利用POSIX互斥锁和条件变量实现的信号量

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这是一篇从http://man.yolinux.com/cgi-bin/man2html?cgi_command=pthread_mutexattr_init转载过来的文章。

利用POSIX互斥锁、条件变量，和内存映射文件，实现的信号量机制，用于进程间的同步。

/* sem.h */struct semaphore{pthread_mutex_t lock;pthread_cond_t nonzero;unsigned count;};typedef struct semaphore semaphore_t;semaphore_t*semaphore_create(char *semaphore_name);semaphore_t*semaphore_open(char *semaphore_name);voidsemaphore_post(semaphore_t *semap);voidsemaphore_wait(semaphore_t *semap);voidsemaphore_close(semaphore_t *semap);/* sem.c */#include <sys/types.h>#include <sys/stat.h>#include <sys/mman.h>#include <fcntl.h>#include <pthread.h>#include "sem.h"semaphore_t* semaphore_create(char *semaphore_name){int fd = open(semaphore_name, O_RDWR | O_CREAT | O_EXCL, 0666);if (fd < 0) return (NULL);(void) ftruncate(fd, sizeof(semaphore_t));pthread_mutexattr_t psharedm;pthread_condattr_t psharedc;(void) pthread_mutexattr_init(&psharedm);(void) pthread_mutexattr_setpshared(&psharedm, PTHREAD_PROCESS_SHARED);(void) pthread_condattr_init(&psharedc);(void) pthread_condattr_setpshared(&psharedc, PTHREAD_PROCESS_SHARED);semaphore_t* semap = (semaphore_t *) mmap(NULL,  sizeof(semaphore_t),  PROT_READ | PROT_WRITE,  MAP_SHARED,  fd,  0);close (fd);(void) pthread_mutex_init(&semap->lock, &psharedm);(void) pthread_cond_init(&semap->nonzero, &psharedc);// 我觉得应该给个大于零的初值semap->count = 0;return (semap);}semaphore_t* semaphore_open(char *semaphore_name){int fd = open(semaphore_name, O_RDWR, 0666);if (fd < 0) return (NULL);semaphore_t* semap = (semaphore_t *) mmap(NULL,  sizeof(semaphore_t),  PROT_READ | PROT_WRITE,  MAP_SHARED,  fd,  0);close (fd);return (semap);}void semaphore_post(semaphore_t *semap){pthread_mutex_lock(&semap->lock);// 计数为零，说明可能有线程已经阻塞在该条件变量上if (semap->count == 0){pthread_cond_signal(&semapx->nonzero);}semap->count++;pthread_mutex_unlock(&semap->lock);}void semaphore_wait(semaphore_t *semap){pthread_mutex_lock(&semap->lock);// 计数为零，说明已无资源，等待while (semap->count == 0){pthread_cond_wait(&semap->nonzero, &semap->lock);}semap->count--;pthread_mutex_unlock(&semap->lock);}void semaphore_close(semaphore_t *semap){munmap((void *) semap, sizeof(semaphore_t));}

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