参考

《gcc源码分析》

词法分析

词法分析的过程就是将源代码识别成一个一个的词法符号，并在词法分析的过程中创建一些树节点，用来保存某些词法符号的值（value）。这些需要创建树节点的词法符号主要包括标识符（CPP_NAME）、关键字（CPP_KEYWORD）、数值（CPP_NUMBER）常量等。在后续的语法分析过程中，会将这些AST节点（或者AST子树）根据语法规则链接起来，形成某个函数完整的AST结构。

语法分析

语法分析就是，提前预读两个词法符号的自顶向下的语法推导过程：包括提前预读词法符号，语法分析状态以及上下文信息。

这些信息通常将保存在c_parser 这个结构体当中：

/* A parser structure recording information about the state andcontext of parsing.  Includes lexer information with up to twotokens of look-ahead; more are not needed for C.  */
typedef struct c_parser GTY(())
{/* The look-ahead tokens.  */c_token tokens[2];/* How many look-ahead tokens are available (0, 1 or 2).  */short tokens_avail;/* True if a syntax error is being recovered from; false otherwise.c_parser_error sets this flag.  It should clear this flag whenenough tokens have been consumed to recover from the error.  */BOOL_BITFIELD error : 1;/* True if we're processing a pragma, and shouldn't automaticallyconsume CPP_PRAGMA_EOL.  */BOOL_BITFIELD in_pragma : 1;/* True if we're parsing the outermost block of an if statement.  */BOOL_BITFIELD in_if_block : 1;/* True if we want to lex an untranslated string.  */BOOL_BITFIELD lex_untranslated_string : 1;/* Objective-C specific parser/lexer information.  */BOOL_BITFIELD objc_pq_context : 1;/* The following flag is needed to contextualize Objective-C lexicalanalysis.  In some cases (e.g., 'int NSObject;'), it isundesirable to bind an identifier to an Objective-C class, evenif a class with that name exists.  */BOOL_BITFIELD objc_need_raw_identifier : 1;
} c_parser;

gcc对c语言进行语法分析的入口函数为c_parse_file ：

/* Parse a single source file.  */void
c_parse_file (void)
{/* Use local storage to begin.  If the first token is a pragma, parse it.If it is #pragma GCC pch_preprocess, then this will load a PCH filewhich will cause garbage collection.  */c_parser tparser;memset (&tparser, 0, sizeof tparser);the_parser = &tparser;if (c_parser_peek_token (&tparser)->pragma_kind == PRAGMA_GCC_PCH_PREPROCESS)c_parser_pragma_pch_preprocess (&tparser);the_parser = GGC_NEW (c_parser);*the_parser = tparser;c_parser_translation_unit (the_parser);the_parser = NULL;
}

预读一个符号

预读一个符号，如果是PRAGMA_GCC_PCH_PREPROCESS，将进行编译的预处理工作，c_parser_pragma_pch_preprocess 为预处理函数。
c_parser_peek_token 会解析出一个token，这时会返回指向下个token的指针：

/* Return a pointer to the next token from PARSER, reading it in ifnecessary.  */static inline c_token *
c_parser_peek_token (c_parser *parser)
{if (parser->tokens_avail == 0){c_lex_one_token (parser, &parser->tokens[0]);parser->tokens_avail = 1;}return &parser->tokens[0];
}

解析一个单独的词法符号时，调用c_lex_one_token

/* Read in and lex a single token, storing it in *TOKEN.  */static void
c_lex_one_token (c_parser *parser, c_token *token)
{timevar_push (TV_LEX);token->type = c_lex_with_flags (&token->value, &token->location, NULL,(parser->lex_untranslated_string? C_LEX_STRING_NO_TRANSLATE : 0));token->id_kind = C_ID_NONE;token->keyword = RID_MAX;token->pragma_kind = PRAGMA_NONE;switch (token->type){case CPP_NAME:{tree decl;bool objc_force_identifier = parser->objc_need_raw_identifier;if (c_dialect_objc ())parser->objc_need_raw_identifier = false;if (C_IS_RESERVED_WORD (token->value)){enum rid rid_code = C_RID_CODE (token->value);if (rid_code == RID_CXX_COMPAT_WARN){warning_at (token->location,OPT_Wc___compat,"identifier %qs conflicts with C++ keyword",IDENTIFIER_POINTER (token->value));}else if (c_dialect_objc ()){if (!objc_is_reserved_word (token->value)&& (!OBJC_IS_PQ_KEYWORD (rid_code)|| parser->objc_pq_context)){/* Return the canonical spelling for this keyword.  */token->value = ridpointers[(int) rid_code];token->type = CPP_KEYWORD;token->keyword = rid_code;break;}}else{token->type = CPP_KEYWORD;token->keyword = rid_code;break;}}decl = lookup_name (token->value);if (decl){if (TREE_CODE (decl) == TYPE_DECL){token->id_kind = C_ID_TYPENAME;break;}}else if (c_dialect_objc ()){tree objc_interface_decl = objc_is_class_name (token->value);/* Objective-C class names are in the same namespace asvariables and typedefs, and hence are shadowed by localdeclarations.  */if (objc_interface_decl&& (global_bindings_p ()|| (!objc_force_identifier && !decl))){token->value = objc_interface_decl;token->id_kind = C_ID_CLASSNAME;break;}}token->id_kind = C_ID_ID;}break;case CPP_AT_NAME:/* This only happens in Objective-C; it must be a keyword.  */token->type = CPP_KEYWORD;token->keyword = C_RID_CODE (token->value);break;case CPP_COLON:case CPP_COMMA:case CPP_CLOSE_PAREN:case CPP_SEMICOLON:/* These tokens may affect the interpretation of any identifiersfollowing, if doing Objective-C.  */if (c_dialect_objc ())parser->objc_need_raw_identifier = false;break;case CPP_PRAGMA:/* We smuggled the cpp_token->u.pragma value in an INTEGER_CST.  */token->pragma_kind = (enum pragma_kind) TREE_INT_CST_LOW (token->value);token->value = NULL;break;default:break;}timevar_pop (TV_LEX);
}

语法分析函数调用关系

重点函数分析

c_parse_file

函数c_parse_filer如上所述，是C语法分析的入口函数，该函数首先对当前的词法符号进行判断，如果该符号的编译制导类型是PRAGMA_GCC_PCH_PREPROCESS，则调用函数c_parser_pragma_pch_preprocess对进行源代码的预处理，否则，调用函数c_parser_translation_unit()进行语法推导。

c_parser_translation_unit

这个函数是进行语法分析的主流程函数：