有同学做类似如下的操作：
| class X { public: X() // 类X的构造函数ctor { _mysql_handler = mysql_init(NULL); }
// 定义类X的全局变量 X g_x;
// 程序入口main函数 int main() { 。。。 。。。 } |
看似简单的代码，但非常不幸，程序运行时，卡在了mysql_init处。语法上看不出任何破绽，原因会是什么了？
他提供了另一个线索：不在构造函数中调用mysql_init则正常，不会卡住。结合起来分析，推断是因为mysql_init中也使用到了全局变量（另一种原因是有越界），而全局变量的初始化顺序程序是无法约定的，很有可能是因为g_x的初始化，发生在mysql_init依赖的全局变量之前。若推论成立，则mysql_init使用了未初始化的值，这是导致它卡住的根本原因。可以使用valgrind验证一下。当然，使用下列的方法应当也能奏效：全局变量相互依赖和初始化顺序的解决办法（http://blog.chinaunix.net/uid-20682147-id-3245149.html），即改成：
| #define g_x x_ref() X& x_ref() { static X x; // 技巧就在这里 return x; } |
当然，良好的习惯是尽量避免使用全局变量，实在无法避免时（如考虑到结构的复杂性），则可以考虑用上述方法规避全局变量互依赖产生的问题。
附1：mysql_init源码
| /**************************************************************************** Init MySQL structure or allocate one ****************************************************************************/
MYSQL * STDCALL mysql_init(MYSQL *mysql) { if (mysql_server_init(0, NULL, NULL)) return 0; if (!mysql) { if (!(mysql=(MYSQL*) my_malloc(sizeof(*mysql),MYF(MY_WME | MY_ZEROFILL)))) { set_mysql_error(NULL, CR_OUT_OF_MEMORY, unknown_sqlstate); return 0; } mysql->free_me=1; } else memset(mysql, 0, sizeof(*(mysql))); mysql->charset=default_client_charset_info; strmov(mysql->net.sqlstate, not_error_sqlstate);
/* Only enable LOAD DATA INFILE by default if configured with --enable-local-infile */
#if defined(ENABLED_LOCAL_INFILE) && !defined(MYSQL_SERVER) mysql->options.client_flag|= CLIENT_LOCAL_FILES; #endif
#ifdef HAVE_SMEM mysql->options.shared_memory_base_name= (char*) def_shared_memory_base_name; #endif
mysql->options.methods_to_use= MYSQL_OPT_GUESS_CONNECTION; mysql->options.report_data_truncation= TRUE; /* default */
/* By default we don't reconnect because it could silently corrupt data (after reconnection you potentially lose table locks, user variables, session variables (transactions but they are specifically dealt with in mysql_reconnect()). This is a change: < 5.0.3 mysql->reconnect was set to 1 by default. How this change impacts existing apps: - existing apps which relyed on the default will see a behaviour change; they will have to set reconnect=1 after mysql_real_connect(). - existing apps which explicitely asked for reconnection (the only way they could do it was by setting mysql.reconnect to 1 after mysql_real_connect()) will not see a behaviour change. - existing apps which explicitely asked for no reconnection (mysql.reconnect=0) will not see a behaviour change. */ mysql->reconnect= 0;
mysql->options.secure_auth= TRUE;
return mysql; } |
附2：mysql_server_init源码
| /* Initialize the MySQL client library
SYNOPSIS mysql_server_init()
NOTES Should be called before doing any other calls to the MySQL client library to initialize thread specific variables etc. It's called by mysql_init() to ensure that things will work for old not threaded applications that doesn't call mysql_server_init() directly.
RETURN 0 ok 1 could not initialize environment (out of memory or thread keys) */
int STDCALL mysql_server_init(int argc __attribute__((unused)), char **argv __attribute__((unused)), char **groups __attribute__((unused))) { int result= 0; if (!mysql_client_init) { mysql_client_init=1; org_my_init_done=my_init_done; if (my_init()) /* Will init threads */ return 1; init_client_errs(); if (mysql_client_plugin_init()) return 1; if (!mysql_port) { char *env; struct servent *serv_ptr __attribute__((unused));
mysql_port = MYSQL_PORT;
/* if builder specifically requested a default port, use that (even if it coincides with our factory default). only if they didn't do we check /etc/services (and, failing on that, fall back to the factory default of 3306). either default can be overridden by the environment variable MYSQL_TCP_PORT, which in turn can be overridden with command line options. */
#if MYSQL_PORT_DEFAULT == 0 if ((serv_ptr= getservbyname("mysql", "tcp"))) mysql_port= (uint) ntohs((ushort) serv_ptr->s_port); #endif if ((env= getenv("MYSQL_TCP_PORT"))) mysql_port=(uint) atoi(env); }
if (!mysql_unix_port) { char *env; #ifdef __WIN__ mysql_unix_port = (char*) MYSQL_NAMEDPIPE; #else mysql_unix_port = (char*) MYSQL_UNIX_ADDR; #endif if ((env = getenv("MYSQL_UNIX_PORT"))) mysql_unix_port = env; } mysql_debug(NullS); #if defined(SIGPIPE) && !defined(__WIN__) (void) signal(SIGPIPE, SIG_IGN); #endif #ifdef EMBEDDED_LIBRARY if (argc > -1) result= init_embedded_server(argc, argv, groups); #endif } else result= (int)my_thread_init(); /* Init if new thread */ return result; } |