线程同步：信号量

#include <semaphore.h>

int sem_init(sem_t *sem, int pshared, unsigned int value);

int sem_wait(sem_t * sem);

int sem_trywait(sem_t * sem);

int sem_timewait(sem_t * sem);

int sem_post(sem_t * sem);

int sem_getvalue(sem_t * sem, int * sval);

int sem_destroy(sem_t * sem);

Description：

Semaphores are counters for resources shared between threads. The basicoperations  on  semaphores  are:  increment the counter atomically, and wait until the counter is non-null and decrement it atomically.

/*******************************************************************
*   sem_init
*
*   Description:
*     initializes the semaphore object pointed to  by  |sem|
*
*   <Additional description>
*     LinuxThreads  currently  does  not support  process-shared semaphores, thus sem_init always returns with
*     error ENOSYS if |pshared| is not zero.
*
*   Parameters:
*     |sem|,        semaphore to be initialozed
*     |pshared|,   indicates whether the  semaphore  is  local  to  the current  process
*                       0: local  to  the current  process
*                       others: shared between several processes
*
*   Returns:
*     OK on success,
*     ERROR on failure
*
*****************************************************************/
int sem_init(sem_t *sem, int pshared,unsigned int value);

/*******************************************************************
*   sem_wait
*
*   Description:
*     Suspends the calling thread until the semaphore  pointed  to by |sem| has non-zero count.
*     It then atomically decreases the semaphore count.
*
*   <Additional description>
*     递减即锁定，否则就阻塞直到sem_post()释放这个锁。
*
*   Parameters:
*
*   Returns:
*     OK on success,
*     ERROR on failure
*
*****************************************************************/
int sem_wait(sem_t * sem);

/*******************************************************************
*   sem_trywait
*
*   Description:
*     A non-blocking variant of !sem_wait!.
*
*   <Additional description>
*     1) If the semaphore pointed  to  by  |sem|  has  non-zero  count,  the count is atomically
*         decreased and sem_trywait immediately returns 0.
*     2) If  the  semaphore count is zero, sem_trywait immediately returns with error EAGAIN.
*
*   Parameters:
*
*   Returns:
*
*****************************************************************/
int sem_trywait(sem_t * sem);

/*******************************************************************
*   sem_timedwait--lock a semaphore
*
*   Description:
*     The sem_timedwait() function shall lock the semaphore referenced by sem as in the sem_wait()
*     function. However, if the semaphore cannot be locked when waiting for another process or
*     thread to unlock the semaphore by performing a sem_post() function, this wait shall be terminated
*     when the specified timeout expires.
*     (注：sem_timedwait() 与 sem_wait() 类似，只不过 abs_timeout 指定一个阻塞的时间上限，如果调用因不
*     能立即执行递减而要阻塞。abs_timeout 参数指向一个指定绝对超时时刻的结构，这个结果由自 Epoch，
*     1970-01-01 00:00:00 +0000(UTC) 秒数和纳秒数构成。这个结构定义如下：
*     struct timespec {
*         time_t tv_sec;        /* 秒 */
*         long   tv_nsec;       /* 纳秒 */
*     };
*     如果调用时超时时刻已经到点，并且信号量不能立即锁定，那么 sem_timedwait() 将失败于超时(errno 设置为 ETIMEDOUT)。
*     如果操作能被立即执行，那么 sem_timedwait() 永远不会失败于超时错误，而不管 abs_timeout 的值。进一步说，abs_timeout
*     的验证在此时没有进行。)
*
*   <Additional description>
*
*   Parameters:
*
*   Returns:
*     OK on success,
*     ERROR on failure
*
*****************************************************************/
int sem_timewait(sem_t * sem);

/*******************************************************************
*   sem_post
*
*   Description:
*     Atomically  increases the count of the semaphore pointed to by |sem|. This  function  never
*     blocks  and  can  safely  be  used  in asynchronous signal handlers.
*
*   <Additional description>
*
*   Parameters:
*
*   Returns:
*     OK on success,
*     ERROR on failure
*
*****************************************************************/
int sem_post(sem_t * sem);

/*******************************************************************
*   sem_getvalue
*
*   Description:
*     stores in the location pointed to by |sval| the current count of the semaphore |sem|.
*
*   <Additional description>
*     LinuxThreads  currently  does  not support  process-shared semaphores, thus sem_init always returns with
*     error ENOSYS if |pshared| is not zero.
*
*   Parameters:
*     |sem|,        semaphore to be initialozed
*     |pshared|,   indicates whether the  semaphore  is  local  to  the current  process
*                       0: local  to  the current  process
*                       others: shared between several processes
*
*   Returns:
*     OK on success,
*     ERROR on failure
*
*****************************************************************/
int sem_getvalue(sem_t * sem, int * sval);

/*******************************************************************
*   sem_destroy
*
*   Description:
*     destroys a semaphore object,  freeing  the  resources  it might  hold.
*
*   <Additional description>
*     No threads should be waiting on the semaphore at the time sem_destroy is  called.
*     In  the  LinuxThreads  implementation,   no resources  are  associated  with  semaphore
*     objects, thus sem_destroy actually does nothing except checking that no thread is waiting
*     on  the semaphore.
*
*   Parameters:
*
*   Returns:
*
*
*****************************************************************/
int sem_destroy(sem_t * sem);

http://manpages.ubuntu.com/manpages/dapper/man3/sem_init.3.html