int pthread_once(pthread_once_t *once_control,void(*init_routine)(void));

参数：

once_control         控制变量

init_routine         初始化函数

返回值：

若成功返回0，若失败返回错误编号。

 类型为pthread_once_t的变量是一个控制变量。控制变量必须使用PTHREAD_ONCE_INIT宏静态地初始化。

pthread_once函数首先检查控制变量，判断是否已经完成初始化，如果完成就简单地返回；否则，pthread_once调用初始化函数，并且记录下初始化被完成。如果在一个线程初始时，另外的线程调用pthread_once，则调用线程等待，直到那个现成完成初始话返回。

下面就是该函数的程序例子： 

#define _MULTI_THREADED
#include <pthread.h>
#include <stdio.h>

#define         NUMTHREADS    3
int             number      = 0;
int             okStatus    = 777;
pthread_once_t  onceControl = PTHREAD_ONCE_INIT;

static void checkResults(char *string, int rc) {
  if (rc) {
    printf("Error on : %s, rc=%d",
           string, rc);
    exit(EXIT_FAILURE);
  }
  return;
}
void initRoutine(void){  printf("In the initRoutine\n");  number++;}void *threadfunc(void *parm){  printf("Inside secondary thread\n");  pthread_once(&onceControl, initRoutine);  return NULL;}int main(int argc, char **argv){  pthread_t             thread[NUMTHREADS];  int                   rc=0;  int                   i=NUMTHREADS;  void                 *status;  printf("Enter Testcase - %s\n", argv[0]);  for (i=0; i < NUMTHREADS; ++i) {    printf("Create thread %d\n",           i);    rc = pthread_create(&thread[i], NULL, threadfunc, NULL);    checkResults("pthread_create()\n", rc);  }  for (i=0; i < NUMTHREADS; ++i) {    printf("Wait for thread %d\n", i);    rc = pthread_join(thread[i], &status);    checkResults("pthread_join()\n", rc);    if ((int)status != okStatus) {      printf("Secondary thread failed\n");      exit(1);    }  }  if (number != 1) {    printf("An incorrect number of 1 one-time init routine was called!\n");    exit(1);  }  printf("One-time init routine called exactly once\n");  printf("Main completed\n");  return 0;}

编译 gcc -o pthread_once -lpthread pthread_once.c

运行结果：Enter Testcase - ./pthread_once
Create thread 0
Create thread 1
Create thread 2
Wait for thread 0
Inside secondary thread
In the initRoutine
Inside secondary thread
Secondary thread failed