利用POSIX互斥锁和条件变量实现的信号量

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);
void			semaphore_post(semaphore_t *semap);
void			semaphore_wait(semaphore_t *semap);
void			semaphore_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));
}