c++ - 即使设置为 PTHREAD_PROCESS_SHARED，Pthread 条件变量也不会发出信号

Question

我在跨文件使用条件变量和互斥锁时遇到共享内存问题。

我的研究让我来到这里 Share condition variable & mutex between processes: does mutex have to locked before?如果您运行两个完全独立的可执行文件，OP 发布的解决方案将不起作用。我尝试了他的方法来解决我自己的问题，但两个单独的进程不会相互发出信号。因此，为了确认 OP 代码确实有效，如下所示，我复制了他的代码并在中途添加了#define，以便您可以编译并作为父亲启动，更改定义并作为儿子启动。如果你像 OP 那样用 fork 运行代码，它就可以工作。如果你作为两个单独的可执行文件运行，它就不起作用......有人有什么想法吗？

背景问题这始于我之前的问题 POSIX Shared Memory Sync Across Processes C++/C++11

测试代码

#include <QCoreApplication>

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

#define OKTOWRITE "/condwrite"
#define MESSAGE "/msg"
#define MUTEX "/mutex_lock"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    pthread_cond_t* condition;
    pthread_mutex_t* mutex;
    char* message;
    int des_cond, des_msg, des_mutex;
    int mode = S_IRWXU | S_IRWXG;

    des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode);

    if (des_mutex < 0) {
        perror("failure on shm_open on des_mutex");
        exit(1);
    }

    if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1) {
        perror("Error on ftruncate to sizeof pthread_cond_t\n");
        exit(-1);
    }

    mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0);

    if (mutex == MAP_FAILED ) {
        perror("Error on mmap on mutex\n");
        exit(1);
    }

    des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode);

    if (des_cond < 0) {
        perror("failure on shm_open on des_cond");
        exit(1);
    }

    if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1) {
        perror("Error on ftruncate to sizeof pthread_cond_t\n");
        exit(-1);
    }

    condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0);

    if (condition == MAP_FAILED ) {
        perror("Error on mmap on condition\n");
        exit(1);
    }

//#define father
#ifdef father


    /* HERE WE GO */
    /**************************************/

        /* set mutex shared between processes */
    pthread_mutexattr_t mutexAttr;
    pthread_mutexattr_init(&mutexAttr);
    pthread_mutexattr_setpshared(&mutexAttr, PTHREAD_PROCESS_SHARED);
    pthread_mutex_init(mutex, &mutexAttr);

    /* set condition shared between processes */
    pthread_condattr_t condAttr;
    pthread_condattr_init(&condAttr);
    pthread_condattr_setpshared(&condAttr, PTHREAD_PROCESS_SHARED);
    pthread_cond_init(condition, &condAttr);

    /*************************************/


    printf("father waits on condition\n");

     pthread_mutex_lock(mutex);
     pthread_cond_wait(condition, mutex);
     pthread_mutex_unlock(mutex);

     printf("Signaled by son process, wake up!!!!!!!!\n");

    pthread_condattr_destroy(&condAttr);
    pthread_mutexattr_destroy(&mutexAttr);
    pthread_mutex_destroy(mutex);
    pthread_cond_destroy(condition);

    shm_unlink(OKTOWRITE);
    shm_unlink(MESSAGE);
    shm_unlink(MUTEX);
#else

//    if (!fork()) {

//        sleep(3);

        pthread_mutex_lock(mutex);
        pthread_cond_signal(condition);
        printf("son signaled\n");
        pthread_mutex_unlock(mutex);
        exit(0);
//    }

//    else {

#endif

//    }

    exit(0);

    return a.exec();
}

Answer 1

您在子进程中不必要地截断了互斥量和条件变量的共享内存。由于这部分曾经在原始代码中发生在 fork() 之前，所以您只为这对夫妇做了一次。但是在两个单独的进程中，您分别在父进程中使用跨进程 pthread 互斥锁和条件变量初始化共享内存，然后在子进程中使用 O_TRUNC 销毁 pthreads 放入其中的所有内容。正确代码如下:

父亲.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

#define OKTOWRITE "/condwrite"
#define MESSAGE "/msg"
#define MUTEX "/mutex_lock"

int main(int argc, char *argv[])
{
    pthread_cond_t* condition;
    pthread_mutex_t* mutex;
    char* message;
    int des_cond, des_msg, des_mutex;
    int mode = S_IRWXU | S_IRWXG;

    des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR | O_TRUNC, mode);

    if (des_mutex < 0) {
        perror("failure on shm_open on des_mutex");
        exit(1);
    }

    if (ftruncate(des_mutex, sizeof(pthread_mutex_t)) == -1) {
        perror("Error on ftruncate to sizeof pthread_cond_t\n");
        exit(-1);
    }

    mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0);

    if (mutex == MAP_FAILED ) {
        perror("Error on mmap on mutex\n");
        exit(1);
    }

    des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR | O_TRUNC, mode);

    if (des_cond < 0) {
        perror("failure on shm_open on des_cond");
        exit(1);
    }

    if (ftruncate(des_cond, sizeof(pthread_cond_t)) == -1) {
        perror("Error on ftruncate to sizeof pthread_cond_t\n");
        exit(-1);
    }

    condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0);

    if (condition == MAP_FAILED ) {
        perror("Error on mmap on condition\n");
        exit(1);
    }

    /* HERE WE GO */
    /**************************************/

        /* set mutex shared between processes */
    pthread_mutexattr_t mutexAttr;
    pthread_mutexattr_init(&mutexAttr);
    pthread_mutexattr_setpshared(&mutexAttr, PTHREAD_PROCESS_SHARED);
    pthread_mutex_init(mutex, &mutexAttr);

    /* set condition shared between processes */
    pthread_condattr_t condAttr;
    pthread_condattr_init(&condAttr);
    pthread_condattr_setpshared(&condAttr, PTHREAD_PROCESS_SHARED);
    pthread_cond_init(condition, &condAttr);

    /*************************************/


    printf("father waits on condition\n");

     pthread_mutex_lock(mutex);
     pthread_cond_wait(condition, mutex);
     pthread_mutex_unlock(mutex);

     printf("Signaled by son process, wake up!!!!!!!!\n");

    pthread_condattr_destroy(&condAttr);
    pthread_mutexattr_destroy(&mutexAttr);
    pthread_mutex_destroy(mutex);
    pthread_cond_destroy(condition);

    shm_unlink(OKTOWRITE);
    shm_unlink(MESSAGE);
    shm_unlink(MUTEX);

    exit(0);
}

儿子.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>

#define OKTOWRITE "/condwrite"
#define MESSAGE "/msg"
#define MUTEX "/mutex_lock"

int main(int argc, char *argv[])
{
    pthread_cond_t* condition;
    pthread_mutex_t* mutex;
    char* message;
    int des_cond, des_msg, des_mutex;
    int mode = S_IRWXU | S_IRWXG;

    des_mutex = shm_open(MUTEX, O_CREAT | O_RDWR, mode);

    if (des_mutex < 0) {
        perror("failure on shm_open on des_mutex");
        exit(1);
    }

    mutex = (pthread_mutex_t*) mmap(NULL, sizeof(pthread_mutex_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_mutex, 0);

    if (mutex == MAP_FAILED ) {
        perror("Error on mmap on mutex\n");
        exit(1);
    }

    des_cond = shm_open(OKTOWRITE, O_CREAT | O_RDWR, mode);

    if (des_cond < 0) {
        perror("failure on shm_open on des_cond");
        exit(1);
    }

    condition = (pthread_cond_t*) mmap(NULL, sizeof(pthread_cond_t),
            PROT_READ | PROT_WRITE, MAP_SHARED, des_cond, 0);

    if (condition == MAP_FAILED ) {
        perror("Error on mmap on condition\n");
        exit(1);
    }

    pthread_mutex_lock(mutex);
    pthread_cond_signal(condition);
    printf("son signaled\n");
    pthread_mutex_unlock(mutex);
    exit(0);
}

测试:

1st terminal: $ ./father
father waits on condition

2nd terminal: $ ./son
son signaled

1st terminal: Signaled by son process, wake up!!!!!!!!
$