<转>linux操作系统编程——共享内存读写（采用信号量进行同步互斥）

http://blog.csdn.net/yanghaoran321/article/details/7872722

程序要求：

     创建一个写端和一个读端，写端写入数据后读端才开始读，读端读完数据后，写端才可以开始写，这样的同步采用信号机制实现，并且写端与读端打开顺序不同也能实现功能；

程序如下：

（1）write.c（写端）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#include "sem.h"

typedef struct
{
    char buf[1024];
}memory;

int main(int argc, const char *argv[])
{
    key_t key;
    memory *p = NULL;
    int shmid;
    int create_flag = 0;
    int sem_id;

    if ((key = ftok(".", 'a')) < 0)
    {
        perror("failed to get key");
        exit(-1);
    }

    if ((sem_id = semget(key, 1, 0666 | IPC_CREAT | IPC_EXCL)) < 0)
    {
        if (errno == EEXIST)
        {
            if ((sem_id = semget(key, 1, 0666)) < 0)
            {
                perror("failed to semget");
                exit(-1);
            }
        }
    }

    init_sem(sem_id, 0);

    if ((shmid = shmget(key, sizeof(memory), 0666 | IPC_CREAT | IPC_EXCL)) < 0)
    {
        if (errno == EEXIST)
        {
            if ((shmid = shmget(key, sizeof(memory), 0666)) < 0)
            {
                perror("failed to shmget memory");
                exit(-1);
            }
        }
        else
        {
            perror("failed to shmget");
            exit(-1);
        }
    }
    else
        create_flag = 1;

    if ((p = shmat(shmid, NULL, 0)) == (void *)(-1))
    {
        perror("failed to shmat memory");
        exit(-1);
    }

    while(1)
    {
        printf(">");
        fgets(p->buf, sizeof(p->buf), stdin);
        p->buf[strlen(p->buf) - 1] = 0;

        sem_v(sem_id);

        if (strncmp(p->buf, "quit", 4) == 0)
            break;
    }

    if (create_flag == 1)
    {
        if (shmdt(p) < 0)
        {
            perror("failed to shmdt memory");
            exit(-1);
        }

        if (shmctl(shmid, IPC_RMID, NULL) == -1)
        {
            perror("failed to delete share memory");
            exit(-1);
        }

        delete_sem(sem_id);
    }

    return 0;
}

（2）read.c（读端）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#include "sem.h"

typedef struct
{
    char buf[1024];
}memory;

int main(int argc, const char *argv[])
{
    key_t key;
    int shmid;
    memory *p = NULL;
    int create_flag = 0;
    int sem_id;

    if ((key = ftok(".", 'a')) < 0)
    {
        perror("failed to get key");
        exit(-1);
    }

    if ((sem_id = semget(key, 1, 0666 | IPC_CREAT | IPC_EXCL)) < 0)
    {
        if (errno == EEXIST)
        {
            if ((sem_id = semget(key, 1, 0666)) < 0)
            {
                perror("failed to semget");
                exit(-1);
            }
        }
    }

    init_sem(sem_id, 0);

    if ((shmid = shmget(key, sizeof(memory), 0666 | IPC_CREAT | IPC_EXCL)) < 0)
    {
        if (errno == EEXIST)
        {
            if ((shmid = shmget(key, sizeof(memory), 0666)) < 0)
            {
                perror("failed to create share memory");
                exit(-1);
            }
        }
        else
        {
            perror("failed to shmget");
            exit(-1);
        }
    }
    else
        create_flag = 1;

    if ((p = shmat(shmid, NULL, 0)) == (void *)(-1))
    {
        perror("failed to shmat");
        exit(-1);
    }

    while(1)
    {
        sem_p(sem_id);

        if (strncmp(p->buf, "quit", 4) == 0)
            break;

        printf("recv: %s
", p->buf);

    }

    if (create_flag == 1)
    {
        if (shmdt(p) < 0)
        {
            perror("failed to shmdt");
            exit(-1);
        }

        if (shmctl(shmid, IPC_RMID, NULL) == -1)
        {
            perror("failed to delete share memory");
            exit(-1);
        }

        delete_sem(sem_id);
    }

    return 0;
}

关于封装信号量函数的头文件：

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <unistd.h>

void init_sem(int , int );
void delete_sem(int );
void sem_p(int );
void sem_v(int );

union semun
{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};

void init_sem(int sem_id, int init_value)
{
    union semun sem_union;

    sem_union.val = init_value;

    if (semctl(sem_id, 0, SETVAL, sem_union) < 0)
    {
        perror("failed to init_sem");
        exit(-1);
    }

    return ;
}

void delete_sem(int sem_id)
{
    union semun sem_union;

    if (semctl(sem_id, 0, IPC_RMID, sem_union) < 0)
    {
        perror("failed to delete_sem");
        exit(-1);
    }

    return ;
}

void sem_p(int sem_id)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;
    sem_b.sem_op = -1;
    sem_b.sem_flg = SEM_UNDO;

    if (semop(sem_id, &sem_b, 1) < 0)
    {
        perror("failed to sem_p");
        exit(-1);
    }

    return;
}

void sem_v(int sem_id)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;
    sem_b.sem_op = 1;
    sem_b.sem_flg = SEM_UNDO;

    if (semop(sem_id, &sem_b, 1) < 0)
    {
        perror("failed to sem_v");
        exit(-1);
    }

    return ;
}