C-嘗試將字符傳遞到pthread_create，發生分段錯誤

Question

好的，所以我有4個線程創建（我創建了4個線程，每個線程都管理一個基本方向，北，南，東或西）：

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; 
} 

出於某種原因，在創建線程，而是通過創造，代碼賽格故障中途。我不確定它在哪裏發生，但我很確定它在threadcode函數中的某處，因爲線程將開始創建，但會失敗。

Answer 1

你正在做的嚴重錯誤與你傳遞給pthread_create()什麼：

1. 它需要一個void *。你正試圖通過它int。這是實現定義的行爲，並且在某些情況下可能可能明智地工作，但最好避免它。傳遞一個實際的指針。

2. 如果做傳遞一個int，那麼你至少應該將其轉換回一個int在你的線程功能。你實際上試圖轉換成一個char *，它甚至不是接近你傳遞的。然後，您嘗試取消引用虛假指針來獲取角色，這就是爲什麼您要進行分段轉換。

下面是你應該做的一個例子。請注意，您不應該在多線程中調用printf()或fprintf()之類的函數，而不進行某種同步（或者對於這個問題perror()，但是對於一個簡單的示例，它將立即退出，我將抓住機會）。你不顯示實現，但有一個公平的機會，你調用的其他一些函數應該被類似的保護。

#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; 
} 

可能的輸出：

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$