tag: 信号 signal sigchld 死锁 堆栈
我们的程序需要捕获信号自己处理，所以尝试对1-32的信号处理（后面33-64的信号不处理）。
但是在调试代码时，发现一个线程死锁的问题。
程序目的：捕获信号，然后打印堆栈。
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伪代码如下：设置捕获信号函数(){ //设置信号处理函数 sigact.sa_sigaction = TsSigHandler; // //这里捕获了很多信号，包括SIGCHLD：子进程结束，父进程会收到该信号 sigaction( SIGSEGV, &sigact, NULL ); .... sigaction( SIGCHLD, &sigact, NULL ); } 信号处理函数：TsSigHandler{ //调用打印堆栈函数 PrintStack();} 打印堆栈函数PrintStack{ //打印堆栈 backtrace(); backtrace_symbols(); //调用system函数执行一些命令 system("xxxxxx");} |
Thread 12 (Thread 0xf7dd2b90 (LWP 5770)):
以下是一个让我觉得奇怪的堆栈，奇怪之处：
1.死锁了：__lll_lock_wait_private
2.获得了2个信号：<signal handler called>，为什么不是一个一个信号处理
堆栈如下：
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#0 0xffffe410 in __kernel_vsyscall ()#1 0x002a0783 in __lll_lock_wait_private () from /lib/libc.so.6#2 0x001f8448 in _L_lock_124 () from /lib/libc.so.6#3 0x001f7f8b in do_system () from /lib/libc.so.6#4 0x001f8412 in system () from /lib/libc.so.6#5 0x00317ead in system () from /lib/libpthread.so.0#6 0x080f95c1 in PrintStack() ()#7 0x080f9844 in TsSigHandler(int, siginfo*, void*) ()#8 <signal handler called>#9 0xffffe410 in __kernel_vsyscall ()#10 0x001eb1a9 in sigprocmask () from /lib/libc.so.6#11 0x001f8132 in do_system () from /lib/libc.so.6#12 0x001f8412 in system () from /lib/libc.so.6#13 0x00317ead in system () from /lib/libpthread.so.0#14 0x080f95c1 in PrintStack() ()#15 0x080f9844 in TsSigHandler(int, siginfo*, void*) ()#16 <signal handler called>#17 0x002338ec in memcpy () from /lib/libc.so.6#18 0x0804fa02 in boom ()#19 0x080dbd9c in RunCmd ()#20 0x080dbf12 in CmdParse ()#21 0x080dc705 in OspTeleDaemon ()#22 0x080f8817 in OspTaskTemplateFunc(void*) ()#23 0x0030f832 in start_thread () from /lib/libpthread.so.0#24 0x00293e0e in clone () from /lib/libc.so.6 |
#18 0x0804fa02 in boom ()
boom()是我写的一个制造崩溃的函数：
char *pBoom = NULL;
memcpy( pBoom, "aaaa", 100 );
#16 <signal handler called>
触发信号
#15 0x080f9844 in TsSigHandler(int, siginfo*, void*) ()
TsSigHandler是信号处理函数。通过以下代码设置：
struct sigaction sigact;
sigemptyset( &sigact.sa_mask );
sigact.sa_flags = SA_ONESHOT | SA_SIGINFO;
sigact.sa_sigaction = TsSigHandler;
信号触发后，由TsSigHandler函数处理
#14 0x080f95c1 in PrintStack() ()
TsSigHandler函数中调用PrintStack函数打印堆栈。
#13 0x00317ead in system () from /lib/libpthread.so.0
PrintStack函数中调用了system函数做一些额外的事情，例如执行gcore（事实证明，这种方法是有点问题的）。
#11 0x001f8132 in do_system () from /lib/libc.so.6
system调用了do_system
#10 0x001eb1a9 in sigprocmask () from /lib/libc.so.6
do_system调用sigprocmask
#8 <signal handler called>
关键来了：这是获取到了另外一个信号：SIGCHLD。
#7 0x080f9844 in TsSigHandler(int, siginfo*, void*) ()
又调用信号处理函数TsSigHandler
#3 0x001f7f8b in do_system () from /lib/libc.so.6
system调用do_system，调用流程和上面当然是一样的
#2 0x001f8448 in _L_lock_124 () from /lib/libc.so.6
#1 0x002a0783 in __lll_lock_wait_private () from /lib/libc.so.6
nice！锁住了。。
分析:
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/* Execute LINE as a shell command, returning its status. */static intdo_system (const char *line){ int status, save; pid_t pid; struct sigaction sa;#ifndef _LIBC_REENTRANT struct sigaction intr, quit;#endif sigset_t omask; sa.sa_handler = SIG_IGN; sa.sa_flags = 0; __sigemptyset (&sa.sa_mask); DO_LOCK (); if (ADD_REF () == 0) { if (__sigaction (SIGINT, &sa, &intr) < 0) { (void) SUB_REF (); goto out; } if (__sigaction (SIGQUIT, &sa, &quit) < 0) { save = errno; (void) SUB_REF (); goto out_restore_sigint; } } DO_UNLOCK (); /* We reuse the bitmap in the 'sa' structure. */ __sigaddset (&sa.sa_mask, SIGCHLD); save = errno; if (__sigprocmask (SIG_BLOCK, &sa.sa_mask, &omask) < 0) {#ifndef _LIBC if (errno == ENOSYS) __set_errno (save); else#endif { DO_LOCK (); if (SUB_REF () == 0) { save = errno; (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL); out_restore_sigint: (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL); __set_errno (save); } out: DO_UNLOCK (); return -1; } }#ifdef CLEANUP_HANDLER CLEANUP_HANDLER;#endif#ifdef FORK pid = FORK ();#else pid = __fork ();#endif if (pid == (pid_t) 0) { /* Child side. */ const char *new_argv[4]; new_argv[0] = SHELL_NAME; new_argv[1] = "-c"; new_argv[2] = line; new_argv[3] = NULL; /* Restore the signals. */ (void) __sigaction (SIGINT, &intr, (struct sigaction *) NULL); (void) __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL); (void) __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL); INIT_LOCK (); /* Exec the shell. */ (void) __execve (SHELL_PATH, (char *const *) new_argv, __environ); _exit (127); } else if (pid < (pid_t) 0) /* The fork failed. */ status = -1; else /* Parent side. */ { /* Note the system() is a cancellation point. But since we call waitpid() which itself is a cancellation point we do not have to do anything here. */ if (TEMP_FAILURE_RETRY (__waitpid (pid, &status, 0)) != pid) status = -1; }#ifdef CLEANUP_HANDLER CLEANUP_RESET;#endif save = errno; DO_LOCK (); if ((SUB_REF () == 0 && (__sigaction (SIGINT, &intr, (struct sigaction *) NULL) | __sigaction (SIGQUIT, &quit, (struct sigaction *) NULL)) != 0) || __sigprocmask (SIG_SETMASK, &omask, (sigset_t *) NULL) != 0) {#ifndef _LIBC /* glibc cannot be used on systems without waitpid. */ if (errno == ENOSYS) __set_errno (save); else#endif status = -1; } DO_UNLOCK (); return status;} |
system()函数执行的大体过程是：fork()->exec()->waitpid()，
waitpid用于等待子进程执行完毕。
但是在子进程执行完毕时，会产生SIGCHLD信号，
而SIGCHLD信号会唤醒wait中的进程，这就是看到了2个信号的原因，
解决方法：
1.忽略SIGCHLD信号：其实这个信号一般情况下应该被忽略，除非你的程序需要对这种情况做非常特殊的处理
2.不要在这里调用system()
to do： 有空了记得补详细些