今天编译mysql的动态库(libodbc.so)编译过了，但当我编译一个应用服务时需链接用到mysql的动态库时就报错：
/home/sc/lib/libodbc.so: undefined reference to `mysql_server_init'
/home/sc/lib/libodbc.so: undefined reference to `mysql_close'
/home/sc/lib/libodbc.so: undefined reference to `mysql_affected_rows'
/home/sc/lib/libodbc.so: undefined reference to `mysql_ping'
/home/sc/lib/libodbc.so: undefined reference to `mysql_real_query'
/home/sc/lib/libodbc.so: undefined reference to `mysql_set_character_set'
/home/sc/lib/libodbc.so: undefined reference to `mysql_free_result'
/home/sc/lib/libodbc.so: undefined reference to `mysql_autocommit'
/home/sc/lib/libodbc.so: undefined reference to `mysql_fetch_field_direct'
/home/sc/lib/libodbc.so: undefined reference to `mysql_real_connect'
/home/sc/lib/libodbc.so: undefined reference to `mysql_store_result'
/home/sc/lib/libodbc.so: undefined reference to `mysql_field_count'
/home/sc/lib/libodbc.so: undefined reference to `mysql_commit'
/home/sc/lib/libodbc.so: undefined reference to `mysql_fetch_row'
/home/sc/lib/libodbc.so: undefined reference to `mysql_error'
/home/sc/lib/libodbc.so: undefined reference to `mysql_server_end'
/home/sc/lib/libodbc.so: undefined reference to `mysql_rollback'
/home/sc/lib/libodbc.so: undefined reference to `mysql_errno'
/home/sc/lib/libodbc.so: undefined reference to `mysql_init'
排错：
1. 查看动态库mysql相关的方法:nm libodbc.so|grep mysql
U mysql_affected_rows
U mysql_autocommit
U mysql_close
U mysql_commit
U mysql_errno
U mysql_error
U mysql_fetch_field_direct
U mysql_fetch_row
U mysql_field_count
U mysql_free_result
U mysql_init
U mysql_ping
U mysql_real_connect
U mysql_real_query
U mysql_rollback
U mysql_server_end
U mysql_server_init
U mysql_set_character_set
U mysql_store_result
其实这些方法前面的 'U' 代表:该符号未定义过，需要自其他对象文件中链接进来;
这时候在看编译mysql动态库所链接的动态库，发现了链接-L$(MYSQL_LIB),链接时(-L是代表库的路径，而我们这个系统现在根本没有这个环境变量)，这个估计是同事在调不同mysql版本时改makefile加上的
将此去掉在重新编译，那些方法前面的 'U' 变成了 'T'代表:该符号放在代码段中，通常是那些全局非静态函数;