进程间通信的例子-共享内存

消费者

root@ubuntu:~/test/SourceCode/ch14# cat shm1.c 
/* Our first program is a consumer. After the headers the shared memory segment
 (the size of our shared memory structure) is created with a call to shmget,
 with the IPC_CREAT bit specified. */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <sys/shm.h>

#include "shm_com.h"

int main()
{
    int running = 1;
    void *shared_memory = (void *)0;
    struct shared_use_st *shared_stuff;
    int shmid;

    srand((unsigned int)getpid());    

    shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

    if (shmid == -1) {
        fprintf(stderr, "shmget failed\n");
        exit(EXIT_FAILURE);
    }

/* We now make the shared memory accessible to the program. */

    shared_memory = shmat(shmid, (void *)0, 0);
    if (shared_memory == (void *)-1) {
        fprintf(stderr, "shmat failed\n");
        exit(EXIT_FAILURE);
    }

    printf("Memory attached at %X\n", (int)shared_memory);

/* The next portion of the program assigns the shared_memory segment to shared_stuff,
 which then prints out any text in written_by_you. The loop continues until end is found
 in written_by_you. The call to sleep forces the consumer to sit in its critical section,
 which makes the producer wait. */

    shared_stuff = (struct shared_use_st *)shared_memory;
    shared_stuff->written_by_you = 0;
    while(running) {
        if (shared_stuff->written_by_you) {
            printf("You wrote: %s", shared_stuff->some_text);
            sleep( rand() % 4 ); /* make the other process wait for us ! */
            shared_stuff->written_by_you = 0;
            if (strncmp(shared_stuff->some_text, "end", 3) == 0) {
                running = 0;
            }
        }
    }

/* Lastly, the shared memory is detached and then deleted. */

    if (shmdt(shared_memory) == -1) {
        fprintf(stderr, "shmdt failed\n");
        exit(EXIT_FAILURE);
    }

    if (shmctl(shmid, IPC_RMID, 0) == -1) {
        fprintf(stderr, "shmctl(IPC_RMID) failed\n");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}

root@ubuntu:~/test/SourceCode/ch14# 

root@ubuntu:~/test/SourceCode/ch14# cat shm_com.h 
/* A common header file to describe the shared memory we wish to pass about. */

#define TEXT_SZ 2048

struct shared_use_st {
    int written_by_you;
    char some_text[TEXT_SZ];
};

生产者

root@ubuntu:~/test/SourceCode/ch14# cat shm2.c 
/* The second program is the producer and allows us to enter data for consumers.
 It's very similar to shm1.c and looks like this. */

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include <sys/shm.h>

#include "shm_com.h"

int main()
{
    int running = 1;
    void *shared_memory = (void *)0;
    struct shared_use_st *shared_stuff;
    char buffer[BUFSIZ];
    int shmid;

    shmid = shmget((key_t)1234, sizeof(struct shared_use_st), 0666 | IPC_CREAT);

    if (shmid == -1) {
        fprintf(stderr, "shmget failed\n");
        exit(EXIT_FAILURE);
    }

    shared_memory = shmat(shmid, (void *)0, 0);
    if (shared_memory == (void *)-1) {
        fprintf(stderr, "shmat failed\n");
        exit(EXIT_FAILURE);
    }

    printf("Memory attached at %X\n", (int)shared_memory);

    shared_stuff = (struct shared_use_st *)shared_memory;
    while(running) {
        while(shared_stuff->written_by_you == 1) {
            sleep(1);            
            printf("waiting for client...\n");
        }
        printf("Enter some text: ");
        fgets(buffer, BUFSIZ, stdin);
        
        strncpy(shared_stuff->some_text, buffer, TEXT_SZ);
        shared_stuff->written_by_you = 1;

        if (strncmp(buffer, "end", 3) == 0) {
                running = 0;
        }
    }

    if (shmdt(shared_memory) == -1) {
        fprintf(stderr, "shmdt failed\n");
        exit(EXIT_FAILURE);
    }
    exit(EXIT_SUCCESS);
}
root@ubuntu:~/test/SourceCode/ch14# 

结果为


说明：使用一个结构，标识written_by_you
当共享内存写入时就设置这个标志，，这个标志被设置时，程序就从共享内存中读取文本