使用读写锁pthread_rwlock_t未定义的问题

今天在程序里使用读写锁包括以下函数：

#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwptr);
int pthread_rwlock_wrlock(pthread_rwlock_t *rwptr);
int pthread_rwlock_unlock(pthread_rwlock_t *rwptr);

int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwptr);
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwptr);

int pthread_rwlock_init(pthread_rwlock_t *rwptr,
                     const pthread_rwlockattr_t *attr);
int pthread_rwlock_destroy(pthread_rwlock_t *rwptr);

int pthread_rwlockattr_init(pthread_rwlockattr_t *attr)
int pthread_rwlockattr_destroy(pthread_rwlockattr_t * attr);

int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t * attr, int *valptr);

int pthread_rwlockattr_setpshared(pthread_rwlockattr_t* attr, int value);
                  都返回：成功 0，出错为正的Exxxx值

测试程序如下：

#include <pthread.h>

pthread_rwlock_t lock = PTHREAD_RWLOCK_INITIALIZER;

int main()

{

       return 0;

}

$ gcc –o lock lock.c
$ lock.c:2: parse error before `lock'

$ lock.c:2: `PTHREAD_RWLOCK_INITIALIZER' undeclared here (not in a function)

$ lock.c:2: warning: data definition has no type or storage class

最后查文档解决方是：
在程序开头加入：
#define _XOPEN_SOURCE 500