信号实现父子进程之间的同步--sigsuspend的作用

男娘i 2022-04-16 03:34 182阅读 0赞

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## 函数原型： ##

**　　\#include <signal.h>**

**　　int sigsuspend(const sigset\_t \*mask);**

## 作用： ##

**　　用于在接收到某个信号之前，临时用mask替换进程的信号掩码，并暂停进程执行，直到收到信号为止。**

**　　The sigsuspend() function replaces the current signal mask of the calling thread with the set of signals pointed to by sigmask and then suspends the thread until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. This will not cause any other signals that may have been pending on the process to become pending on the thread.**

**　　If the action is to terminate the process then sigsuspend() will never return. If the action is to execute a signal-catching function, thensigsuspend() will return after the signal-catching function returns, with the signal mask restored to the set that existed prior to thesigsuspend() call.**

**　　It is not possible to block signals that cannot be ignored. This is enforced by the system without causing an error to be indicated.**

**　　也就是说，sigsuspend后，进程就挂在那里，等待着开放的信号的唤醒。系统在接收到信号后，马上就把现在的信号集还原为原来的，然后调用处理函数。**

**返回值：**

**　　sigsuspend返回后将恢复调用之前的的信号掩码。信号处理函数完成后，进程将继续执行。该系统调用始终返回-1，并将errno设置为EINTR.**

**　　Since sigsuspend() suspends process execution indefinitely, there is no successful completion return value. If a return occurs, -1 is returned and errno is set to indicate the error.**

**　　The sigsuspend() function will fail if:**

**　　\[EINTR\]**

**　　A signal is caught by the calling process and control is returned from the signal-catching function.**

**也就是说，sigsuspend后，进程就挂在那里，等待着开放的信号的唤醒。系统在接受到信号后，马上就把现在的信号集还原为原来的，然后调用处理函数。**

**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。**

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：

$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。**

#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");  }

![20130511140243302][]

**父子进程同步到方法如下：**

#include "apue.h"static volatile sig_atomic_t sigflag; /* set nonzero by sig handler */static sigset_t newmask, oldmask, zeromask;static voidsig_usr(int signo) /* one signal handler for SIGUSR1 and SIGUSR2 */{ sigflag = 1;}voidTELL_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");}voidTELL_PARENT(pid_t pid){ kill(pid, SIGUSR2);  /* tell parent we're done */}voidWAIT_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");}voidTELL_CHILD(pid_t pid){ kill(pid, SIGUSR1);   /* tell child we're done */}voidWAIT_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");}

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[20130511140243302]: /images/20220416/484b62a35f5e4d7f84c3ddae6e173ebd.png
[SouthEast]: /images/20220416/f9d182546a0c415187bbf979cfc4981e.png