Question

我正在尝试使用POSIX命名信号量来确保只能运行我的可执行文件的一个实例。但我遇到了麻烦;信号量的值始终为0，因此它总是阻塞。

#include <semaphore.h>  /* required for semaphores */
#include <stdio.h>
#include <unistd.h>     /* usleep */
#include <fcntl.h>      // O_EXCL
#include <assert.h>
#include <stdlib.h>     /* exit, EXIT_FAILURE */


int main(int argc, char *argv[])
{
  int ret;
  int i;
  sem_t* semaphore;

  semaphore = sem_open("/mysemaphore", O_EXCL, 0777 /*0644*/, 2);
  printf("sem_open returned %p at line %u\n", semaphore, __LINE__);

  // if it exists, open with "open", and parameters will be ignored (therefore given as 0)
  if(!semaphore)
  {
    semaphore = sem_open("/mysemaphore", O_CREAT, 0, 0);
    printf("sem_open returned %p at line %u\n", semaphore, __LINE__);
  }

  // either of the above calls should have given us a valid semaphore
  assert(semaphore);

  // read its value time and again
  ret = sem_getvalue(semaphore, &i);
  printf("sem_getvalue returned %i at line %u, value is %i\n", ret, __LINE__, i);

// ....

输出：

sem_open returned 0x8003a4e0 at line 36
sem_getvalue returned 0 at line 50, value is 0

平台：Cygwin 1.7.33-2

使用此命令构建：

gcc Main.c -o Main -lpthread

非常感谢帮助！

Answer 1

使用sem_post(semaphore)增加，sem_wait(semaphore)减少。

此外，使用O_CREAT时，应将模式和值指定为有用的东西：

semaphore = sem_open("/mysemaphore", O_CREAT, 0777, 0);

命名POSIX信号量的问题

1 个答案: