えられています。 Googles FPL Labのフラットバファーズチームは、フィードバックを提供し、多くの質問に答えて、可能な限り最高の互換性を確保するのに非常に役立ちました。 flatcc （Flatbuffers Cコンパイラ）vs Google flatcという名前に注目してください。
JSON形式は、Googles flatcツールと互換性があります。 flatcツールは、スキーマとバッファーを入力として使用してコマンドラインからJSONを変換します。 flatcc 、実行時にJSONを読み書きするためのスキーマ固有のコードを生成します。 flatccアプローチははるかに高速で展開しやすくなりますが、 flatcアプローチは、ゲームシーンの編集などのJSONを手動で作業する場合、より便利になる可能性があります。どちらのツールにもその場所があります。
注：ビッグエンディアンプラットフォームは、リリース0.4.0の時点でのみサポートされています。
Mesonビルドシステムのサポートを追加することと見なされていますが、問題＃56を介してこれについてのフィードバックに適しています
可能であれば、メインプログラムを備えた短い再現可能なスキーマとソースファイルを提供してください。できればhexdumpを生成し、バッファー検証剤を呼び出して、入力が有効であることを確認し、デバッグライブラリflatccrt_dとリンクします。
バッファーのデバッグとreadfile.hも参照してください。
例：
サンプル/バグラポート
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 リリース0.6.2（開発中）は主にバグ修正リリースです。詳細については、Changelogを参照してください。 _create_as_rootへの呼び出しの前にオブジェクトが作成された場所で、バッファ端がバッファ内で見られる最大のオブジェクトにパッドがパッドにされている場合、オブジェクトが作成された場所で長年のバグが修正されました。 Clang Debug Buildsの場合、-fsanitize =未定義が追加されており、これにはそのフラグを使用してリンカーシンボルの欠落を避けるために依存するソースコードが必要になる場合があります。この機能は、cmakelists.txtで無効にすることができます。
リリース0.6.1には、主にバグの修正とコミュニティからの多数の貢献が含まれ、プラットフォームエッジケースを処理します。さらに、GCCが攻撃的すぎ、頻繁に破壊され、携帯性に反して機能するため、Clangに代わりに依存して、Pendantic GCCの警告が無効になります。既存のC ++テストケースは、Cコードが一般的なC ++コンパイラでも動作することを保証しますが、一部の環境を破ることができるため、すべてのテストを無効にすることなくそのテストを無効にするフラグがあります。フラットバッファ形式のオプションのスカラー値のサポートが追加されました。また、さまざまなプラットフォームでメモリ割り当てを抽象化するためのサポートが改善されています。 <table>_identifier 、 identifier容易に名前の競合につながるため、生成コードで<table>_file_identifier支持して廃止されました。生成されたコードのfile_extension定数は、プレフィックスドット（。）がありません。
リリース0.6.0は、キー属性と一緒に使用する「プライマリ」属性を導入して、検索とソートにデフォルトキーを選択します。プライマリが存在しない場合、最も低いIDのキーがプライマリになります。テーブルとベクトルは、プライマリキーで再帰的に並べ替えることができます。ブレイキング：以前は、最低のIDではなく最初にリストされていたものが主キーになります。また、構造フィールドに固定長のスカラーアレイを導入します（structおよびenum要素はサポートされていません）。構造体は、チャーアレイを含む固定長アレイフィールドをサポートします。空の構造体は完全に機能することはなく、もはやサポートされていません。また、Flatcによってもサポートされなくなりました。注：charアレイは現在、Googleの一部ではありません。フラットコンパイラ-INT8アレイは代わりに使用できます。ブレイキング：空の構造体はサポートされなくなりました - Googles Flatcコンパイラでも無効です。追加の変更については、changelogを参照してください。非推奨： flatcc_accessors.hの関数からの低レベルのcast_to/from castインターフェイスがいくつかの珍しいプラットフォームでフロート変換を破壊するため、 read/write_from/toを支持して削除されます。これは通常の使用には影響しませんが、このリリースでは有効なままです。
0.5.3のリリース0.5.3さまざまなバグ修正（Changelogを参照）と1つの破壊と低い影響の変更：破壊：0.5.3変更ビルダーの動作コールの作成は、 MyGame_Example_Monster_create()のmonster_test.fbs （）の場合、id属性が使用されている場合、常にフィールドIDによって注文されます（＃81）。数値キーフィールドでテーブルをソートするときに未定義の動作を修正します。
リリース0.5.2は、オプションの_getサフィックスを読者のメソッドに導入します。 flatcc -gを使用することにより、 _getメソッドのみが有効です。これにより、一部のフィールド名が潜在的な名前が削除されます。 0.5.2は、テーブルとベクトルの待望のクローン操作も導入します。 C ++ Smoketestを追加して、忍び込み続ける数のポインター割り当てエラーを減らすために追加されました。ランタイムライブラリには、追加のファイルrefmap.cが必要になりました。
リリース0.5.1 JSONプリンターでバッファオーバーランを修正し、C ++と組み込みnewlib Standard Libraryとのポータブルライブラリ<stdalign.h>互換性を改善します。 JSONの印刷と解析は、test_json.cのテストドライバーに見られるように、スキーマルート以外のテーブルを解析して印刷するのに役立つように一貫性がありました。 Monster_test.fbsファイルは、MonsterテーブルをGoogles Flatcバージョンとより一致させるために再編成され、結果としてマイナーなスキーマネームスペースの矛盾が解決されました。ポータブルヘッダーへの明示的な参照は、生成されたソースから移動されました。 Extern "C" c ++ガードは、生成されたヘッダーの周りに追加されました。 0.5.1も低レベルの組合インターフェイスをクリーンアップして、{type、value}という用語が{type、member}および{type、member}で一貫して使用されます。
beブランチで有効になります。プロジェクトが安定すると頻繁に更新する計画はありませんが、特にさまざまなターゲットプラットフォームでのテストに関して、コミュニティからの入力が常に歓迎され、関連するリリースに含まれます。
このリストはやや時代遅れであり、最近のコンパイラバージョンが追加され、CIプラットフォームがサポートされなくなった場合には古いバージョンが削除されますが、サポートされているターゲットの大部分は変更されていません。 MSVC 2010は将来的に非推奨になる可能性があります。
CI-MOREブランチは追加のコンパイラをテストします：
C11/C ++ 11は、常に機能すると予想される参照です。
GCC --pedanticコンパイラオプションは、GCC-8+の時点ではサポートされていません。これは、ポータブルではないコードの変更を強制し、新しいGCCリリースごとにコードベースを破る傾向があるためです。
MSVC 2017は、CI環境がMSVC 2010をサポートしないため、常にテストされるとは限りません。
C ++コンパイラの古い/非標準のバージョンは問題を引き起こします。Static_Assertとalignas static_assertどおりに欠席したり完全に機能したりしない奇妙な方法で動作します。多くの場合、回避策がありますが、 -std=c++11または-std=c++14を使用する方が信頼できます。
ポータブルライブラリは、GCC 4.7の前のstdalign.hおよびstdint.hのc ++制限を回避していないため、PortableライブラリはGCC C ++ pre 4.7をサポートしていません。これは修正できますが、優先事項ではありません。
CI環境が更新されると、以前のTESTETコンパイラバージョンが廃止された可能性があります。現在の構成については、 ci-moreブランチの.travis.ymlおよびappveyor.yml参照してください。
Monsterのサンプルは、C99スタイルコードを使用してC ++バージョンをより適切に追跡するため、MSVC 2010で動作しません。
ビルドオプションFLATCC_TEST使用して、問題があるプラットフォームでflatccコンパイルを作成する可能性のあるすべてのテストを無効にすることができます。 BULDオプションFLATCC_CXX_TEST 、C ++テスト（生成されたCコードを含む単純なC ++ファイル）用に特異的に無効にすることができます。
他のまたは古いコンパイラをサポートできない理由はありませんが、ビルド構成で何らかの作業を必要とし、ポータブルライブラリの更新が必要になる場合があります。上記は、単にテストおよび構成されたものです。
ポータビリティレイヤーには、エンディアンハンドリングなどにとって一般的に重要な機能があり、オプションのC11機能と欠落している機能の互換性を提供するものもあります。一緒になって、これはほとんどのCコンパイラをサポートするはずですが、成熟のためのコミュニティフィードバックに依存しています。
ランタイムの必要なサイズには、-std = C11を使用してinclude/flatcc/reflection （多くの数値解析サポートが必要）をreflection/reflection.fbsすることで、 include/flatcc/supportを大幅に削減できます。必要なファイルの正確なセットは、リリースからリリースに変更される可能性があり、コンパイルされたコードサイズに関しては実際には重要ではありません。
優先事項は、合理的に高速で、サーバーと埋め込みデバイスの両方に適した使いやすいCビルダーインターフェイスを設計することでしたが、絶対パフォーマンスよりも使いやすさ - それでも小さなバッファ出力レートは1秒間に測定され、大まかな推定から1秒あたり10〜100ミリのバッファーを読み取ります。フラットバッファーを読むことは、それらを構築するよりも数桁以上速いです。
1000モンスター、動的に拡張されたモンスター名、モンスターベクトル、およびインベントリベクトルを備えた100MBバッファーの場合、帯域幅は2.2GHz Haswell Core I7 CPUで約2.2GB/sと45ms/バッファーに達します。これには、すべてのデータの読み取りと検証が含まれます。いくつかの重要なフィールドのみを読むと、帯域幅が2.7GB/sと37ms/opに増加します。 10MBバッファーの場合、帯域幅は高くなる可能性がありますが、最終的には小さなバッファーが頭上にあるため、小さなバッファーをエンコードする約150ns/OPで300MB/sに低下します。これらの数字は、大まかなガイドラインにすぎません。明らかに、ハードウェア、コンパイラ、およびエンコードされたデータに依存しています。測定は、初期ウォームアップステップを除外します。
生成されたJSONパーサーは、CまたはC ++で直接フラットバッファーを構築するよりも約4倍遅く、700バイトJSONメッセージの場合は約2200NS対600NSです。したがって、JSONの解析は、Google Flatbuffersベンチマークページで報告されているように、同等のプロトコルバッファーを読むよりも約2桁高速です。 LZ4圧縮により、JSON解析の全体的な処理時間が2倍になります。 JSON印刷は解析よりも高速ですが、それほど大きくはありません。 JSONは、大きなバッファーで圧縮されたフラットバッファーの約半分のサイズに圧縮されますが、小さなバッファーではより悪化します（まったく圧縮していない場合は言うまでもありません）。
フラットバッファー読み取りパフォーマンスは、JSONパーサーとプロトコルバッファーが本質的に本質的に含める検証を除外していることに注意する必要があります。検証はベンチマークされていませんが、大規模なバッファーのほんの一部のみを読み取らない限り、50％未満の読み取りオーバーヘッドを追加するでしょう。
ベンチマークも参照してください。
クライアントCコードは、ビルダースタックがオブジェクトをコミットする前に拡張可能なアリーナを提供するため、ほぼすべての種類の割り当てを避けるためにバッファーを構築することを回避できます。スタックは、リトルエンディアンプラットフォーム上の整数アレイからのベクトルなど、完全なオブジェクトを直接構築できる場合にほとんどバイパスされます。
読者のインターフェイスは、パフォーマンスの改善の余地が少ないため、かなり速くなければなりません。また、ビルダーよりもはるかに簡単です。
また、ユーザビリティは、可能な限り少ない生成されたソースコードとコンパイル時間よりも優先されています。コンパイルされたサイズに大いに影響しないはずです。
コンパイルされたバイナリ出力は、最も制限的なマイクロコントローラーを除くすべてのものに対してかなり小さい必要があります。 33Kモンスターソーステストファイル（生成されたヘッダーとビルダーライブラリに加えて）は、PrintFステートメントやその他のサポートロジックのオーバーヘッド、またはビルダーライブラリを除く30Kオブジェクトファイルを含む50K未満の最適化されたバイナリ実行可能ファイルになります。
読み取り専用のバイナリは小さいですが、作業が少ないことを考慮すると、ビルダーよりもはるかに小さいわけではありません。互換性テストは、事前に生成されたバイナリモンスターmonsterdata_test.goldenモンスターファイルを読み取り、すべてのコンテンツが予想通りであることを確認します。これにより、13K最適化されたバイナリ実行可能ファイルまたは6Kオブジェクトファイルが作成されます。このチェックのソースは、ヘッダーファイルを除く5Kです。読者はライブラリにリンクする必要はありません。
JSONパーサーは、パーサーの決定ツリーにインラインするため、純粋なフラットバッファの使用と比較して、コンパイルされたCバイナリを膨らませます。 Monster.FBのJSONパーサーは、実行可能なバイナリに100K +/-最適化設定を追加する場合があります。
フラットバッファを構築し、フラットバッファを解析および印刷するための生成されたコードはすべて、 include/flatccアクセスが必要です。読者はライブラリに依存していませんが、他のすべての生成されたファイルはlibflatccrt.aランタイムライブラリに依存しています。 libflatcc.a 、FlatCCコンパイラ自体がライブラリとして必要な場合にのみ必要であることに注意してください。
リーダーとビルダーは、生成された共通リーダーとビルダーヘッダーファイルに依存しています。これらの共通ファイルにより、グローバルネームスペースを変更し、基本タイプ（ uoffset_tなど）を再定義することができます。将来的には、これはライブラリコードに移動し、これらの抽象化にマクロを使用し、最終的には標準の32ビットの非署名オフセット（ uoffset_t ）を超えたタイプの事前定義されたファイルのセットを持つ可能性があります。ランタイムライブラリは、タイプ定義のセットに固有です。
使用に関する詳細なガイダンスについては、monster_test.cと生成されたファイルを参照してください。このプロジェクトで使用されているモンスタースキーマは、いくつかの追加のエッジケースをテストするために、オリジナルへのわずかな適応です。
フラットバッファを構築するには、別のビルダーヘッダーファイルがスキーマごとに生成されます。コンパイラと小さなランタイムライブラリlibflatccrt.aによって生成されるflatbuffers_common_builder.hファイルが必要です。読者とビルダー生成コードが別々に保たれるのは、この要件のためです。典型的な使用は、Monster_test.cファイルで見ることができます。ビルダーは、コンテンツをベクトルまたは文字列に繰り返し押すことができますが、含まれるテーブルが更新され、外部形式の解析が簡素化されます。ネストされたバッファーをインラインで構築することも可能です - 最初はこれは過度に聞こえるかもしれませんが、ネットワークインターフェイスでバッファーの結合を包むときに役立ち、すべてのバッファーレベルの適切なアライメントを保証します。
フラットバッファを確認するために、 myschema_verifier.hが生成されます。ランタイムライブラリとリーダーヘッダーに依存します。
JSONパーサーとプリンターは、スキーマファイルごとに1つのファイルを生成し、含まれているスキーマには、パーサーやプリンターを含む独自のパーサーとプリンターがあり、ビルダーの仕組みと類似しています。
低レベル注：ビルダーは、各テーブルの前にアドホックするのではなく、バッファーの端にあるすべてのvtableを生成しますが、それ以外の場合はvtableの同じ重複排除を行います。これにより、vtableをホットキャッシュでクラスター化したり、バッファーを部分的に送信するときにすべてのvtableが利用可能であることを確認できます。この動作は、ランタイムフラグによって無効にすることができます。
一部のユースケースには非常に制約された埋め込みデバイスが含まれる場合があるため、ビルダーライブラリはアロケーターオブジェクトとバッファエミッタオブジェクトでカスタマイズできます。個別のエミッターにより、必要に応じて、メモリに完全なバッファーをメモリに存在させることなく、バッファーを構築できます。
Tipeless Builder Libraryはflatcc_builder.hおよびflatcc_emitter.hに文書化されていますが、C for C for C for Cの生成されたビルダーAPIはビルダーインターフェイスリファレンスに記載されています。
時には、生成されたコードで使用されるマクロの密な性質について懸念が提起されます。これらのマクロは、実際に利用可能な機能を理解することを困難にします。ビルダーインターフェイスリファレンスは、一般的な方法で操作を文書化しようとします。より詳細な情報を取得するために、生成された関数プロトタイプはscripts/flatcc-doc.shスクリプトで抽出できます。
また、マクロが「危険」であることに関心がある人もいます。マクロは、静的または静的なインライン関数を生成するため、FlatCCで使用する場合、安全ではありません。これらは、直接Cコードの場合と同じ拡張に誤って使用された場合、タイムエラーをコンパイルするトリガーをトリガーします。
拡張により、生成された出力が係数10を超えることにより、ソース制御下のコードが爆発せず、生成されたコードのバージョンを意味のある方法で比較し、意図したスキーマと一致するかどうかを確認することができます。マクロは、ポータブルヘッダーを介してプラットフォームの抽象化を扱うためにも重要です。
それでも、ビルドシステムによって直接サポートされていないものの、生成された出力を見ることができます。例として、 include/flatcc/reflection反射スキーマの事前に生成されたヘッダーファイルが含まれています。 clangコンパイラツールチェーンを使用して拡張出力を確認するには、実行してください。
clang -E -DNDEBUG -I include
include/flatcc/reflection/reflection_reader.h |
clang-format
Clangをサポートしていないプラットフォームでは、他の同様のコマンドが利用できる可能性があります。
コンパイラは、生成されたコードのほぼすべてを最適化し、関数が静的または静的インラインであるため、エンドユーザーコードによって実際に参照されるロジックのみを使用することに注意してください。残りの部分は通常、アプリケーションコードに効率的にインラインで、合理的に小さなバイナリコードサイズをもたらします。
詳細については、＃88をご覧ください
生成されたコードの拡張は、特定のオブジェクトタイプのドキュメントを取得するために使用できます。
次のスクリプトがこのプロセスを自動化します。
scripts/flatcc-doc.sh <schema-file> <name-prefix> [<outdir>]
機能プロトタイプを<outdir>/<name-prefix>.docに書きます。
スクリプトにはClangコンパイラとClang-Formatツールが必要であるが、スクリプトは他のツールチェーンにも適応する可能性があることに注意してください。
スクリプトの背後にある原則は、反射スキーマを例として説明することができます。ここでは、オブジェクトテーブルのドキュメントが抽出されます。
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'
Clang-formatのWebKitスタイルにより、パラメーターとリターンタイプがすべて同じ行に配置されることが保証されます。 Grepは、同じラインで開始する関数ヘッダーを抽出し、ストリップ関数ボディをカットします。 SEDストリップ__tmpマクロ名の競合を回避するために使用されるパラメーター名の__TMPサフィックス。グレップストリップ);冗長なフォワード宣言を削除し、SEDを追加します。各ラインを有効なCプロトタイプにするため。
上記は、出力が変化する可能性があるため、常に機能することは保証されていませんが、長い道のりを歩む必要があります。
flatcc-v0.5.2のように、出力の小さな抽出物
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);
...
リフレクションとモンスタースキーマを使用して、次のスクリプトで例を示します。
scripts/reflection-doc-example.sh
scripts/monster-doc-example.sh
モンスタードキュメントの例は、基本的に呼び出します。
scripts/flatcc-doc.sh samples/monster/monster.fbs MyGame_Sample_Monster_
ファイルMyGame_Sample_Monster_.docになります：
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);
...
フラットバッファネイティブタイプも抽出できます。たとえば、文字列操作：
scripts/flatcc-doc.sh samples/monster/monster.fbs flatbuffers_string_
flatbuffers_string_.docになります：
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);
...
詳細については、 flatcc -hを参照してください。
ここにリストされているオンラインバージョン：flatcc -help.mdしかし、最新の参照にはflatcc -hを使用してください。
コンパイラは、読み取り（デフォルト）とライティング（-W）フラットバッファの両方をサポートする、またはサポートの有無にかかわらず、すべてのスキーマと共通ファイルのヘッダーファイルまたはヘッダーを生成できます。最も簡単なオプションは、すべての人に（-a）を使用し、 myschema_builder.hファイルを含めることです。
（-a）または（-v）は、検証剤ファイルも生成します。
Flatbuffer Buildersをコンパイルするときに、compilerにinclude Compilerに含まれるフォルダーの下にあるflatccがcompilerに表示されていることを確認してください。
flatcc （-i）を含むパスは、同じベース名（ケースインセンティブ）が同一であるすべてのスキーマファイルを想定し、最初のファイルのみが含まれます。生成されたすべてのファイルは入力ベースネームを使用し、ワーキングディレクトリまたは（-O）によって設定されたパスに着陸します。
ファイルは、（-stdout）を使用してstdoutに生成できます。 Cヘッダーは注文および連結されますが、それ以外の場合は別のファイル出力と同一です。それぞれに含まれるステートメントはガードされているため、これは欠落しないインクルードファイルにつながりません。
生成されたコードは、特に-stdoutと組み合わされたすべてが大きく表示される場合がありますが、実際に使用される部品のみが最終的な実行可能ファイルまたはオブジェクトファイルをスペースに導きます。最新のコンパイラインラインで、静的にリンクされたビルダーライブラリの必要な部分のみを含めます。
JSONプリンターとパーサーは、-jsonフラグまたは-json-printerまたはjson-parserを使用して生成できます。シンボリックな酵素を印刷することができる特定のランタイムライブラリコンパイルタイムフラグがいくつかありますが、これらは実行時にも無効にすることができます。
libflatccrt （スキーマコンパイラ）ではなく、 libflatcc （ランタイムのRT）にリンクしてください。そうしないと、ビルダーは利用できません。また、includccプロジェクトのルートの「include」を含むパスに「含める」ことを確認してください。
FLATCCは、バッファを読み取ったり検証したりするときに、バッファーのfile_identifierデフォルトで期待します。
バッファーは、オフセット4に予期しない4バイト識別子を持つか、識別子が存在しない可能性があります。
すべての言語インターフェイスがバッファーのファイル識別子をサポートするわけではなく、もしそうなら、古いバージョンではそうしないかもしれません。ユーザーは、PythonとLuaの両方のインターフェイスで問題を報告していますが、これは簡単に解決できます。
検証剤の返品値を確認してください。
int ret;
char *s;
ret = MyTable_verify_as_root(buf, size);
if (ret) {
s = flatcc_verify_error_string(ret);
printf("buffer failed: %sn", s);
}
識別子のないバッファーを確認するか、別の識別子を無視するには、null識別子を使用して検証器の_with_identifierバージョンを使用します。
char *identifier = 0;
MyTable_verify_as_root_with_identifier(buf, size, identifier);
バッファの使用を読むには：
MyTable_as_root_with_identifier(buf, 0);
識別子を使用せずにバッファを構築するには：
MyTable_start_as_root_with_identifier(builder, 0);
...
MyTable_end_as_root_with_identifier(builder, 0);
他のいくつかのas_root呼び出しには、JSON Printingを含むas_root_with_identifierバージョンがあります。
flatcc toolを構築した後、Binariesはbinおよびlib Directoriesにflatccソースツリーの下にあります。
Googles Flatbuffersチュートリアルに従うモンスターの例に直接ジャンプするか、以下のQuickStartガイドに沿って読むことができます。 Monsterチュートリアルをフォローしている場合は、次のように、FlatCCをクローンして構築し、別のプロジェクトディレクトリにソースをコピーすることをお勧めします。
git clone https://github.com/dvidelabs/flatcc.git
flatcc/scripts/setup.sh -a mymonster
cd mymonster
scripts/build.sh
build/mymonster
scripts/setup.sh 、最小限のリンクとして、ライブラリとツールをカスタムmymonsterにリンクします。 （-a）では、単純なビルドスクリプトを追加し、例をコピーし、 .gitignore scripts/setup.sh -hを参照してください。セットアップはFlatCCを構築することもできますが、システム用にビルド環境が構成されていることを確認する必要があります。
独自のスキーマファイルを記述するには、スキーマファイルの作成に関するメインフラットバッファーズプロジェクトのドキュメントに従ってください。
ビルダーインターフェイスの参照は、モンスターサンプルと以下のクイックスタートを研究した後に役立つ場合があります。
ベクトルのソートやキーによる要素を見つけるなどの高度な例を探している場合、 test/monster_testプロジェクトでこれらを見つける必要があります。
次のQuickStartガイドは、 test/monster_testプロジェクトの幅広い簡素化です。スキーマはチュートリアルとはわずかに異なることに注意してください。焦点は、一般的なフラットバッファーの概念ではなく、C固有のフレームワークにあります。
セットアップツールを使用して空のプロジェクトを作成してフォローすることもできますが、以下のテキストにはそれについての仮定はありません。
ここでは、Monster Flatbufferへの読み取り専用アクセスの簡単な例を提供します。これは、Monster_test.cファイルの適応された抽出物です。
まず、common（-c）サポートヘッダーを使用してスキーマの読み取り専用をコンパイルし、monster_test.fbsには他のファイルが含まれているため、再帰を追加します。
flatcc -cr --reader test/monster_test/monster_test.fbs
簡単にするために、プロジェクトルートフォルダーにプロジェクトのサンプルを構築すると仮定しますが、Praxisでは、いくつかのパスを変更することをお勧めします。
mkdir -p build/example
flatcc -cr --reader -o build/example test/monster_test/monster_test.fbs
cd build/example
わかりました：
flatbuffers_common_reader.h
include_test1_reader.h
include_test2_reader.h
monster_test_reader.h
（よりシンプルなsamples/monster/monster.fbsもありますが、スキーマファイルが含まれていることはありません）。
名前空間は長い場合があるため、オプションでマクロを使用してこれを管理します。
#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() {...} */
上記のファイルがmonster_example.cであると仮定すると、以下は、読み取り専用のプロジェクトをコンパイルするいくつかの方法です - ランタイムライブラリとのコンパイルは後で表示されます。
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
インクルードパスまたはソースパスは異なる可能性があります。 include/flatcc/portableの一部のファイルは常に使用されますが、 -D FLATCC_PORTABLEフラグには、C11機能のないコンパイラをサポートするための追加ファイルが含まれています。
注：一部のClang/GCCプラットフォームでは、リンカーがposix_memalign見つけられない場合は、-std = gnu99または-std = gnu11を使用する必要がある場合があります。Paligned_alloc.hのコメントも参照してください。
ここでは、バッファを構築する方法の非常に限られた例を提供します - いくつかのフィールドのみが更新されます。詳細については、Monster_test.cとDoc Directoryを参照してください。
まず、ファイルを生成する必要があります。
flatcc -a monster_test.fbs
これが生成します：
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
注：バッファーを読み取るだけでない限り、実際には以前に表示されている読み物の生成を行いません。ビルダーの生成は常に読み取りACCEを生成します。
"monster_test_builder.h"を含めることにより、他のすべてのファイルが自動的に含まれます。 Cコンパイラには、プロジェクトルートが現在のディレクトリであると仮定して、 flatcc/flatcc_builder.h 、 flatcc/flatcc_verifier.h 、およびその他のファイルにアクセスするため-I include必要です。
検証剤は必要ありません。 -Aオプションを怠lazに選択したために作成されました。
ビルダーは、バッファーを効率的に構築するために必要なランタイム環境をセットアップするには、最初に初期化する必要があります。ビルダーは、実際のバッファーを構築するためにエミッタオブジェクトに依存します。ここでは、デフォルトを暗黙的に使用します。それを手に入れると、ビルダーをハンドルと見なし、バッファを完成させるまでフラットバッファ生成されたAPIに集中できます（つまり、結果にアクセスします）。非自明な用途の場合、カスタムエミッターを提供し、たとえば、すべてのページをflatcc_builder_finalize_buffer flatcc_builder_get_direct_buffer使用してすべてのページを単一のバッファーにマージするのではなく、ネットワーク上でページを排出することをお勧めします。 flatcc_builder.hおよびflatcc_emitter.hのドキュメントコメントも参照してください。 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:
コンパイラ：
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)
ランタイム：
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.
走る
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.
構成
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
