sched_yield()这个函数可以使用另一个级别等于或高于当前线程的线程先运行。如果没有符合条件的线程，那么这个函数将会立刻返回然后继续执行当前线程的程序。  

在成功完成之后返回零，否则返回-1.

看下面一个实例

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

#define            LOOPCONSTANT     1000
#define            THREADS          3

pthread_mutex_t    mutex = PTHREAD_MUTEX_INITIALIZER;
int                i,j,k,l;
static void checkResults(char *string, int rc) {
  if (rc) {
    printf("Error on : %s, rc=%d",
           string, rc);
    exit(EXIT_FAILURE);
  }
  return;
}
void *threadfunc(void *parm){  int   loop = 0;  int   localProcessingCompleted = 0;  int   numberOfLocalProcessingBursts = 0;  int   processingCompletedThisBurst = 0;  int   rc;  printf("Entered secondary thread\n");  for (loop=0; loop<LOOPCONSTANT; ++loop) {    rc = pthread_mutex_lock(&mutex);    checkResults("pthread_mutex_lock()\n", rc);    /* Perform some not so important processing */    i++, j++, k++, l++;    rc = pthread_mutex_unlock(&mutex);    checkResults("pthread_mutex_unlock()\n", rc);    /* This work is not too important. Also, we just released a lock       and would like to ensure that other threads get a chance in       a more co-operative manner. This is an admittedly contrived       example with no real purpose for doing the sched_yield().       */    sched_yield();  }  printf("Finished secondary thread\n");  return NULL;}int main(int argc, char **argv){  pthread_t             threadid[THREADS];  int                   rc=0;  int                   loop=0;  printf("Enter Testcase - %s\n", argv[0]);  rc = pthread_mutex_lock(&mutex);  checkResults("pthread_mutex_lock()\n", rc);  printf("Creating %d threads\n", THREADS);  for (loop=0; loop<THREADS; ++loop) {    rc = pthread_create(&threadid[loop], NULL, threadfunc, NULL);    checkResults("pthread_create()\n", rc);  }  sleep(1);  rc = pthread_mutex_unlock(&mutex);  checkResults("pthread_mutex_unlock()\n", rc);  printf("Wait for results\n");  for (loop=0; loop<THREADS; ++loop) {    rc = pthread_join(threadid[loop], NULL);    checkResults("pthread_join()\n", rc);  }  pthread_mutex_destroy(&mutex);  printf("Main completed\n");  return 0;}

运行结果 Enter Testcase - ./sched_yield

Creating 3 threads
Entered secondary thread
Entered secondary thread
Entered secondary thread
Wait for results
Fi