第一次这么近距离的接触源码，有点兴奋，又有点不知所措，尼玛随便一个源文件都有几千行，你说怎么看，还有一些数不清的数据结构，函数，类等乱七八糟的东西。
我装的mysql版本是5.7.12，然后运行平台是centos 6.5 x64 (应该是x64)。
启动gdb
gdb --args /usr/local/mysql-5.7.12/bin/mysqld也可以用如下，如果你设置了链接的话
gdb --args mysqld
然后你就进入了gdb的环境中
在此呢，我们可以用b 函数名来添加断点
这个个断点是我在mysqld.cc的源文件中找的一个函数，随便瞎找的一个。
然后用下面的这个命令来启动mysqld
然后就是gdb c，c的意思是继续的意思，刚刚run --defaults-file=/etc/my.cnf --user=root已经运行了，而且到达设置的断点mysqld_main
如果此时我们用另一个客户端service mysql status一下的话
发现mysql的服务并没有启动
于是我们继续在gdb中输入命令c
...
并且这里是等待模式，没有出现(gdb)
然后在另一可以客户端继续service mysql status一下的话
会发现mysql的服务已经启动了
继续在这个客户端操作，登录mysql
mysql -uroot -hlocalhost -p
发现可以成功的登录进去了
然后再看看调试环境下
可以发现出现了一个新的线程，应该是客户端连接的线程吧。
然后退出mysql，exit，发现gdb环境下没啥变化，没有说线程结束了什么的。
但是如果我们重新开一个客户端登录mysql，又会多出一个线程
关闭了客户端仍然没有发现这两个线程退出的迹象。
当在一个客户端关闭mysql服务的时候，service mysql stop,
会发现在gdb出现了这个
然后
Shutting down MySQL......................................这个点点无限循环下去
于是在gdb环境下，输入命令c
(gdb) c
Continuing.
2016-05-03T10:49:28.524672Z 0 [Note] InnoDB: page_cleaner: 1000ms intended loop took 180093ms. The settings might not be optimal. (flushed=0 and evicted=0, during the time.)
[Thread 0x7fff957fb700 (LWP 4113) exited]
[Thread 0x7fffb8c82700 (LWP 4162) exited]
2016-05-03T10:49:28.569814Z 0 [Note] Giving 0 client threads a chance to die gracefully
2016-05-03T10:49:28.569875Z 0 [Note] Shutting down slave threads
2016-05-03T10:49:28.569896Z 0 [Note] Forcefully disconnecting 0 remaining clients
2016-05-03T10:49:28.569912Z 0 [Note] Event Scheduler: Purging the queue. 0 events
[Thread 0x7fffb8cc3700 (LWP 4135) exited]
2016-05-03T10:49:28.578091Z 0 [Note] Binlog end
2016-05-03T10:49:28.581076Z 0 [Note] Shutting down plugin 'ngram'
2016-05-03T10:49:28.581127Z 0 [Note] Shutting down plugin 'ARCHIVE'
2016-05-03T10:49:28.581135Z 0 [Note] Shutting down plugin 'partition'
2016-05-03T10:49:28.581139Z 0 [Note] Shutting down plugin 'BLACKHOLE'
2016-05-03T10:49:28.581170Z 0 [Note] Shutting down plugin 'CSV'
2016-05-03T10:49:28.581180Z 0 [Note] Shutting down plugin 'MEMORY'
2016-05-03T10:49:28.581189Z 0 [Note] Shutting down plugin 'MyISAM'
2016-05-03T10:49:28.581240Z 0 [Note] Shutting down plugin 'MRG_MYISAM'
2016-05-03T10:49:28.581250Z 0 [Note] Shutting down plugin 'PERFORMANCE_SCHEMA'
2016-05-03T10:49:28.581321Z 0 [Note] Shutting down plugin 'INNODB_SYS_VIRTUAL'
2016-05-03T10:49:28.581337Z 0 [Note] Shutting down plugin 'INNODB_SYS_DATAFILES'
2016-05-03T10:49:28.581341Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESPACES'
2016-05-03T10:49:28.581345Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN_COLS'
2016-05-03T10:49:28.581349Z 0 [Note] Shutting down plugin 'INNODB_SYS_FOREIGN'
2016-05-03T10:49:28.581352Z 0 [Note] Shutting down plugin 'INNODB_SYS_FIELDS'
2016-05-03T10:49:28.581356Z 0 [Note] Shutting down plugin 'INNODB_SYS_COLUMNS'
2016-05-03T10:49:28.581363Z 0 [Note] Shutting down plugin 'INNODB_SYS_INDEXES'
2016-05-03T10:49:28.581367Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLESTATS'
2016-05-03T10:49:28.581370Z 0 [Note] Shutting down plugin 'INNODB_SYS_TABLES'
2016-05-03T10:49:28.581374Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_TABLE'
2016-05-03T10:49:28.581377Z 0 [Note] Shutting down plugin 'INNODB_FT_INDEX_CACHE'
2016-05-03T10:49:28.581381Z 0 [Note] Shutting down plugin 'INNODB_FT_CONFIG'
2016-05-03T10:49:28.581411Z 0 [Note] Shutting down plugin 'INNODB_FT_BEING_DELETED'
2016-05-03T10:49:28.581417Z 0 [Note] Shutting down plugin 'INNODB_FT_DELETED'
2016-05-03T10:49:28.581420Z 0 [Note] Shutting down plugin 'INNODB_FT_DEFAULT_STOPWORD'
2016-05-03T10:49:28.581424Z 0 [Note] Shutting down plugin 'INNODB_METRICS'
2016-05-03T10:49:28.581428Z 0 [Note] Shutting down plugin 'INNODB_TEMP_TABLE_INFO'
2016-05-03T10:49:28.581431Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_POOL_STATS'
2016-05-03T10:49:28.581435Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE_LRU'
2016-05-03T10:49:28.581438Z 0 [Note] Shutting down plugin 'INNODB_BUFFER_PAGE'
2016-05-03T10:49:28.581442Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX_RESET'
2016-05-03T10:49:28.581445Z 0 [Note] Shutting down plugin 'INNODB_CMP_PER_INDEX'
2016-05-03T10:49:28.581449Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM_RESET'
2016-05-03T10:49:28.581452Z 0 [Note] Shutting down plugin 'INNODB_CMPMEM'
2016-05-03T10:49:28.581456Z 0 [Note] Shutting down plugin 'INNODB_CMP_RESET'
2016-05-03T10:49:28.581459Z 0 [Note] Shutting down plugin 'INNODB_CMP'
2016-05-03T10:49:28.581463Z 0 [Note] Shutting down plugin 'INNODB_LOCK_WAITS'
2016-05-03T10:49:28.581466Z 0 [Note] Shutting down plugin 'INNODB_LOCKS'
2016-05-03T10:49:28.581470Z 0 [Note] Shutting down plugin 'INNODB_TRX'
2016-05-03T10:49:28.581473Z 0 [Note] Shutting down plugin 'InnoDB'
[Thread 0x7fffb8d04700 (LWP 4112) exited]
2016-05-03T10:49:28.581822Z 0 [Note] InnoDB: FTS optimize thread exiting.
2016-05-03T10:49:28.590938Z 0 [Note] InnoDB: Starting shutdown...
[Thread 0x7fff96bfd700 (LWP 4110) exited]
[Thread 0x7fff975fe700 (LWP 4109) exited]
2016-05-03T10:49:28.691896Z 0 [Note] InnoDB: Dumping buffer pool(s) to /usr/local/mysql-5.7.12/data/ib_buffer_pool
2016-05-03T10:49:28.692817Z 0 [Note] InnoDB: Buffer pool(s) dump completed at 160503 18:49:28
[Thread 0x7fffb0dbd700 (LWP 4101) exited]
[Thread 0x7fff961fc700 (LWP 4111) exited]
[Thread 0x7fffb17be700 (LWP 4100) exited]
[Thread 0x7fff97fff700 (LWP 4108) exited]
[Thread 0x7fffabfff700 (LWP 4102) exited]
[Thread 0x7fffa8dfa700 (LWP 4107) exited]
[Thread 0x7fffaabfd700 (LWP 4104) exited]
[Thread 0x7fffa97fb700 (LWP 4106) exited]
[Thread 0x7fffaa1fc700 (LWP 4105) exited]
[Thread 0x7fffb23e5700 (LWP 4098) exited]
[Thread 0x7fffab5fe700 (LWP 4103) exited]
[Thread 0x7fffb37e7700 (LWP 4096) exited]
[Thread 0x7fffb2de6700 (LWP 4097) exited]
[Thread 0x7fffb87ef700 (LWP 4088) exited]
[Thread 0x7fffb41e8700 (LWP 4095) exited]
[Thread 0x7fffb4be9700 (LWP 4094) exited]
[Thread 0x7fffb55ea700 (LWP 4093) exited]
[Thread 0x7fffb69ec700 (LWP 4091) exited]
[Thread 0x7fffb5feb700 (LWP 4092) exited]
[Thread 0x7fffb73ed700 (LWP 4090) exited]
[Thread 0x7fffb7dee700 (LWP 4089) exited]
2016-05-03T10:49:29.967622Z 0 [Note] InnoDB: Shutdown completed; log sequence number 2739027
2016-05-03T10:49:29.970363Z 0 [Note] InnoDB: Removed temporary tablespace data file: "ibtmp1"
2016-05-03T10:49:29.970416Z 0 [Note] Shutting down plugin 'sha256_password'
2016-05-03T10:49:29.970422Z 0 [Note] Shutting down plugin 'mysql_native_password'
2016-05-03T10:49:29.970683Z 0 [Note] Shutting down plugin 'binlog'
2016-05-03T10:49:29.971711Z 0 [Note] /usr/local/mysql-5.7.12/bin/mysqld: Shutdown complete
[Thread 0x7fffeeeec700 (LWP 4087) exited]
Program exited normally.
(gdb)
然后即使就是程序退出是的各种动作，哗啦啦一片。
然后再看那个关闭mysql服务的客户端
可以发现mysql服务已经彻底关闭了。
上面是初次用gdb来运行mysqld的过程，只能算是初涉吧。