如果主线程完成但对等线程没有完成会发生什么？

Question

我是 C 和多线程编程的新手，只是一个问题，如果主线程完成但对等线程没有完成会发生什么，进程是否仍然等待所有对等线程完成或进程在主线程完成后立即终止？

Answer 1

When the main function returns, or you call exit , then the system will terminate the process and all threads running in that process.

If you have background (detached) threads that you want to keep running, you have to end the "main" thread only, not the process. How to do that is platform-dependent, but on POSIX platforms (eg Linux or macOS) then you call pthread_exit instead of exit .

If you don't have any background threads then you need to join all the running threads to keep them from being forcibly terminated.

Answer 2

Here is an example program which illustrates the preceding answer on GLIBC/Linux operating system. The program creates a detached secondary thread and if it is not passed a parameter, the main thread exits otherwise it calls pthread_exit():

#include <pthread.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>


void *th_entry(void *p)
{
  int i;

  printf("Secondary thread starting...\n");

  for (i = 0; i < 5; i ++) {
    printf(".");
    fflush(stdout);
    sleep(1);
  }

  printf("\nSecondary thread exiting\n");

  return NULL;
}


int main(int ac, char *av[])
{
  int rc;
  pthread_t tid;

  printf("Main thread starting...\n");

  rc = pthread_create(&tid, NULL, th_entry, NULL);
  if (rc != 0) {
    errno = rc;
    fprintf(stderr, "pthread_create(): '%m' (%d)\n", errno);
    return 1;
  }

  // Detach the secondary thread
  rc = pthread_detach(tid);
  if (rc != 0) {
    errno = rc;
    fprintf(stderr, "pthread_detach(): '%m' (%d)\n", errno);
    return 1;
  }

  // Some dummy work
  sleep(1);

  if (av[1]) {
    printf("Main thread exiting\n");
    return 0;
  } else {
    printf("Main thread calling pthread_exit()\n");
    pthread_exit(NULL);
  }

  return 0;

}

Compile it:

$ gcc td.c -l pthread

When the program is not passed any parameter, the main thread calls pthread_exit() and the process ends when the secondary thread finishes.

$ ./a.out
Main thread starting...
Secondary thread starting...
.Main thread calling pthread_exit()
....
Secondary thread exiting
$

When the program is passed a parameter, the main thread calls exit() (it returns 0 but it is equivalent) and the process ends immediately terminating the secondary thread:

$ ./a.out 1
Main thread starting...
Secondary thread starting...
.Main thread exiting
$