可以在fork函数调用之前，通过指令设置gdb调试工具跟踪父进程或子进程。
默认情况下gdb是跟踪父进程的。
set follow-fork-mode child 命令设置gdb在fork之后跟踪子进程。
set follow-fork-mode parent设置跟踪父进程。
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
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：
#include <stdio.h>
#include <pthread.h>
void processA();
void processB();
void * processAworker(void *arg);
int main(int argc, const char *argv[])
{
int pid;
pid = fork();
if(pid != 0)
processA();
else
processB();
return 0;
}
void processA()
{
pid_t pid = getpid();
char prefix[] = "ProcessA: ";
char tprefix[] = "thread ";
int tstatus;
pthread_t pt;
printf("%s%lu %s\n", prefix, pid, "step1");
tstatus = pthread_create(&pt, NULL, processAworker, NULL);
if( tstatus != 0 )
{
printf("ProcessA: Can not create new thread.");
}
processAworker(NULL);
sleep(1);
}
void * processAworker(void *arg)
{
pid_t pid = getpid();
pthread_t tid = pthread_self();
char prefix[] = "ProcessA: ";
char tprefix[] = "thread ";
printf("%s%lu %s%lu %s\n", prefix, pid, tprefix, tid, "step2");
printf("%s%lu %s%lu %s\n", prefix, pid, tprefix, tid, "step3");
return NULL;
}
void processB()
{
pid_t pid = getpid();
char prefix[] = "ProcessB: ";
printf("%s%lu %s\n", prefix, pid, "step1");
printf("%s%lu %s\n", prefix, pid, "step2");
printf("%s%lu %s\n", prefix, pid, "step3");
}
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
GDB调试——启动调试程序
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