本课的核心内容如下run 命令continue 命令break 命令backtrace 与 frame 命令info break、enable、disable 和 delete 命令list 命令print 和 ptype 命令为了结合实践这里以调试 Redis 源码为例来介绍每一个命令这里先介绍一些常用命令的基础用法某些命令的高级用法会在后面讲解。5.1 redis 源码下载与 debug 版本编译Redis 的最新源码下载地址可以在 Redis 官网获得使用 wget 命令将 Redis 源码文件下载下来[rootlocalhost gdbtest]# wget http://download.redis.io/releases/redis-4.0.11.tar.gz --2018-09-08 13:08:41-- http://download.redis.io/releases/redis-4.0.11.tar.gz Resolving download.redis.io (download.redis.io)... 109.74.203.151 Connecting to download.redis.io (download.redis.io)|109.74.203.151|:80... connected. HTTP request sent, awaiting response... 200 OK Length: 1739656 (1.7M) [application/x-gzip] Saving to: ‘redis-4.0.11.tar.gz’ 54% [ ] 940,876 65.6KB/s eta 9s解压[rootlocalhost gdbtest]# tar zxvf redis-4.0.11.tar.gz进入生成的 redis-4.0.11 目录使用makefile命令进行编译。makefile命令是 Linux 程序编译基本的命令由于本课程的重点是 Linux 调试如果读者不熟悉 Linux 编译可以通过互联网或相关书籍补充一下相关知识。为了方便调试我们需要生成调试符号并且关闭编译器优化选项操作如下[rootlocalhost gdbtest]# cd redis-4.0.11 [rootlocalhost redis-4.0.11]# make CFLAGS-g -O0 -j 4注意由于 redis 是纯 C 项目使用的编译器是 gcc所以这里设置编译器的选项时使用的是CFLAGS选项如果项目使用的语言是 C那么使用的编译器一般是 g相对应的编译器选项是CXXFLAGS。这点请读者注意区别。另外这里makefile使用了-j选项其值是4表示开启4个进程同时编译加快编译速度。编译成功后会在 src 目录下生成多个可执行程序其中 redis-server 和 redis-cli 是我们需要调试的程序。进入 src 目录使用 GDB 启动 redis-server 这个程序[rootlocalhost src]# gdb redis-server Reading symbols from /root/gdbtest/redis-4.0.11/src/redis-server...done.5.2 run 命令默认情况下前面的课程中我们说gdb filename命令只是附加的一个调试文件并没有启动这个程序需要输入run命令简写为 r启动这个程序(gdb) r Starting program: /root/gdbtest/redis-4.0.11/src/redis-server [Thread debugging using libthread_db enabled] Using host libthread_db library /lib64/libthread_db.so.1. 46455:C 08 Sep 13:43:43.957 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo 46455:C 08 Sep 13:43:43.957 # Redis version4.0.11, bits64, commit00000000, modified0, pid46455, just started 46455:C 08 Sep 13:43:43.957 # Warning: no config file specified, using the default config. In order to specify a config file use /root/gdbtest/redis-4.0.11/src/redis-server /path/to/redis.conf 46455:M 08 Sep 13:43:43.957 * Increased maximum number of open files to 10032 (it was originally set to 1024). [New Thread 0x7ffff07ff700 (LWP 46459)] [New Thread 0x7fffefffe700 (LWP 46460)] [New Thread 0x7fffef7fd700 (LWP 46461)] _._ _.-__ -._ _.- . _. -._ Redis 4.0.11 (00000000/0) 64 bit .- .-. \/ _.,_ -._ ( , .- | , ) Running in standalone mode |-._-...- __...-.-._| _.-| Port: 6379 | -._ ._ / _.- | PID: 46455 -._ -._ -./ _.- _.- |-._-._ -.__.- _.-_.-| | -._-._ _.-_.- | http://redis.io -._ -._-.__.-_.- _.- |-._-._ -.__.- _.-_.-| | -._-._ _.-_.- | -._ -._-.__.-_.- _.- -._ -.__.- _.- -._ _.- -.__.- 46455:M 08 Sep 13:43:43.965 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128. 46455:M 08 Sep 13:43:43.965 # Server initialized 46455:M 08 Sep 13:43:43.965 # WARNING overcommit_memory is set to 0! Background save may fail under low memory condition. To fix this issue add vm.overcommit_memory 1 to /etc/sysctl.conf and then reboot or run the command sysctl vm.overcommit_memory1 for this to take effect. 46455:M 08 Sep 13:43:43.965 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command echo never /sys/kernel/mm/transparent_hugepage/enabled as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled. 46455:M 08 Sep 13:43:43.965 * Ready to accept connections这就是 redis-server 启动界面假设程序已经启动再次输入 run 命令则是重启程序。我们在 GDB 界面按 Ctrl C 快捷键让 GDB 中断下来再次输入 r 命令GDB 会询问我们是否重启程序输入 yes 确认重启。^C Program received signal SIGINT, Interrupt. 0x00007ffff73ee923 in epoll_wait () from /lib64/libc.so.6 Missing separate debuginfos, use: debuginfo-install glibc-2.17-196.el7_4.2.x86_64 (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) yes Starting program: /root/gdbtest/redis-4.0.11/src/redis-server5.3 continue 命令当 GDB 触发断点或者使用 Ctrl C 命令中断下来后想让程序继续运行只要输入continue命令即可简写为 c。当然如果continue命令继续触发断点GDB 就会再次中断下来。^C Program received signal SIGINT, Interrupt. 0x00007ffff73ee923 in epoll_wait () from /lib64/libc.so.6 (gdb) c Continuing.5.4 break 命令break命令简写为 b即我们添加断点的命令可以使用以下方式添加断点break functionname在函数名为 functionname 的入口处添加一个断点break LineNo在当前文件行号为 LineNo 处添加一个断点break filename:LineNo在 filename 文件行号为 LineNo 处添加一个断点。这三种方式都是我们常用的添加断点的方式。举个例子对于一般的 Linux 程序来说main() 函数是程序入口函数redis-server 也不例外我们知道了函数的名字就可以直接在 main() 函数处添加一个断点(gdb) b main Breakpoint 1 at 0x423450: file server.c, line 3709.添加好了以后使用 run 命令重启程序就可以触发这个断点了GDB 会停在断点处。(gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /root/gdbtest/redis-4.0.11/src/redis-server [Thread debugging using libthread_db enabled] Using host libthread_db library /lib64/libthread_db.so.1. Breakpoint 1, main (argc1, argv0x7fffffffe648) at server.c:3709 3709 int main(int argc, char **argv) { (gdb)redis-server 默认端口号是 6379 我们知道这个端口号肯定是通过操作系统的 socket API bind() 函数创建的通过文件搜索找到调用这个函数的文件其位于 anet.c 441 行。我们使用break命令在这个地方加一个断点(gdb) b anet.c:441 Breakpoint 3 at 0x426cf0: file anet.c, line 441由于程序绑定端口号是 redis-server 启动时初始化的为了能触发这个断点再次使用 run 命令重启下这个程序GDB 第一次会触发 main() 函数处的断点输入 continue 命令继续运行接着触发 anet.c:441 处的断点(gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /root/gdbtest/redis-4.0.11/src/redis-server [Thread debugging using libthread_db enabled] Using host libthread_db library /lib64/libthread_db.so.1. Breakpoint 1, main (argc1, argv0x7fffffffe648) at server.c:3709 3709 int main(int argc, char **argv) { (gdb) c Continuing. 46699:C 08 Sep 15:30:31.403 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo 46699:C 08 Sep 15:30:31.403 # Redis version4.0.11, bits64, commit00000000, modified0, pid46699, just started 46699:C 08 Sep 15:30:31.403 # Warning: no config file specified, using the default config. In order to specify a config file use /root/gdbtest/redis-4.0.11/src/redis-server /path/to/redis.conf 46699:M 08 Sep 15:30:31.404 * Increased maximum number of open files to 10032 (it was originally set to 1024). Breakpoint 3, anetListen (err0x746bb0 server560 , s10, sa0x75edb0, len28, backlog511) at anet.c:441 441 if (bind(s,sa,len) -1) { (gdb)anet.c:441 处的代码如下现在断点停在第 441 行所以当前文件就是 anet.c可以直接使用“break 行号”添加断点。例如可以在第 444 行、450 行、452 行分别加一个断点看看这个函数执行完毕后走哪个 return 语句退出则可以执行440 static int anetListen(char *err, int s, struct sockaddr *sa, socklen_t len, int backlog) { 441 if (bind(s,sa,len) -1) { 442 anetSetError(err, bind: %s, strerror(errno)); 443 close(s); 444 return ANET_ERR; (gdb) l 445 } 446 447 if (listen(s, backlog) -1) { 448 anetSetError(err, listen: %s, strerror(errno)); 449 close(s); 450 return ANET_ERR; 451 } 452 return ANET_OK; 453 } 454 (gdb) b 444 Breakpoint 3 at 0x426cf5: file anet.c, line 444. (gdb) b 450 Breakpoint 4 at 0x426d06: file anet.c, line 450. (gdb) b 452 Note: breakpoint 4 also set at pc 0x426d06. Breakpoint 5 at 0x426d06: file anet.c, line 452. (gdb)添加好这三个断点以后我们使用continue命令继续运行程序发现程序运行到第 452 行中断下来即触发 Breakpoint 5(gdb) c Continuing. Breakpoint 5, anetListen (err0x746bb0 server560 , s10, sa0x7e34e0, len16, backlog511) at anet.c:452 452 return ANET_OK;说明 redis-server 绑定端口号并设置侦听listen成功我们可以再打开一个 SSH 窗口验证一下发现 6379 端口确实已经处于侦听状态了[rootlocalhost src]# lsof -i -Pn | grep redis redis-ser 46699 root 10u IPv6 245844 0t0 TCP *:6379 (LISTEN)5.5 backtrace 与 frame 命令backtrace命令简写为 bt用来查看当前调用堆栈。接上redis-server 现在中断在 anet.c:452 行可以通过backtrace命令来查看当前的调用堆栈(gdb) bt #0 anetListen (err0x746bb0 server560 , s10, sa0x7e34e0, len16, backlog511) at anet.c:452 #1 0x0000000000426e35 in _anetTcpServer (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, afafentry10, backlog511) at anet.c:487 #2 0x000000000042793d in anetTcp6Server (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, backlog511) at anet.c:510 #3 0x000000000042b0bf in listenToPort (port6379, fdsfdsentry0x746ae4 server356, countcountentry0x746b24 server420) at server.c:1728 #4 0x000000000042fa77 in initServer () at server.c:1852 #5 0x0000000000423803 in main (argc1, argv0x7fffffffe648) at server.c:3862 (gdb)这里一共有 6 层堆栈最顶层是 main() 函数最底层是断点所在的 anetListen() 函数堆栈编号分别是 #0 ~ #5 如果想切换到其他堆栈处可以使用 frame 命令简写为 f该命令的使用方法是“frame 堆栈编号编号不加 #”。在这里依次切换至堆栈顶部然后再切换回 #0 练习一下(gdb) f 1 #1 0x0000000000426e35 in _anetTcpServer (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, afafentry10, backlog511) at anet.c:487 487 if (anetListen(err,s,p-ai_addr,p-ai_addrlen,backlog) ANET_ERR) s ANET_ERR; (gdb) f 2 #2 0x000000000042793d in anetTcp6Server (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, backlog511) at anet.c:510 510 return _anetTcpServer(err, port, bindaddr, AF_INET6, backlog); (gdb) f 3 #3 0x000000000042b0bf in listenToPort (port6379, fdsfdsentry0x746ae4 server356, countcountentry0x746b24 server420) at server.c:1728 1728 fds[*count] anetTcp6Server(server.neterr,port,NULL, (gdb) f 4 #4 0x000000000042fa77 in initServer () at server.c:1852 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) (gdb) f 5 #5 0x0000000000423803 in main (argc1, argv0x7fffffffe648) at server.c:3862 3862 initServer(); (gdb)通过查看上面的各个堆栈可以得出这里的调用层级关系即main() 函数在第 3862 行调用了 initServer() 函数initServer() 在第 1852 行调用了 listenToPort() 函数listenToPort() 在第 1728 行调用了 anetTcp6Server() 函数anetTcp6Server() 在第 510 行调用了 _anetTcpServer() 函数_anetTcpServer() 函数在第 487 行调用了 anetListen() 函数当前断点正好位于 anetListen() 函数中5.6 info break、enable、disable 和 delete 命令在程序中加了很多断点而我们想查看加了哪些断点时可以使用info break命令简写为 info b(gdb) info b Num Type Disp Enb Address What 1 breakpoint keep y 0x0000000000423450 in main at server.c:3709 breakpoint already hit 1 time 2 breakpoint keep y 0x000000000049c1f0 in _redisContextConnectTcp at net.c:267 3 breakpoint keep y 0x0000000000426cf0 in anetListen at anet.c:441 breakpoint already hit 1 time 4 breakpoint keep y 0x0000000000426d05 in anetListen at anet.c:444 breakpoint already hit 1 time 5 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:450 breakpoint already hit 1 time 6 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:452 breakpoint already hit 1 time通过上面的内容片段可以知道目前一共增加了 6 个断点除了断点 2 以外其他的断点均被触发一次其他信息比如每个断点的位置所在的文件和行号、内存地址、断点启用和禁用状态信息也一目了然。如果我们想禁用某个断点使用“disable 断点编号”就可以禁用这个断点了被禁用的断点不会再被触发同理被禁用的断点也可以使用“enable 断点编号”重新启用。(gdb) disable 1 (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep n 0x0000000000423450 in main at server.c:3709 breakpoint already hit 1 time 2 breakpoint keep y 0x000000000049c1f0 in _redisContextConnectTcp at net.c:267 3 breakpoint keep y 0x0000000000426cf0 in anetListen at anet.c:441 breakpoint already hit 1 time 4 breakpoint keep y 0x0000000000426d05 in anetListen at anet.c:444 breakpoint already hit 1 time 5 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:450 breakpoint already hit 1 time 6 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:452 breakpoint already hit 1 time使用disable 1以后第一个断点的 Enb 一栏的值由 y 变成 n重启程序也不会再次触发(gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /root/gdbtest/redis-4.0.11/src/redis-server [Thread debugging using libthread_db enabled] Using host libthread_db library /lib64/libthread_db.so.1. 46795:C 08 Sep 16:15:55.681 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo 46795:C 08 Sep 16:15:55.681 # Redis version4.0.11, bits64, commit00000000, modified0, pid46795, just started 46795:C 08 Sep 16:15:55.681 # Warning: no config file specified, using the default config. In order to specify a config file use /root/gdbtest/redis-4.0.11/src/redis-server /path/to/redis.conf 46795:M 08 Sep 16:15:55.682 * Increased maximum number of open files to 10032 (it was originally set to 1024). Breakpoint 3, anetListen (err0x746bb0 server560 , s10, sa0x75edb0, len28, backlog511) at anet.c:441 441 if (bind(s,sa,len) -1) {如果disable命令和enable命令不加断点编号则分别表示禁用和启用所有断点(gdb) disable (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep n 0x0000000000423450 in main at server.c:3709 2 breakpoint keep n 0x000000000049c1f0 in _redisContextConnectTcp at net.c:267 3 breakpoint keep n 0x0000000000426cf0 in anetListen at anet.c:441 breakpoint already hit 1 time 4 breakpoint keep n 0x0000000000426d05 in anetListen at anet.c:444 5 breakpoint keep n 0x0000000000426d16 in anetListen at anet.c:450 6 breakpoint keep n 0x0000000000426d16 in anetListen at anet.c:452 (gdb) enable (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep y 0x0000000000423450 in main at server.c:3709 2 breakpoint keep y 0x000000000049c1f0 in _redisContextConnectTcp at net.c:267 3 breakpoint keep y 0x0000000000426cf0 in anetListen at anet.c:441 breakpoint already hit 1 time 4 breakpoint keep y 0x0000000000426d05 in anetListen at anet.c:444 5 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:450 6 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:452 (gdb)使用“delete 编号”可以删除某个断点如delete 2 3则表示要删除的断点 2 和断点 3(gdb) delete 2 3 (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep y 0x0000000000423450 in main at server.c:3709 4 breakpoint keep y 0x0000000000426d05 in anetListen at anet.c:444 5 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:450 6 breakpoint keep y 0x0000000000426d16 in anetListen at anet.c:452同样的道理如果输入 delete 不加命令号则表示删除所有断点。5.7 list 命令list命令和后面介绍的print命令都是 GDB 调试中用到的频率最高的命令list命令简写为 l可以查看当前断点处的代码。使用frame命令切换到刚才的堆栈 #3 处然后输入list命令看下效果(gdb) f 4 #4 0x000000000042fa77 in initServer () at server.c:1852 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) (gdb) l 1847 } 1848 server.db zmalloc(sizeof(redisDb)*server.dbnum); 1849 1850 /* Open the TCP listening socket for the user commands. */ 1851 if (server.port ! 0 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) 1853 exit(1); 1854 1855 /* Open the listening Unix domain socket. */ 1856 if (server.unixsocket ! NULL) { (gdb)断点停在第 1852 行输入 list 命令以后会显示第 1852 行前后的 10 行代码再次输入 list 命令试一下(gdb) l 1857 unlink(server.unixsocket); /* dont care if this fails */ 1858 server.sofd anetUnixServer(server.neterr,server.unixsocket, 1859 server.unixsocketperm, server.tcp_backlog); 1860 if (server.sofd ANET_ERR) { 1861 serverLog(LL_WARNING, Opening Unix socket: %s, server.neterr); 1862 exit(1); 1863 } 1864 anetNonBlock(NULL,server.sofd); 1865 } 1866 (gdb) l 1867 /* Abort if there are no listening sockets at all. */ 1868 if (server.ipfd_count 0 server.sofd 0) { 1869 serverLog(LL_WARNING, Configured to not listen anywhere, exiting.); 1870 exit(1); 1871 } 1872 1873 /* Create the Redis databases, and initialize other internal state. */ 1874 for (j 0; j server.dbnum; j) { 1875 server.db[j].dict dictCreate(dbDictType,NULL); 1876 server.db[j].expires dictCreate(keyptrDictType,NULL);代码继续往后显示 10 行也就是说第一次输入list命令会显示断点处前后的代码继续输入list指令会以递增行号的形式继续显示剩下的代码行一直到文件结束为止。当然 list 指令还可以往前和往后显示代码命令分别是“list 加号”和“list -减号”(gdb) list - 1857 unlink(server.unixsocket); /* dont care if this fails */ 1858 server.sofd anetUnixServer(server.neterr,server.unixsocket, 1859 server.unixsocketperm, server.tcp_backlog); 1860 if (server.sofd ANET_ERR) { 1861 serverLog(LL_WARNING, Opening Unix socket: %s, server.neterr); 1862 exit(1); 1863 } 1864 anetNonBlock(NULL,server.sofd); 1865 } 1866 (gdb) l - 1847 } 1848 server.db zmalloc(sizeof(redisDb)*server.dbnum); 1849 1850 /* Open the TCP listening socket for the user commands. */ 1851 if (server.port ! 0 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) 1853 exit(1); 1854 1855 /* Open the listening Unix domain socket. */ 1856 if (server.unixsocket ! NULL) {list默认显示多少行可以通过修改相关的 GDB 配置由于我们一般不会修改这个默认显示行数这里就不再浪费篇幅介绍了。list不仅可以显示当前断点处的代码也可以显示其他文件某一行的代码更多的用法可以在 GDB 中输入help list查看也可以通过(gdb) help list List specified function or line. With no argument, lists ten more lines after or around previous listing. list - lists the ten lines before a previous ten-line listing. One argument specifies a line, and ten lines are listed around that line. Two arguments with comma between specify starting and ending lines to list. Lines can be specified in these ways: LINENUM, to list around that line in current file, FILE:LINENUM, to list around that line in that file, FUNCTION, to list around beginning of that function, FILE:FUNCTION, to distinguish among like-named static functions. *ADDRESS, to list around the line containing that address. With two args if one is empty it stands for ten lines away from the other arg.上面的帮助信息中介绍了可以使用list FILE:LINENUM来显示某个文件的某一行处的代码这里不再演示是因为我觉得实用性不大。使用 GDB 的目的是调试因此更关心的是断点附近的代码而不是通过 GDB 阅读代码GDB 并不是一个好的阅读工具。以我自己为例调试 Redis 时用 GDB 调试而阅读代码使用的却是 Visual Studio 和 VSCode。5.8 print 和 ptype 命令通过print命令简写为 p我们可以在调试过程中方便地查看变量的值也可以修改当前内存中的变量值。切换当前断点到堆栈 #4 然后打印以下三个变量。(gdb) bt #0 anetListen (err0x746bb0 server560 , s10, sa0x7e34e0, len16, backlog511) at anet.c:447 #1 0x0000000000426e35 in _anetTcpServer (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, afafentry10, backlog511) at anet.c:487 #2 0x000000000042793d in anetTcp6Server (errerrentry0x746bb0 server560 , portportentry6379, bindaddrbindaddrentry0x0, backlog511) at anet.c:510 #3 0x000000000042b0bf in listenToPort (port6379, fdsfdsentry0x746ae4 server356, countcountentry0x746b24 server420) at server.c:1728 #4 0x000000000042fa77 in initServer () at server.c:1852 #5 0x0000000000423803 in main (argc1, argv0x7fffffffe648) at server.c:3862 (gdb) f 4 #4 0x000000000042fa77 in initServer () at server.c:1852 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) (gdb) l 1847 } 1848 server.db zmalloc(sizeof(redisDb)*server.dbnum); 1849 1850 /* Open the TCP listening socket for the user commands. */ 1851 if (server.port ! 0 1852 listenToPort(server.port,server.ipfd,server.ipfd_count) C_ERR) 1853 exit(1); 1854 1855 /* Open the listening Unix domain socket. */ 1856 if (server.unixsocket ! NULL) { (gdb) p server.port $15 6379 (gdb) p server.ipfd $16 {0 repeats 16 times} (gdb) p server.ipfd_count $17 0这里使用print命令分别打印出 server.port 、server.ipfd 、server.ipfd_count 的值其中 server.ipfd 显示“{0 repeats 16 times}”这是 GDB 显示字符串或字符数据特有的方式当一个字符串变量或者字符数组或者连续的内存值重复若干次GDB 就会以这种模式来显示以节约空间。print命令不仅可以显示变量值也可以显示进行一定运算的表达式计算结果值甚至可以显示一些函数的执行结果值。举个例子我们可以输入p server.port来输出 server.port 的地址值如果在 C 对象中可以通过 p this 来显示当前对象的地址也可以通过 p *this 来列出当前对象的各个成员变量值如果有三个变量可以相加 假设变量名分别叫 a、b、c 可以使用p a b c来打印这三个变量的结果值。假设 func() 是一个可以执行的函数p func() 命令可以输出该变量的执行结果。举一个最常用的例子某个时刻某个系统函数执行失败了通过系统变量 errno 得到一个错误码则可以使用 p strerror(errno) 将这个错误码对应的文字信息打印出来这样就不用费劲地去 man 手册上查找这个错误码对应的错误含义了。print 命令不仅可以输出表达式结果同时也可以修改变量的值我们尝试将上文中的端口号从 6379 改成 6400 试试(gdb) p server.port6400 $24 6400 (gdb) p server.port $25 6400 (gdb)当然一个变量值修改后能否起作用要看这个变量的具体位置和作用举个例子对于表达式 int a b / c ; 如果将 c 修改成 0 那么程序就会产生除零异常。再例如对于如下代码int j 100; for (int i 0; i j; i) { printf(i %d\n, i); }如果在循环的过程中利用print命令将 j 的大小由 100 改成 1000 那么这个循环将输出 i 的值 1000 次。总结起来利用print命令我们不仅可以查看程序运行过程中的各个变量的状态值也可以通过临时修改变量的值来控制程序的行为。GDB 还有另外一个命令叫ptype顾名思义其含义是“print type”就是输出一个变量的类型。例如我们试着输出 Redis 堆栈 #4 的变量 server 和变量 server.port 的类型(gdb) ptype server type struct redisServer { pid_t pid; char *configfile; char *executable; char **exec_argv; int hz; redisDb *db; ...省略部分字段... (gdb) ptype server.port type int可以看到对于一个复合数据类型的变量ptype 不仅列出了这个变量的类型 这里是一个名叫 redisServer 的结构体而且详细地列出了每个成员变量的字段名有了这个功能我们在调试时就不用刻意去代码文件中查看某个变量的类型定义了。5.9 小结本节课介绍了run、continue、break、backtrace、frame、info break、list、print和ptype等命令这些都是 GDB 调试过程中非常常用的命令尤其是一些复合命令如info、break是调试多线程程序的核心命令请读者务必掌握。