進程間通信（IPC）:共享內存

共享內存允許兩個不相關的進程訪問同一個邏輯內存。

共享內存的定義如下：

#include<sys/shm.h>

void *shmat(shm_id,const void *shm_addr, int shmflg);//連接到一個進程的地址空間中
int shmctl(int shm_id, int cmd, struct shmid_ds *buf);//共享內存的控制函數
int shmdt(const void *shm_addr);//將共享內存從當前進程中分離
int shmget(key_t key, size_t size, int shmflg);//創建共享內存

第一個程序創建一個共享內存段，然後把寫到它裏面的數據都顯示出來。第二個程序將連接一個已有的共享內存段，並允許我們向其中輸入數據。

/* 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) {

    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);
}