1. 默认设置下，在调试多进程程序时GDB只会调试主进程。但是GDB（>V7.0）支持多进程的分别以及同时调试，换句话说，GDB可以同时调试多个程序。只需要设置follow-fork-mode(默认值：parent)和detach-on-fork（默认值：on）即可。
follow-fork-mode detach-on-fork 说明
parent on 只调试主进程（GDB默认）
child on 只调试子进程
parent off 同时调试两个进程，gdb跟主进程，子进程block在fork位置
child off 同时调试两个进程，gdb跟子进程，主进程block在fork位置
child on 只调试子进程
parent off 同时调试两个进程，gdb跟主进程，子进程block在fork位置
child off 同时调试两个进程，gdb跟子进程，主进程block在fork位置
设置方法：set follow-fork-mode [parent|child] set detach-on-fork [on|off]
查询正在调试的进程：info inferiors
切换调试的进程： inferior <infer number>
添加新的调试进程： add-inferior [-copies n] [-exec executable] ,可以用file executable来分配给inferior可执行文件。
其他：remove-inferiors infno， detach inferior
2. GDB默认支持调试多线程，跟主线程，子线程block在create thread。
查询线程：info threads
切换调试线程：thread <thread number>
例程：
1 #include <stdio.h> 2 #include <pthread.h> 3 4 void processA(); 5 void processB(); 6 void * processAworker(void *arg); 7 8 int main(int argc, const char *argv[]) 9 { 10 int pid; 11 12 pid = fork(); 13 14 if(pid != 0) 15 processA(); 16 else 17 processB(); 18 19 return 0; 20 } 21 22 void processA() 23 { 24 pid_t pid = getpid(); 25 char prefix[] = "ProcessA: "; 26 char tprefix[] = "thread "; 27 int tstatus; 28 pthread_t pt; 29 30 printf("%s%lu %s ", prefix, pid, "step1"); 31 32 tstatus = pthread_create(&pt, NULL, processAworker, NULL); 33 if( tstatus != 0 ) 34 { 35 printf("ProcessA: Can not create new thread."); 36 } 37 38 processAworker(NULL); 39 sleep(1); 40 } 41 42 void * processAworker(void *arg) 43 { 44 pid_t pid = getpid(); 45 pthread_t tid = pthread_self(); 46 char prefix[] = "ProcessA: "; 47 char tprefix[] = "thread "; 48 49 printf("%s%lu %s%lu %s ", prefix, pid, tprefix, tid, "step2"); 50 printf("%s%lu %s%lu %s ", prefix, pid, tprefix, tid, "step3"); 51 52 return NULL; 53 } 54 55 void processB() 56 { 57 pid_t pid = getpid(); 58 char prefix[] = "ProcessB: "; 59 printf("%s%lu %s ", prefix, pid, "step1"); 60 printf("%s%lu %s ", prefix, pid, "step2"); 61 printf("%s%lu %s ", prefix, pid, "step3"); 62 63 }
输出：
[cnwuwil@centos c-lab]$ ./test ProcessA: 802 step1 ProcessB: 803 step1 ProcessB: 803 step2 ProcessB: 803 step3 ProcessA: 802 thread 3077555904 step2 ProcessA: 802 thread 3077555904 step3 ProcessA: 802 thread 3077553008 step2 ProcessA: 802 thread 3077553008 step3
调试：
1. 调试主进程，block子进程。
(gdb) set detach-on-fork off (gdb) show detach-on-fork Whether gdb will detach the child of a fork is off. (gdb) catch fork Catchpoint 1 (fork) (gdb) r [Thread debugging using libthread_db enabled] Catchpoint 1 (forked process 3475), 0x00110424 in __kernel_vsyscall () Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.47.el6.i686 (gdb) break test.c:14 Breakpoint 2 at 0x8048546: file test.c, line 14. (gdb) cont [New process 3475] [Thread debugging using libthread_db enabled] Breakpoint 2, main (argc=1, argv=0xbffff364) at test.c:14 Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.47.el6.i686 (gdb) info inferiors Num Description Executable 2 process 3475 /home/cnwuwil/labs/c-lab/test * 1 process 3472 /home/cnwuwil/labs/c-lab/test
2. 切换到子进程：
(gdb) inferior 2 [Switching to inferior 2 [process 3475] (/home/cnwuwil/labs/c-lab/test)] [Switching to thread 2 (Thread 0xb7fe86c0 (LWP 3475))] #0 0x00110424 in ?? () (gdb) info inferiors Num Description Executable * 2 process 3475 /home/cnwuwil/labs/c-lab/test 1 process 3472 /home/cnwuwil/labs/c-lab/test (gdb) inferior 1 [Switching to inferior 1 [process 3472] (/home/cnwuwil/labs/c-lab/test)] [Switching to thread 1 (Thread 0xb7fe86c0 (LWP 3472))] #0 main (argc=1, argv=0xbffff364) at test.c:14 (gdb) info inferiors Num Description Executable 2 process 3475 /home/cnwuwil/labs/c-lab/test * 1 process 3472 /home/cnwuwil/labs/c-lab/test
3. 设断点继续调试主进程，主进程产生两个子线程：
(gdb) break test.c:50 Breakpoint 3 at 0x804867d: file test.c, line 50. (2 locations) (gdb) cont ProcessA: 3472 step1 [New Thread 0xb7fe7b70 (LWP 3562)] ProcessA: 3472 thread 3086911168 step2 Breakpoint 3, processAworker (arg=0x0) at test.c:50 (gdb) info inferiors Num Description Executable 2 process 3475 /home/cnwuwil/labs/c-lab/test * 1 process 3472 /home/cnwuwil/labs/c-lab/test (gdb) info threads 3 Thread 0xb7fe7b70 (LWP 3562) 0x00110424 in __kernel_vsyscall () 2 Thread 0xb7fe86c0 (LWP 3475) 0x00110424 in ?? () * 1 Thread 0xb7fe86c0 (LWP 3472) processAworker (arg=0x0) at test.c:50
4. 切换到主进程中的子线程，注意：线程2为前面产生的子进程
(gdb) thread 3 [Switching to thread 3 (Thread 0xb7fe7b70 (LWP 3562))]#0 0x00110424 in __kernel_vsyscall () (gdb) cont ProcessA: 3472 thread 3086911168 step3 ProcessA: 3472 thread 3086908272 step2 [Switching to Thread 0xb7fe7b70 (LWP 3562)] Breakpoint 3, processAworker (arg=0x0) at test.c:50 (gdb) info threads * 3 Thread 0xb7fe7b70 (LWP 3562) processAworker (arg=0x0) at test.c:50 2 Thread 0xb7fe86c0 (LWP 3475) 0x00110424 in ?? () 1 Thread 0xb7fe86c0 (LWP 3472) 0x00110424 in __kernel_vsyscall () (gdb) thread 1