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这篇文章主要介绍“PostgreSQL中Declarations的作用是什么”，在日常操作中，相信很多人在PostgreSQL中Declarations的作用是什么问题上存在疑惑，小编查阅了各式资料，整理出简单好用的操作方法，希望对大家解答”PostgreSQL中Declarations的作用是什么”的疑惑有所帮助！接下来，请跟着小编一起来学习吧！
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Flex输入文件由四部分组成:
%{
Declarations
%}
Definitions
%%
Rules
%%
User subroutines由%{和%}包含的部分为Declarations部分,这一部分都是C代码,会原封不动的copy到lex.yy.c文件中.
比较重要的定义包括:
YYSTYPE-Bison使用一个union联合体来存储所有可能类型的值,全局变量yyvalue的类型是YYSTYPE.
%top{
/*-------------------------------------------------------------------------
*
* scan.l
* lexical scanner for PostgreSQL
* PostgreSQL的词法扫描器
*
* NOTE NOTE NOTE:
* 特别特别特别注意:
* The rules in this file must be kept in sync with src/fe_utils/psqlscan.l!
* 这个文件中的规则必须与src/fe_utils/psqlscan.l文件中的规则保持一致!!!
*
* The rules are designed so that the scanner never has to backtrack,
* in the sense that there is always a rule that can match the input
* consumed so far (the rule action may internally throw back some input
* with yyless(), however). As explained in the flex manual, this makes
* for a useful speed increase --- about a third faster than a plain -CF
* lexer, in simple testing. The extra complexity is mostly in the rules
* for handling float numbers and continued string literals. If you change
* the lexical rules, verify that you haven't broken the no-backtrack
* property by running flex with the "-b" option and checking that the
* resulting "lex.backup" file says that no backing up is needed. (As of
* Postgres 9.2, this check is made automatically by the Makefile.)
* 之所以设计这一的规则是便于扫描器不需要回溯,确保对于输入一定有一条规则与其匹配
* (但是,规则动作可能在内部用yyless() throw back一些输入).
* 正如Flex手册中所说明的,这可以提升性能 --
* 在简单测试的情况下,相对于普通的-CF词法分析器,大概有1/3的性能提升.
* 额外的复杂性主要体现在处理浮点数和连续字符串文字的规则中.
* 如果修改了词法规则,通过以-b选项执行Flex以确保没有打破无回溯的约定,
* 并且坚持结果文件"lex.backup"以确认无需备份.
* (在PG 9.2,该检查通过Makefile自动执行)
*
* Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* src/backend/parser/scan.l
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include
#include
#include "common/string.h"
#include "parser/gramparse.h"
#include "parser/parser.h" /* only needed for GUC variables */
#include "parser/scansup.h"
#include "mb/pg_wchar.h"
}
//------------------ 声明部分
%{
/* LCOV_EXCL_START */
/* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
//在扫描器出现致命错误时,避免调用exit()直接退出
#undef fprintf
#define fprintf(file, fmt, msg) fprintf_to_ereport(fmt, msg)
static void
fprintf_to_ereport(const char *fmt, const char *msg)
{
ereport(ERROR, (errmsg_internal("%s", msg)));
}
/*
* GUC variables. This is a DIRECT violation of the warning given at the
* head of gram.y, ie flex/bison code must not depend on any GUC variables;
* as such, changing their values can induce very unintuitive behavior.
* But we shall have to live with it until we can remove these variables.
* GUC参数变量.这直接违反了gram.y中提出的约定,如flex/bison代码不能依赖GUC变量;
* 因此,改变他们的值会导致未知的后果.
* 但在去掉这些变量前,不得不"活下去"
*/
int backslash_quote = BACKSLASH_QUOTE_SAFE_ENCODING;
bool escape_string_warning = true;
bool standard_conforming_strings = true;
/*
* Set the type of YYSTYPE.
* 设置YYSTYPE.
* 在Bison中,全局变量yylval的类型为YYSTYPE,默认为int
* Internally, bison declares each value as a C union that includes all of the types.
* You list all of the types in %union declarations.
* Bison turns them into a typedef for a union type called YYSTYPE.
*/
#define YYSTYPE core_YYSTYPE
/*
* Set the type of yyextra. All state variables used by the scanner should
* be in yyextra, *not* statically allocated.
* 设置yyextra的数据类型.所有扫描器使用的状态变量应在yyextra中,不是静态分配的.
*/
#define YY_EXTRA_TYPE core_yy_extra_type *
/*
* Each call to yylex must set yylloc to the location of the found token
* (expressed as a byte offset from the start of the input text).
* When we parse a token that requires multiple lexer rules to process,
* this should be done in the first such rule, else yylloc will point
* into the middle of the token.
* 每一次调用yylex必须设置yylloc指向发现的token所在的位置.
* (从输入文本开始计算的字节偏移量)
* 在分析一个需要多个词法规则进行处理的token时,
* 在第一次应用规则时就应该完成这个动作,否则的话yylloc会指向到token的中间位置.
*/
#define SET_YYLLOC() (*(yylloc) = yytext - yyextra->scanbuf)
/*
* Advance yylloc by the given number of bytes.
* 通过给定的字节数调整yylloc的位置
*/
#define ADVANCE_YYLLOC(delta) ( *(yylloc) += (delta) )
#define startlit() ( yyextra->literallen = 0 )
static void addlit(char *ytext, int yleng, core_yyscan_t yyscanner);
static void addlitchar(unsigned char ychar, core_yyscan_t yyscanner);
static char *litbufdup(core_yyscan_t yyscanner);
static char *litbuf_udeescape(unsigned char escape, core_yyscan_t yyscanner);
static unsigned char unescape_single_char(unsigned char c, core_yyscan_t yyscanner);
static int process_integer_literal(const char *token, YYSTYPE *lval);
static bool is_utf16_surrogate_first(pg_wchar c);
static bool is_utf16_surrogate_second(pg_wchar c);
static pg_wchar surrogate_pair_to_codepoint(pg_wchar first, pg_wchar second);
static void addunicode(pg_wchar c, yyscan_t yyscanner);
static bool check_uescapechar(unsigned char escape);
#define yyerror(msg) scanner_yyerror(msg, yyscanner)
#define lexer_errposition() scanner_errposition(*(yylloc), yyscanner)
static void check_string_escape_warning(unsigned char ychar, core_yyscan_t yyscanner);
static void check_escape_warning(core_yyscan_t yyscanner);
/*
* Work around a bug in flex 2.5.35: it emits a couple of functions that
* it forgets to emit declarations for. Since we use -Wmissing-prototypes,
* this would cause warnings. Providing our own declarations should be
* harmless even when the bug gets fixed.
* Flex 2.5.35存在一个bug:忽略了函数但没有忽略函数声明.
* 因为使用了-Wmissing-prototypes选项,这会导致警告出现.
* 就算bug修复,提供PG的声明也可能会存在问题.
*/
extern int core_yyget_column(yyscan_t yyscanner);
extern void core_yyset_column(int column_no, yyscan_t yyscanner);
%} 到此，关于“PostgreSQL中Declarations的作用是什么”的学习就结束了，希望能够解决大家的疑惑。理论与实践的搭配能更好的帮助大家学习，快去试试吧！若想继续学习更多相关知识，请继续关注创新互联网站，小编会继续努力为大家带来更多实用的文章！