C-尝试将字符传递到pthread_create，发生分段错误

Question

好的，所以我有4个线程创建（我创建了4个线程，每个线程都管理一个基本方向，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); 

，我有我的threadcode功能

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，在创建线程，而是通过创造，代码赛格故障中途。我不确定它在哪里发生，但我很确定它在threadcode函数中的某处，因为线程将开始创建，但会失败。

Answer 1

你正在做的严重错误与你传递给pthread_create()什么：

1. 它需要一个void *。你正试图通过它int。这是实现定义的行为，并且在某些情况下可能可能明智地工作，但最好避免它。传递一个实际的指针。

2. 如果做传递一个int，那么你至少应该将其转换回一个int在你的线程功能。你实际上试图转换成一个char *，它甚至不是接近你传递的。然后，您尝试取消引用虚假指针来获取角色，这就是为什么您要进行分段转换。

下面是你应该做的一个例子。请注意，您不应该在多线程中调用printf()或fprintf()之类的函数，而不进行某种同步（或者对于这个问题perror()，但是对于一个简单的示例，它将立即退出，我将抓住机会）。你不显示实现，但有一个公平的机会，你调用的其他一些函数应该被类似的保护。

#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$