Question

每当我的孩子进程死亡时，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函数并不总是第二次被调用。有时虽然如此，我有点困惑。

我的代码就是这个：

  #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;
  }

当我尝试运行它时，我的结果有时候是：

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并不总是接收我的SIGCHLD信号。当我运行valgrind时，我收到以下错误：

    ==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)

但我不确定这意味着什么。

Answer 1

最简单的方法是初始化psa为零初始化它：

struct sigaction psa = {0};

但是，有一些细微之处。您不能从信号处理程序中调用异步信号不安全函数，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)来避免这种情况：

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

此外，您在创建另一个子进程之前对每个子进程执行wait。因此，任何时候都只有一个孩子在跑步。如果您修改程序以允许多个子进程同时运行且子进程在同一个进程中死亡，则不会捕获所有SIGCHLD信号。例如，如果SIGCHLD在信号处理程序正在处理之前到达是先前的SIGCHLD，那么它将丢失。因为待处理的信号将被丢弃，除非它们是不同的信号，除了实时信号。例如，如果SIGUSR1在处理SIGCHLD时到达（通过信号处理程序），则会被阻止;但是另一个SIGCHLD将被忽略。

多次使用sigaction时出错

1 个答案: