【基础知识】进程通信之共享内存+信号量

1. 信号量函数的介绍

如果用semget创建了一个新的信号量集对象时，则semid_ds结构成员变量的值设置如下：

    sem_otime设置为0。sem_ctime设置为当前时间。msg_qbytes设成系统的限制值。sem_nsems设置为nsems参数的数值。semflg的读写权限写入sem_perm.mode中。sem_perm结构的uid和cuid成员被设置成当前进程的有效用户ID，gid和cuid成员被设置成当前进程的有效组ID。

sops为指向sembuf数组，定义所要进行的操作序列。下面是信号量操作举例。

struct sembuf sem_get={0,-1,IPC_NOWAIT}; /将信号量对象中序号为0的信号量减1/

struct sembuf sem_get={0,1,IPC_NOWAIT}; /将信号量对象中序号为0的信号量加1/

struct sembuf sem_get={0,0,0}; /进程被阻塞，直到对应的信号量值为0/

flag一般为0，若flag包含IPC_NOWAIT，则该操作为非阻塞操作。若flag包含SEM_UNDO，则当进程退出的时候会还原该进程的信号量操作，这个标志在某些情况下是很有用的，比如某进程做了P操作得到资源，但还没来得及做V操作时就异常退出了，此时，其他进程就只能都阻塞在P操作上，于是造成了死锁。若采取SEM_UNDO标志，就可以避免因为进程异常退出而造成的死锁。

参考网址：https://blog.csdn.net/guoping16/article/details/6584043

2. 示例代码

新建semaphore.h文件

#ifndef __SEM_H__
#define __SEM_H__#include<stdio.h>
#include<stdlib.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include<sys/types.h>#define PATHNAME "./"
#define PROJ_ID 0x666union semun
{int val; //单个信号的值 struct semid_ds *buf;unsigned short *array;struct seminfo *__buf;
};int create_sem_set(int nsem);int get_sem_set(int nsem);int init_sem_set(int sem_set_id, int which_sem);int sem_p(int sem_set_id, int which_sem);int sem_v(int sem_set_id, int which_sem);int destroy_sem_set(int sem_set_id);#endif

新建semaphore.c文件

#include "semaphore.h"static int sem_set(int nsem, int semflag)
{key_t key = ftok(PATHNAME, PROJ_ID);if (key < 0){perror("ftok failure");exit(-1);}int sem_set_id = semget(key, nsem, semflag);if (sem_set_id < 0){perror("semget failure");exit(-1);}return sem_set_id;
}int create_sem_set(int nsem)
{if (nsem <= 0){perror("nsem <= 0");exit(-1);}return sem_set(nsem, IPC_CREAT | IPC_EXCL | 0666);
}int get_sem_set(int nsem)
{if (nsem <= 0){perror("nsem <= 0");exit(-1);}return sem_set(nsem, IPC_CREAT);
}int init_sem_set(int sem_set_id, int which_sem)
{union semun my_semun;my_semun.val = 1;//semun中val的值去设置信号集（sem_set）中单个sem的值int ret = semctl(sem_set_id, which_sem, SETVAL, my_semun);if (ret == -1){perror("semctl error");exit(-1);}return 0;
}static int sem_operatons(int sem_set_id, int which_sem, int op)
{struct sembuf sb;//sembuf is as follows://unsigned short sem_num//short sem_op//short sem_flgsb.sem_num = which_sem;sb.sem_op = op;sb.sem_flg = SEM_UNDO;return semop(sem_set_id, &sb, 1);
}int sem_p(int sem_set_id, int which_sem)
{ return sem_operatons(sem_set_id, which_sem, -1);
}int sem_v(int sem_set_id, int which_sem)
{ return sem_operatons(sem_set_id, which_sem, +1);
}int destroy_sem_set(int sem_set_id)
{int ret = semctl(sem_set_id, 0, IPC_RMID, NULL); if (ret == -1){perror("semctl failure");exit(-1);}return ret;
}

新建shm.h文件

#ifndef __SHM__
#define __SHM__#include<stdlib.h>
#include<stdio.h>
#include<sys/ipc.h>
#include<sys/shm.h>
#include<sys/types.h>#define PATHNAME "./"
#define PROJ_ID 0x666int create_shm(int sz);int get_shm(int sz);void* malloc_addr(int shm_id);int free_addr(const void* addr);int destroy_shm(int shm_id);#endif

新建shm.c文件

#include"shm.h"static int shm(int sz, int flag)
{key_t key = ftok(PATHNAME, PROJ_ID);if (key == -1){perror("ftok failure");exit(-1);}int shm_id = shmget(key, sz, flag);if (shm_id < 0){perror("shmget failure");exit(-1);}return shm_id;
}int create_shm(int sz)
{return shm(sz, IPC_CREAT | IPC_EXCL | 0666);
}int get_shm(int sz)
{return shm(sz, IPC_CREAT);
}void* malloc_addr(int shm_id)
{return shmat(shm_id, NULL, 0);
}int free_addr(const void* addr)
{return shmdt(addr);
}int destroy_shm(int shm_id)
{return shmctl(shm_id, IPC_RMID, NULL);
}

新建test.c文件

#include"shm.h"
#include<unistd.h>
#include<string.h>
#include<sys/wait.h>
#include"semaphore.h"
typedef struct cPicInfo{int size;char state;
}cPicInfo;typedef struct cPicBuf{char *picbuf;int *picbuf_size;char *picbuf_state; //0 :not use 1: will using 2: is using
}cPicBuf;#define SHM_SIZE       (4096 + sizeof(cPicInfo))
#define PICBUF_NUM      (10)cPicBuf* getfreebuf_frome_mem(char *sharebuf, int perpic_size)
{int i;cPicBuf picbuf[PICBUF_NUM];char *psharebuf = sharebuf + 1;for( i = 0; i <PICBUF_NUM; i++){picbuf[i].picbuf = psharebuf + i * SHM_SIZE;picbuf[i].picbuf_size = (int *)(picbuf[i].picbuf + ( SHM_SIZE - sizeof(cPicInfo)));picbuf[i].picbuf_state = (char *)(picbuf[i].picbuf_size) + sizeof(int);if(*picbuf[i].picbuf_state == 0){*picbuf[i].picbuf_state = 2;break;}}printf("get free i:%d\n",i);return i < PICBUF_NUM ?  &picbuf[i] : NULL;
}cPicBuf* getcolbuf_frome_mem(char *sharebuf, int perpic_size)
{int i;cPicBuf picbuf[PICBUF_NUM];char *psharebuf = sharebuf + 1;for( i = 0; i <PICBUF_NUM; i++){picbuf[i].picbuf = psharebuf + i * SHM_SIZE;picbuf[i].picbuf_size = (int *)(picbuf[i].picbuf + ( SHM_SIZE - sizeof(cPicInfo)));picbuf[i].picbuf_state = (char *)(picbuf[i].picbuf_size) + sizeof(int);if(*picbuf[i].picbuf_state == 2){*picbuf[i].picbuf_state = 3;break;}}printf("get col i:%d\n",i);return i < PICBUF_NUM ?  &picbuf[i] : NULL;
}int main()
{int ret;//创建信号量并初始化int sem_set_id = create_sem_set(2);init_sem_set(sem_set_id, 0);init_sem_set(sem_set_id, 1);//创建共享内存int shm_id = create_shm(SHM_SIZE * PICBUF_NUM + 1);//创建子进程pid_t pid = fork();if (pid < 0){perror("fork failure");exit(-1);}else if (pid == 0) //子进程{char* buf = (char*)malloc_addr(shm_id);int count = 0;cPicBuf* freebuf;buf[0] = 1;while(1){//if(buf[0] == 1){    int ret = semctl(sem_set_id, which_sem, SETVAL, my_semun);if( 1 == (ret =semctl(sem_set_id, 1, GETVAL, NULL))){printf("zi ret: %d\n", ret);
;               count++;sem_p(sem_set_id, 0);    freebuf = getfreebuf_frome_mem(buf, SHM_SIZE);if(freebuf != NULL)*freebuf->picbuf_size = count;buf[0] = 2;sem_v(sem_set_id, 0); sem_p(sem_set_id, 1);}if(count == 10)break;}free_addr(buf);}else //父进程{char* buf = (char*)malloc_addr(shm_id);//usleep(500000);//让子进程先P操作cPicBuf* colbuf;int count1 = 0;while(1){//if(buf[0] == 2){if( 0 == (ret = semctl(sem_set_id, 1, GETVAL, NULL))){printf("fu ret: %d\n", ret);count1++;sem_p(sem_set_id, 0); colbuf = getcolbuf_frome_mem(buf, SHM_SIZE);if(colbuf != NULL){printf("picbuf_size :%d \n",*colbuf->picbuf_size);}buf[0] = 1;sem_v(sem_set_id, 0);  sem_v(sem_set_id, 1);                }if(count1 == 10)break;}free_addr(buf);destroy_shm(shm_id);destroy_sem_set(sem_set_id);}return 0;
}

新建Makefile文件

test:test.c semaphore.c shm.cgcc -o $@ $^
.PHONY:clean
clean:rm -f test