前一段领导开发了一个内核的模块，测试的过程中，发现导致MYSQL客户端无法连接服务器。
经过查询文档，追根溯源，终于找到了MYSQL实现链接客户端的代码，在源文件sql-common/client.c里的 CLI_MYSQL_REAL_CONNECT 函数。
但是代码很长，一时半会儿肯定看不明白。这个时候发现，发现代码当中有很多这样的代码：
DBUG_ENTER("mysql_real_connect");
说明只要以调试模式启动MYSQL，就可以跟踪代码的执行。
经过查询文档和测试，只要在 cmake 的时候，增加参数 cmake -WITH_DEBUG=1 就可以了。
然后启动一个MYSQL客户端程序之前，更改一个环境变量：
export MYSQL_DEBUG=d:t:O,/tmp/client.trace
好牛逼啊！我们不禁感叹。之余，学习了一下这个调试模式的具体实现：
在 CMakeList.txt 中，有这么一段代码：
IF(WITH_DEBUG)
SET(CMAKE_BUILD_TYPE "Debug" CACHE STRING ${BUILDTYPE_DOCSTRING} FORCE)
IF(NOT CMAKE_BUILD_TYPE
AND NOT CMAKE_GENERATOR MATCHES "Visual Studio"
AND NOT CMAKE_GENERATOR MATCHES "Xcode")
# This is the case of no CMAKE_BUILD_TYPE choosen, typical for VS and Xcode
# or if custom C flags are set. In VS and Xcode for non-Debug configurations
# DBUG_OFF is already correctly set. Use DBUG_OFF for Makefile based projects
# without build type too, unless user specifically requests DBUG.
IF(NOT CMAKE_C_FLAGS MATCHES "-DDBUG_ON")
ADD_DEFINITIONS(-DDBUG_OFF)
ENDIF()
ENDIF()
如果没有设置 CMAKE_BUILD_TYPE ，那么就会执行下面一段代码。如果这时候也没有设置一个名为-DDBUG_ON的CFLAGS的环境变量的话，就会增加一个CFLAGS：-DDBUG_OFF。
之后，当编译器编译的时候，根据编译器的参数增加的宏来决定 怎么定义 DBUG_ENTER 之类的函数是空代码，还是实际的报错代码。
/*
* These macros provide a user interface into functions in the
* dbug runtime support library. They isolate users from changes
* in the MACROS and/or runtime support.
*
* The symbols "__LINE__" and "__FILE__" are expanded by the
* preprocessor to the current source file line number and file
* name respectively.
*
* WARNING --- Because the DBUG_ENTER macro allocates space on
* the user function's stack, it must precede any executable
* statements in the user function.
*
*/
# ifdef DBUG_OFF
# define DBUG_ENTER(a1)
# define DBUG_RETURN(a1) return(a1)
# define DBUG_VOID_RETURN return
# define DBUG_EXECUTE(keyword,a1)
# define DBUG_PRINT(keyword,arglist)
# define DBUG_2(keyword,format) /* Obsolete */
# define DBUG_3(keyword,format,a1) /* Obsolete */
# define DBUG_4(keyword,format,a1,a2) /* Obsolete */
# define DBUG_5(keyword,format,a1,a2,a3) /* Obsolete */
# define DBUG_PUSH(a1)
# define DBUG_POP()
# define DBUG_PROCESS(a1)
# define DBUG_FILE (stderr)
# define DBUG_SETJMP setjmp
# define DBUG_LONGJMP longjmp
# define DBUG_DUMP(keyword,a1)
# else
# define DBUG_ENTER(a)
auto char *_db_func_; auto char *_db_file_; auto int _db_level_;
auto char **_db_framep_;
_db_enter_ (a,__FILE__,__LINE__,&_db_func_,&_db_file_,&_db_level_,
&_db_framep_)
# define DBUG_LEAVE
(_db_return_ (__LINE__, &_db_func_, &_db_file_, &_db_level_))
# define DBUG_RETURN(a1) return (DBUG_LEAVE, (a1))
# define DBUG_VOID_RETURN {DBUG_LEAVE; return;}
# define DBUG_EXECUTE(keyword,a1)
{if (_db_on_) {if (_db_keyword_ (keyword)) { a1 }}}
# define DBUG_PRINT(keyword,arglist)
{if (_db_on_) {_db_pargs_(__LINE__,keyword); _db_doprnt_ arglist;}}
# define DBUG_2(keyword,format)
DBUG_PRINT(keyword,(format)) /* Obsolete */
# define DBUG_3(keyword,format,a1)
DBUG_PRINT(keyword,(format,a1)) /* Obsolete */
# define DBUG_4(keyword,format,a1,a2)
DBUG_PRINT(keyword,(format,a1,a2)) /* Obsolete */
# define DBUG_5(keyword,format,a1,a2,a3)
DBUG_PRINT(keyword,(format,a1,a2,a3)) /* Obsolete */
# define DBUG_PUSH(a1) _db_push_ (a1)
# define DBUG_POP() _db_pop_ ()
# define DBUG_PROCESS(a1) (_db_process_ = a1)
# define DBUG_FILE (_db_fp_)
# define DBUG_SETJMP(a1) (_db_setjmp_ (), setjmp (a1))
# define DBUG_LONGJMP(a1,a2) (_db_longjmp_ (), longjmp (a1, a2))
# define DBUG_DUMP(keyword,a1,a2) _db_dump_(__LINE__,keyword,a1,a2)
# endif