C++ Linux下使用共享内存

• 在Linux下可以使用System V共享内存段实现共享内存，一共有4个API:
  • 创建共享内存段或使用已经创建的共享内存段-shmget()
  • 将进程附加到已经创建的共享内存段-shmat()
  • 从已连接的共享内存段中分离进程-shmdt()
  • 共享内存段上的控制操作-shmctl()
• 使用System V共享内存段需要引入头文件:

#include <sys/shm.h>

• 多读者和一个写者的样例

#include <sys/shm.h>
#include <unistd.h>
#include <error.h>
#include <iostream>
#include <string>
using namespace std;

#define BUF_SIZE 1024
#define SHM_KEY 114514

void reader(int seq) {
    cout << "reader " << seq << endl;
    int shm_id = shmget(SHM_KEY, BUF_SIZE, IPC_CREAT);
    if (shm_id == -1) {
        perror("Fail to get shared memory id");
        return;
    }

    // Attach to the segment to get a pointer to it.
    char* shm_buf = static_cast<char*>(shmat(shm_id, NULL, 0));
    if (shm_buf == nullptr) {
        perror("Fail to attach shared memory");
        return;
    }

    /* Transfer blocks of data from shared memory to stdout*/
    while (1) {
        cout << "buf[" << seq << "] = " << shm_buf << endl;
        if (string(shm_buf) == to_string(seq)) {
            break;
        }
        sleep(3);
    }
    cout << "Detaching shared memory" << endl;
    if (shmdt(shm_buf) == -1) {
        perror("Fail to detaching shared memory");
        return;
    }
}

void writer() {
    int shm_id = shmget(SHM_KEY, BUF_SIZE, IPC_CREAT);
    if (shm_id == -1) {
        perror("Fail to get shared memory id");
        return;
    }

    // Attach to the segment to get a pointer to it.
    char* shm_buf = static_cast<char*>(shmat(shm_id, NULL, 0));
    if (shm_buf == nullptr) {
        perror("Fail to attach shared memory");
        return;
    }
    /* Transfer blocks of data from shared memory to stdout*/
    while (1) {
        cin >> shm_buf;
    }
    cout << "Detaching shared memory" << endl;
    if (shmdt(shm_buf) == -1) {
        perror("Fail to detaching shared memory");
        return;
    }
}

int main(int argc, char* argv[]) {
    if (argc < 2) {
        return 0;
    }
    srand(::time(nullptr));
    cout << argv[1] << endl;
    string argv1 = argv[1];
    if (argv1 == "reader") {
        reader(rand() % 1000);
    } else {
        writer();
    }
    return 0;
}

• 将文件命名为shm_test.cpp，编译后运行

./shm_test reader
./shm_test writer

• 可以看到读者能够读取到写者写入的数据
• 如果报错显示
  • Fail to get shared memory id: Permission denied
• 那么执行

sudo ./shm_test reader
sudo ./shm_test writer