C- 尝试将字符传递到 pthread_create，发生段错误

Question

好吧，我已经创建了 4 个 pthread(我正在创建 4 个线程，每个线程将管理一个基本方向，北、南、东或西):

if ((rc = pthread_create(&tidn, NULL, threadcode, (void *)'n')) != 0)
    fprintf(stderr, "thread create failed (%s)\n", strerror(rc)), exit(1);
if ((rc = pthread_create(&tids, NULL, threadcode, (void *)'s')) != 0)
    fprintf(stderr, "thread create failed (%s)\n", strerror(rc)), exit(1);
if ((rc = pthread_create(&tide, NULL, threadcode, (void *)'e')) != 0)
    fprintf(stderr, "thread create failed (%s)\n", strerror(rc)), exit(1);
if ((rc = pthread_create(&tidw, NULL, threadcode, (void *)'w')) != 0)
    fprintf(stderr, "thread create failed (%s)\n", strerror(rc)), exit(1);

我已经有了我的线程代码函数

void *threadcode(void* dir)
{
char* direction;
struct cart_t *cart = NULL;

direction = (char*)dir;

printf("casted direction %c\n", *direction);

fprintf(stderr, "thread for direction %c starts\n", *direction);
cart = q_getCart(*direction);
while (cart != NULL) {
    fprintf(stderr, "thread for direction %c gets cart %i\n",
        *direction, cart->num);
    monitor_arrive(cart);
    monitor_cross(cart);
    monitor_leave(cart);
    cart = q_getCart(*direction);
}
fprintf(stderr, "thread for direction %c exits\n", *direction);

return NULL;
}

出于某种原因，线程已创建，但在创建的中途，代码段出错。我不确定它发生在哪里，但我很确定它在 threadcode 函数中的某个地方，因为线程将开始创建但随后会失败。

Answer 1

您传递给 pthread_create() 的内容出现严重错误:

1. 它需要一个 void *。您正在尝试将 int 传递给它。这是实现定义的行为，在某些情况下可能 可以正常工作，但最好避免这种情况。传递给它一个实际的指针。

2. 如果您确实传递了一个int，那么您至少应该在您的线程函数中将它转换回一个int。您实际上尝试转换为 char *，它甚至与您传递的内容相去甚远。然后，您尝试取消引用该虚假指针以获取一个字符，这就是您出现段错误的原因。

下面是您应该做什么的示例。请注意，如果没有某种同步(或 perror())，您不应该在多线程中调用 printf() 或 fprintf() 等函数> 就此而言，但对于一个无论如何都会立即退出的简单示例，我会捕获机会)。您没有显示实现，但您调用的其他一些函数很有可能应该受到类似的保护。

#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

pthread_mutex_t io_mtx = PTHREAD_MUTEX_INITIALIZER;

void * threadcode(void * arg)
{
    const char c = *((char *) arg);

    if ( pthread_mutex_lock(&io_mtx) != 0 ) {
        perror("couldn't acquire mutex");
        exit(EXIT_FAILURE);
    }

    printf("Entered thread for '%c'\n", c);

    if ( pthread_mutex_unlock(&io_mtx) != 0 ) {
        perror("couldn't release mutex");
        exit(EXIT_FAILURE);
    }

    return NULL;
}

int main(void)
{
    pthread_t tidn, tids, tide, tidw;
    static char * dirs = "nsew";

    /*  Create threads  */

    if ( pthread_create(&tidn, NULL, threadcode, &dirs[0]) != 0 ) {
        perror("couldn't create north thread");
        return EXIT_FAILURE;
    }
    if ( pthread_create(&tids, NULL, threadcode, &dirs[1]) != 0 ) {
        perror("couldn't create south thread");
        return EXIT_FAILURE;
    }
    if ( pthread_create(&tide, NULL, threadcode, &dirs[2]) != 0 ) {
        perror("couldn't create east thread");
        return EXIT_FAILURE;
    }
    if ( pthread_create(&tidw, NULL, threadcode, &dirs[3]) != 0 ) {
        perror("couldn't create west thread");
        return EXIT_FAILURE;
    }

    /*  State that threads are created  */

    if ( pthread_mutex_lock(&io_mtx) != 0 ) {
        perror("couldn't acquire mutex");
        exit(EXIT_FAILURE);
    }

    printf("Threads created\n");

    if ( pthread_mutex_unlock(&io_mtx) != 0 ) {
        perror("couldn't release mutex");
        exit(EXIT_FAILURE);
    }

    /*  Join threads  */

    if ( pthread_join(tidn, NULL) != 0 ) {
        perror("couldn't join with north thread");
        return EXIT_FAILURE;
    }
    if ( pthread_join(tids, NULL) != 0 ) {
        perror("couldn't join with south thread");
        return EXIT_FAILURE;
    }
    if ( pthread_join(tide, NULL) != 0 ) {
        perror("couldn't join with east thread");
        return EXIT_FAILURE;
    }
    if ( pthread_join(tidw, NULL) != 0 ) {
        perror("couldn't join with west thread");
        return EXIT_FAILURE;
    }

    return 0;
}

可能的输出:

paul@horus:~/Documents/src/sandbox$ ./thread
Entered thread for 'n'
Entered thread for 's'
Entered thread for 'w'
Threads created
Entered thread for 'e'
paul@horus:~/Documents/src/sandbox$