多次使用sigaction时出错

Question

每次我的子进程死亡时，我都试图调用一个函数。这是通过捕获 execve() 发送的 SIGCHLD 来完成的。我的问题是我的处理程序函数并不总是被第二次调用。有时是这样，所以我有点困惑。

我的代码是这个：

  #include <stdio.h>
  #include <stdlib.h>
  #include <signal.h> // sigaction()
  #include <time.h>
  #include <errno.h>
  #include <unistd.h>
  #define MAX_LENGTH 1000
  #define BILLION 1E9

  struct timespec start,stop;
  float totalTime = 0;

  void runCommand(int signo)
  {
    printf("Run command called\n");

    float processTime = 0;

    //Get the time when the process ended
    if(clock_gettime(CLOCK_MONOTONIC,&stop) == -1){
      perror(NULL);
    }

    processTime = stop.tv_sec - start.tv_sec + ( stop.tv_nsec - start.tv_nsec ) / BILLION;
    totalTime += processTime;
  }


  int main(int argc, char const *argv[]) {

    int counter = 0;
    int pid = 0;
    int status;
    char *args[4];
    char line[MAX_LENGTH];

    //Setup behaviour for SIGCHLD
    struct sigaction psa;
    psa.sa_handler = runCommand;
    sigaction(SIGCHLD, &psa, NULL);

    FILE *fp = fopen("script_file.txt","r");
    if(!fp)
    {
      return 0;
    }

    //Setup argsgcc
    args[0] = "/bin/sh";
    args[1] = "-c";
    args[3] = NULL;

    while(fgets(line, MAX_LENGTH, fp))
    {
      printf("The command number %d is %s \n", counter, line);

      //Get the time when the process started
      if(clock_gettime(CLOCK_MONOTONIC,&start) == -1){
        perror(NULL);
      }

      //Create a new process
      pid = fork();

      //Child process
      if(pid == 0)
      {
        args[2] = line;
        execve("/bin/sh", args, NULL);
      }
      //Parent process
      else
      {
        wait(&status);
      }

      counter++;
    }

    printf("The overall time was %f seconds.\n", totalTime);

    fclose(fp);


    return 0;
  }

当我尝试运行时，有时我的结果是：

The command number 0 is mkdir test/

Run command called
The command number 1 is sleep 0.5

Run command called
The command number 2 is sleep 1

Run command called
The command number 3 is rmdir test/

Run command called
The overall time was 1.544909 seconds.

这似乎是正确的。但在其他情况下，我的结果是：

The command number 0 is mkdir test/

Run command called
The command number 1 is sleep 0.5

The command number 2 is sleep 1

The command number 3 is rmdir test/

The overall time was 0.009810 seconds.

这让我相信 sigaction 并不总是收到我的 SIGCHLD 信号。当我运行 valgrind 时，出现以下错误：

    ==5407== All heap blocks were freed -- no leaks are possible
    ==5407== 
    ==5407== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)
    ==5407== 
    ==5407== 1 errors in context 1 of 2:
    ==5407== Syscall param rt_sigaction(act->sa_flags) points to uninitialised byte(s)
    ==5407==    at 0x4E6F5AE: __libc_sigaction (sigaction.c:62)
    ==5407==    by 0x40098B: main (iv.c:40)
    ==5407==  Address 0xffefff498 is on thread 1's stack
    ==5407==  Uninitialised value was created by a stack allocation
    ==5407==    at 0x400931: main (iv.c:29)
    ==5407== 
    ==5407== 
    ==5407== 1 errors in context 2 of 2:
    ==5407== Syscall param rt_sigaction(act->sa_mask) points to uninitialised byte(s)
    ==5407==    at 0x4E6F5AE: __libc_sigaction (sigaction.c:62)
    ==5407==    by 0x40098B: main (iv.c:40)
    ==5407==  Address 0xffefff4a8 is on thread 1's stack
    ==5407==  Uninitialised value was created by a stack allocation
    ==5407==    at 0x400931: main (iv.c:29)
    ==5407== 
    ==5407== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

但我不确定这是什么意思。

Answer 1

最简单的初始化方式psa就是零初始化：

struct sigaction psa = {0};

但是，也有一些微妙之处。您不能从信号处理程序调用 async-signal-unsafe 函数，例如 printf。有关允许的功能列表，请参阅 signal safty。您可以将 printf 替换为 write(2) 调用来避免这种情况：

write(STDOUT_FILENO, "Run command called\n", sizeof "Run command called\n" - 1);

此外，在创建另一个 child 进程之前，您还要对每个进程执行 wait。因此，任何时候都只有一个 child 运行ning。如果您修改程序以允许多个 child 进程同时运行并且 child 进程大致相同，那么并非所有 SIGCHLD 信号可以被捕获。例如，如果信号处理程序正在处理前一个 SIGCHLD 时 SIGCHLD 到达，那么它将丢失。因为挂起的信号将被丢弃，除非它们是 不同的 信号，real-time 信号除外。例如，如果 SIGUSR1 在 SIGCHLD 正在处理（由信号处理程序）时到达，它将被阻塞；但另一个 SIGCHLD 将被忽略。

多次使用sigaction时出错

Error using sigaction multiple times

c

linux

signals

fork

sigaction