我们的一个服务器软件在线上环境运行时出现了内存缓慢增长的问题。
用valgrind测试 MySQL的C客户端mysqlclient发现，它在正常的使用中会被valgrind报出存在内存泄漏。
1 正常使用场景
下面的代码是使用mysqlclient读取数据的最常用的代码
#include #include int main(){ MYSQL *conn; MYSQL_RES *result; MYSQL_ROW row; char *w; conn = mysql_init(NULL); mysql_real_connect(conn, "127.0.0.1", "root", "", "db_test", 3306, NULL, 0); mysql_query(conn, "select id from test"); result = mysql_store_result(conn); if (result == NULL) { printf("%d:%sn", mysql_errno(conn), mysql_error(conn)); goto out; } while ((row = mysql_fetch_row(result))) { w = row[0]; } out: mysql_free_result(result); mysql_close(conn); mysql_library_end(); return 0;}
上面的代码用valgrind检测内存泄漏输出如下：
cobbliu@ubuntu:~/dev/test$ gcc -o mysql mysql.c -lmysqlclient
cobbliu@ubuntu:~/dev/test$ valgrind --leak-check=full --show-reachable=yes ./mysql
==4497== Memcheck, a memory error detector
==4497== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==4497== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==4497== Command: ./mysql
==4497== in use at exit: 73,872 bytes in 21 blocks
==4497== total heap usage: 84 allocs, 63 frees, 128,626 bytes allocated
==4497== definitely lost: 0 bytes in 0 blocks
==4497== indirectly lost: 0 bytes in 0 blocks
==4497== possibly lost: 0 bytes in 0 blocks
==4497== still reachable: 73,872 bytes in 21 blocks
==4497== suppressed: 0 bytes in 0 blocks
==4497== For counts of detected and suppressed errors, rerun with: -v
==4497== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
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可以看出，在正常的使用场景下，mysqlclient有一部分内存在 mysql_close() 之后并没有被释放。
2 解决方法
stackoverflow上的一篇文章提出了解决方法：在 mysql_close() 之后调用 mysql_library_end() 来释放 剩余的内存空间 MySQL开发手册对 mysql_library_end() 的描述如下：
This function finalizes the MySQL library.
Call it when you are done using the library (for example, after disconnecting from the server).
The action taken by the call depends on whether your application is linked to the MySQL client library or the MySQL embedded server library.
For a client program linked against the libmysqlclient library by using the -lmysqlclient flag, mysql_library_end() performs some memory management to clean up.
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在上面的示例代码中加入 mysql_library_end() 函数：
mysql_free_result(result); mysql_close(conn); mysql_library_end();
cobbliu@ubuntu:~/dev/test$ valgrind --leak-check=full --show-reachable=yes ./mysql
==4513== Memcheck, a memory error detector
==4513== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==4513== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==4513== Command: ./mysql
==4513== in use at exit: 0 bytes in 0 blocks
==4513== total heap usage: 84 allocs, 84 frees, 128,626 bytes allocated
==4513== All heap blocks were freed -- no leaks are possible
==4513== For counts of detected and suppressed errors, rerun with: -v
==4513== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
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所以对于daemon进程，如果频繁地调用 mysql_init 和 mysql_close 的话，记得在 mysql_close 之后调用 mysql_library_end() 来释放未被释放的内存
Date: 2014-05-05 13:42:21 CST
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原文始发于微信公众号（汇编语言）：MySQL C 客户端的内存泄漏问题
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