Stevens在《Unix环境高级编程》一书中是如是回答的“If a signal is caught and if the signal handler returns, then sigsuspend returns and the signal mask of the process is set to its value before the call to sigsuspend.”,由于sigsuspend是原子操作,所以这句给人的感觉就是先调用signal handler先返回,然后sigsuspend再返回。
[cpp] view plain copy print ? int main(void) { sigset_t newmask, oldmask, zeromask; if (signal(SIGINT, sig_int) == SIG_ERR) err_sys("signal(SIGINT) error"); sigemptyset(&zeromask); sigemptyset(&newmask); sigaddset(&newmask, SIGINT); /* block SIGINT and save current signal mask */ if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0) err_sys("SIG_BLOCK error"); /* critical region of code */ pr_mask("in critical region: "); /* allow all signals and pause */ if (sigsuspend(&zeromask) != -1) err_sys("sigsuspend error"); pr_mask("after return from sigsuspend: "); /* reset signal mask which unblocks SIGINT */ if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0) err_sys("SIG_SETMASK error"); /* and continue processing ... */ exit(0); } static void sig_int(int signo) { pr_mask("\nin sig_int: "); return; } 结果:
[cpp] view plain copy print ? $a.out in critical region: SIGINT ^C in sig_int: SIGINT after return from sigsuspend: SIGINT 如果按照sig_handler先返回,那么SIGINT是不该被打印出来的,因为那时屏蔽字还没有恢复,所有信号都是不阻塞的。那么是Stevens说错了么?当然没有,只是Stevens没有说请在sigsuspend的原子操作中到底做了什么? sigsuspend的整个原子操作过程为: (1) 设置新的mask阻塞当前进程; (2) 收到信号,恢复原先mask; (3) 调用该进程设置的信号处理函数; (4) 待信号处理函数返回后,sigsuspend返回。
[cpp] view plain copy print ? #include <stdio.h> #include <signal.h> void checkset(); void func(); void main() { sigset_tblockset,oldblockset,zeroset,pendmask; printf("pid:%ld\n",(long)getpid()); signal(SIGINT,func); sigemptyset(&blockset); sigemptyset(&zeroset); sigaddset(&blockset,SIGINT); sigprocmask(SIG_SETMASK,&blockset,&oldblockset); checkset(); sigpending(&pendmask); if(sigismember(&pendmask,SIGINT)) printf("SIGINTpending\n"); if(sigsuspend(&zeroset)!= -1) { printf("sigsuspenderror\n"); exit(0); } printf("afterreturn\n"); sigprocmask(SIG_SETMASK,&oldblockset,NULL); printf("SIGINTunblocked\n"); } void checkset() { sigset_tset; printf("checksetstart:\n"); if(sigprocmask(0,NULL,&set)<0) { printf("checksetsigprocmask error!!\n"); exit(0); } if(sigismember(&set,SIGINT)) printf("sigint\n"); if(sigismember(&set,SIGTSTP)) printf("sigtstp\n"); if(sigismember(&set,SIGTERM)) printf("sigterm\n"); printf("checksetend\n"); } void func() { printf("hellofunc\n"); }
父子进程同步到方法如下:
[cpp] view plain copy print ? <span style="font-size:18px;"><strong>#include "apue.h" static volatile sig_atomic_t sigflag; /* set nonzero by sig handler */ static sigset_t newmask, oldmask, zeromask; static void sig_usr(int signo) /* one signal handler for SIGUSR1 and SIGUSR2 */ { sigflag = 1; } void TELL_WAIT(void) { if (signal(SIGUSR1, sig_usr) == SIG_ERR) err_sys("signal(SIGUSR1) error"); if (signal(SIGUSR2, sig_usr) == SIG_ERR) err_sys("signal(SIGUSR2) error"); sigemptyset(&zeromask); sigemptyset(&newmask); sigaddset(&newmask, SIGUSR1); sigaddset(&newmask, SIGUSR2); /* * Block SIGUSR1 and SIGUSR2, and save current signal mask. */ if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0) err_sys("SIG_BLOCK error"); } void TELL_PARENT(pid_t pid) { kill(pid, SIGUSR2); /* tell parent we're done */ } void WAIT_PARENT(void) { while (sigflag == 0) sigsuspend(&zeromask); /* and wait for parent */ sigflag = 0; /* * Reset signal mask to original value. */ if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0) err_sys("SIG_SETMASK error"); } void TELL_CHILD(pid_t pid) { kill(pid, SIGUSR1); /* tell child we're done */ } void WAIT_CHILD(void) { while (sigflag == 0) sigsuspend(&zeromask); /* and wait for child */ sigflag = 0; /* * Reset signal mask to original value. */ if (sigprocmask(SIG_SETMASK, &oldmask, NULL) < 0) err_sys("SIG_SETMASK error"); } </strong></span>