内存信号量sem_init函数解析

intsem_init(sem_t *sem, int pshared, unsigned int value);return 0 on success,or -1 on error

本文着重解析sem_init第二个参数 pshared ，下面摘自《The Linux Programming Interface》

这一段大体意思：如果pshared是0 则可以在线程间共享

如果非0，则可以在进程间共享，但是有个条件：信号量必须存在于共享内存中

也就是说并非pshared非0就可以实现进程间共享，示例如下：

#include <stdio.h>

#include <errno.h>

#include <semaphore.h>

#include <fcntl.h>

sem_t sem; 

static void mysem(char *str)

{

 int i = 0;

 while('\0' != str[i])

 {

  printf("%c\n", str[i++]);

  sleep(1);

 }

}

int main(void) 

{

 pid_t pid = -1;

 int ret = -1;

 int status = -1;

 int semvalue;

                                                                                                       sem_init(&sem,0,1);

 pid = fork();

 if(-1 == (ret = pid))

 {

  perror("fork failed: ");

  goto _OUT;

 }

                                                                                                                                                                                                                                                                                                                                                                       

 if(0 == pid)

 {

  mysem("abcd");

printf("2sem address is %x\r\n",&sem);

sem_getvalue(&sem,&ret);

printf("1sem ret is %d\r\n",ret);

  ret = sem_post(&sem);

  sem_getvalue(&sem,&ret);

printf("2sem ret is %d\r\n",ret);

  ret = sem_post(&sem);

  sem_getvalue(&sem,&ret);

printf("2.5sem ret is %d\r\n",ret);

 }

 if(0 < pid)

 {

printf("1sem address is %x\r\n",&sem);

  wait(&status);

  sem_getvalue(&sem,&ret);

printf("3sem ret is %d\r\n",ret);

  ret = sem_wait(&sem);

  mysem("1234");

  ret = sem_wait(&sem);

  sem_getvalue(&sem,&ret);

printf("4sem ret is %d\r\n",ret);

  mysem("5678");

  sem_destroy(&sem);

 }

_OUT:

 return ret;

}

[meteor@localhost sem]$ ./sem_ram_syn

1sem address is 804a044

a

b

c

d

2sem address is 804a044

1sem ret is 1

2sem ret is 2

2.5sem ret is 3

3sem ret is 1

1

2

3

4

^C

仔细分析打印结果可以发现，父进程（pid>0）中sem的值只与sem_init初始化的值有关，不管子进程中

有多少次post，父进程依然会锁住，并不会打印“5678”

具体进程间共享暂未实现，有时间再补上

线程间共享：

#include <stdio.h>

#include <errno.h>

#include <semaphore.h>

#include <fcntl.h>

#include <pthread.h>

#include <unistd.h>

#define SEM_NAME "mysem"

#define OPEN_FLAG O_RDWR|O_CREAT

#define OPEN_MODE 00777

#define INIT_V 0

 sem_t sem;

static void mysem(char *str)

{

 int i = 0;

 while('\0' != str[i])

 {

  printf("%c\n", str[i++]);

  sleep(1);

 }

}

void *thrd_func()

{

 int ret = -1;

       

 mysem("abcd");

 sem_getvalue(&sem,&ret);

 printf("1sem ret is %d\r\n",ret);

 ret = sem_post(&sem);

 sem_getvalue(&sem,&ret);

 printf("2sem ret is %d\r\n",ret);

 ret = sem_post(&sem);

 sem_getvalue(&sem,&ret);

 printf("2.5sem ret is %d\r\n",ret);

}

int main(void)

{

 pid_t pid = -1;

 int ret = -1;

 int status = -1;

 int semvalue;

 pthread_t tid;

 

 sem_init(&sem,0,0);

  pthread_create(&tid,NULL, thrd_func, NULL);

sleep(10);

 sem_getvalue(&sem,&ret);

printf("3sem ret is %d\r\n",ret);

 ret = sem_wait(&sem);

 sem_getvalue(&sem,&ret);

printf("4sem ret is %d\r\n",ret);

 mysem("1234");

 ret = sem_wait(&sem);

 sem_getvalue(&sem,&ret);

printf("5sem ret is %d\r\n",ret);

 mysem("5678");

 sem_destroy(&sem);

 return 0;

}

[meteor@localhost sem]$ ./sem_ram_thread

a

b

c

d

1sem ret is 0

2sem ret is 1

2.5sem ret is 2

3sem ret is 2

4sem ret is 1

1

2

3

4

5sem ret is 0

5

6

7

8

[meteor@localhost sem]$

线程间用锁

#include <stdio.h>

#include <errno.h>

#include <semaphore.h>

#include <fcntl.h>

#include <pthread.h>

#include <unistd.h>

#define SEM_NAME "mysem"

#define OPEN_FLAG O_RDWR|O_CREAT

#define OPEN_MODE 00777

#define INIT_V 0

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

 sem_t sem;

static void mysem(char *str)

{

 int i = 0;

 while('\0' != str[i])

 {

  printf("%c\n", str[i++]);

  sleep(1);

 }

}

void *thrd_func()

{

 int ret = -1;

 pthread_mutex_unlock(&mutex);

 pthread_mutex_unlock(&mutex);

 mysem("abcd");

sleep(20);

 pthread_mutex_unlock(&mutex);

}

int main(void)

{

 pid_t pid = -1;

 int ret = -1;

 int status = -1;

 int semvalue;

 pthread_t tid;

 

 pthread_create(&tid,NULL, thrd_func, NULL);

sleep(10);

 pthread_mutex_lock(&mutex);

 mysem("1234");

 pthread_mutex_lock(&mutex);

 mysem("5678");

 sem_destroy(&sem);

 return 0;

}

[meteor@localhost sem]$ ./lock_thread

a

b

c

d

1

2

3

4

5

6

7

8

[meteor@localhost sem]$

可见，跟信号量不同的一点，子线程不管解锁多少次，主线程一次lock就可以了，后面不会执行

只有子进程（延迟解锁）解锁，才会继续进行