在android下使用信号量时，需要链接libc库。
但是在libc库中，函数sem_open、sem_close、sem_unlink只有声明，并未实现。
可以从一下代码中找到证据。
代码路径： bionic/libc/include/semaphore.h
代码语言：javascript复制/*
* Copyright (C) 2008 The Android Open Source Project
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#ifndef _SEMAPHORE_H
#define _SEMAPHORE_H
#include <sys/cdefs.h>
__BEGIN_DECLS
struct timespec;
typedef struct {
unsigned int count;
#ifdef __LP64__
int __reserved[3];
#endif
} sem_t;
#define SEM_FAILED __BIONIC_CAST(reinterpret_cast, sem_t*, 0)
int sem_destroy(sem_t* __sem);
int sem_getvalue(sem_t* __sem, int* __value);
int sem_init(sem_t* __sem, int __shared, unsigned int __value);
int sem_post(sem_t* __sem);
int sem_timedwait(sem_t* __sem, const struct timespec* __ts);
/*
* POSIX only supports using sem_timedwait() with CLOCK_REALTIME, however that is typically
* inappropriate, since that clock can change dramatically, causing the timeout to either
* expire earlier or much later than intended. This function is added to use a timespec based
* on CLOCK_MONOTONIC that does not suffer from this issue.
*/
int sem_timedwait_monotonic_np(sem_t* __sem, const struct timespec* __ts) __INTRODUCED_IN(28);
int sem_trywait(sem_t* __sem);
int sem_wait(sem_t* __sem);
/* These aren't actually implemented. */
sem_t* sem_open(const char* __name, int _flags, ...);
int sem_close(sem_t* __sem);
int sem_unlink(const char* __name);
__END_DECLS
#endif后记： 安卓中通信这块和linux还是有很大不同的。
