c - 时间相关函数和多线程

Question

我在 Linux(带有 libc-2.12.so 的 CentOS 6.3 发行版)上工作。

我想知道C库函数time()、localtime()、mktime()是否可以在多线程环境下使用。

仅举个例子(不一定是我的项目代码):

#include <stdio.h>  
#include <time.h>

int main()
{

  time_t timep;

  struct tm *p;

  for (int i = 0; i < 1000; ++i)
  {
    time(&timep);
    printf("time() : %d \n", timep);
    p = localtime(&timep);
    timep = mktime(p);

    printf("time()->localtime()->mktime():%d\n", timep);
  }

  return 0;    
}

如果我用 OpenMP 包装上面的代码会怎样？也就是放在多线程环境下。

#include <stdio.h>  
#include <time.h>
#include <omp.h>

int main()
{

  time_t timep;

  struct tm *p;

#pragma omp parallel for
  for (int i = 0; i < 1000; ++i)
  {
    time(&timep);
    printf("time() : %d \n", timep);
    p = localtime(&timep);
    timep = mktime(p);

    printf("time()->localtime()->mktime():%d\n", timep);
  }

  return 0;
}

Answer 1

来自 manual localtime :

The four functions asctime(), ctime(), gmtime() and localtime() return a pointer to static data and hence are not thread-safe. Thread-safe versions asctime_r(), ctime_r(), gmtime_r() and localtime_r() are specified by SUSv2, and available since libc 5.2.5.