是否可以以某种方式在 Docker 容器之间或容器与主机之间使用 POSIX 信号量？

Question

我正在尝试将多进程应用程序迁移到 Docker。不同的进程会被放置在不同的Docker容器中。

应用程序使用共享内存来交换数据并使用信号量来进行同步。我已经重新编译了 Docker，以便不使用 IPC 命名空间，并且我有效地检查了sudo ipcs -m共享内存缓冲区是否可以从不同的容器访问。

问题是信号量不起作用。我编写了这些简单的程序来检查 Docker 中 POSIX 信号量的行为：

/*  To be compiled with -lpthread */

#include <stdio.h>
#include <fcntl.h>
#include <semaphore.h>

int main(void) {

    int ret, val;
    sem_t * mySem;

    printf("[ONE] Opening the semaphore...\n");

    mySem = sem_open("sem1", O_CREAT, 0777, 0);
    if (mySem == SEM_FAILED) {
        printf("[ONE] Error on sem_open()\n");
        return -1;
    }

    ret = sem_post(mySem);

    getchar(); // Awful way to block execution of [ONE] for a while...

    printf("[ONE] Waiting for [TWO]...\n");
    ret = sem_wait(mySem);
    printf("[ONE] Wait ended\n");

    ret = sem_unlink("sem1");
    printf("[ONE] Semaphore destroyed\n");

    return 0;
}

第二个程序是：

/* To be compiled with -lpthread */

#include <stdio.h>
#include <fcntl.h>
#include <semaphore.h>

int main(void) {

    int ret;
    int val;
    sem_t * mySem;

    printf("[TWO] Opening the semaphore...\n");

    mySem = sem_open("sem1", O_CREAT, 0777, 0);
    if (mySem == SEM_FAILED) {
        printf("[TWO] Error on sem_open()\n");
        return -1;
    }

    ret = sem_getvalue(mySem, &val);
    printf("[TWO] Semaphore's value is %d\n", val);

    printf("[TWO] Waiting for [ONE]...\n");
    ret = sem_wait(mySem);
    printf("[TWO] Wait ended\n");

    printf("[ONE] Doing sem_post() on semaphore...\n");
    ret = sem_post(mySem);

    ret = sem_close(mySem);
    printf("[TWO] Semaphore closed\n");

    return 0;
}

在这两种情况下，我都省略了很多控件，例如if (ret != 0) {...}为了保持问题的可读性。

我在主机上运行第一个程序，在 Docker 容器中运行第二个程序。结果是第二个程序永远等待......

问题是：是否可以通过某种方式在 Docker 容器之间或容器与主机之间使用 POSIX 信号量？

Answer 1

我使用 Docker 容器之间的共享内存解决了这个问题，如本问题所述。

以下代码是本教程的修改版本。

文件 server.c

/* To be compiled with -lpthread */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <semaphore.h>

#define SHM_SIZE     1000

int main(void) {

    int shmid;
    key_t key;
    char *shm;

    sem_t * mySem;

    /* We'll name our shared memory segment "5678" */
    key = 5678;

    /* Create the segment.*/
    if ((shmid = shmget(key, SHM_SIZE, IPC_CREAT | IPC_EXCL | 0666)) < 0) {
        perror("shmget");
        exit(1);
    }

    /* Now we attach the segment to our data space */
    if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);
    }

    /* Create a new semaphore */
    mySem = sem_open("sem1", O_CREAT, 0777, 0);

    /* Copy the semaphore on the shared memory segment */  
    memcpy(shm, mySem, sizeof(*mySem));

    /* Do stuff ... */

    /* REMEMBER TO USE THE SHARED MEMORY SEGMENT */
    /* AND NOT THE LOCAL mySem, USE (sem_t*)shm INSTEAD! */

    /* Finally, we wait until the other process 
     * changes the first character of our memory
     * to '*', indicating that it has read what 
     * we put there.
     */
    while (*shm != '*')
        sleep(1);

    /* Mark the memory segment to be destroyed */
    shmctl(shmid, IPC_RMID, NULL);

    /* Detach of the memory segment */
    shmdt(&shm);

    sem_unlink("sem1");

    return 0;
}

文件 client.c

/* To be compiled with -lpthread */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <fcntl.h>
#include <semaphore.h>

#define SHM_SIZE     1000

int main(void) {

    int shmid;
    key_t key;
    char *shm;

    int ret, val;

    key = 5678;

    if ((shmid = shmget(key, SHM_SIZE, 0666)) < 0) {
        perror("shmget");
        exit(1);
    }

    if ((shm = shmat(shmid, NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);
    }

    /* SEMAPHORE IS IN THE SHARED MEMORY SEGMENT */
    /* USE (sem_t*)shm TO ACCESS IT */

    *shm = '*';

    shmdt(&shm);

    sem_close("sem1");

    return 0;
}

由于可读性的原因，代码示例遗漏了许多控件。

我在主机上运行服务器，在 Docker 容器内运行客户端，并检查信号量是否可以从两个进程访问。