試圖在C中使用線程順序打印數字

Question

我想了解有關線程和鎖定，並編寫了一個程序，以使用兩個線程分別打印偶數和奇數的序列來打印數字。但似乎進入了僵局。有人能告訴我我做錯了什麼嗎？

/* 
    threads printing even odd number without shared variable 
*/ 
#include <stdio.h> 
#include <pthread.h> 

pthread_mutex_t mux = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t even = PTHREAD_COND_INITIALIZER; 
pthread_cond_t odd = PTHREAD_COND_INITIALIZER; 

void print_even() 
{ 
    int i = 0; 
    do { 
     pthread_mutex_lock(&mux); 
     pthread_cond_wait(&odd, &mux); 
     i+=2; 
     printf("%d ", i); 
     pthread_cond_signal(&even); 
     pthread_mutex_unlock(&mux); 
    } while (i<100); 
} 

void print_odd() 
{ 
    int i = 1; 
    do { 
     pthread_mutex_lock(&mux); 
     pthread_cond_wait(&even, &mux); 
     i+=2; 
     printf("%d ", i); 
     pthread_cond_signal(&odd); 
     pthread_mutex_unlock(&mux); 
    } while (i<100); 
} 

int main() 
{ 
    pthread_t podd, peven; 
    pthread_mutex_init(&mux, NULL); 
    printf("The values are:\n"); 
    pthread_create(&podd, NULL, (void *)&print_odd, NULL); 
    pthread_create(&peven, NULL, (void *)&print_even, NULL); 
    pthread_mutex_lock(&mux); 
    pthread_cond_signal(&even); 
    pthread_mutex_unlock(&mux); 
    pthread_join(podd, NULL); 
    pthread_join(peven, NULL); 
    printf("\ndone\n"); 
    return 0; 
} 

Answer 1

在使用條件變量的方式中存在缺陷。對你而言，他們自己似乎是溝通的渠道，需要完成某些事情。相反，使用它們作爲指標來檢查當前狀態是否需要做任何事情。

僞代碼：

mutex m 
condition_var odd 
condition_var even 
integer i 

function handle odd numbers: 
    lock(m) 
    while true: 
     if i is odd: 
     print i 
     increment i 
     signal even 
     wait odd 

function handle even numbers: 
    lock(m) 
    while true: 
     if i is even: 
     print i 
     increment i 
     signal odd 
     wait even 

main: 
    i = 0 
    start thread to handle odd numbers 
    start thread to handle even numbers 

正如所看到的，等待之前，該線程檢查該共享計數器是奇數還是偶數，然後相應地動作。但是，它不需要該事件，特別是在啓動main（）中的線程和發信號事件之間沒有競爭條件。

聲明：這是如何使用條件變量，但它絕不是好的MT編程！確保你閱讀並理解給你的問題的評論！

Answer 2

作爲user3386109由主程序發送的信號提及@是太快（前print_odd線程準備好了），因此丟失導致被僵持兩個線程。將編輯程序並儘快發佈代碼。

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

pthread_mutex_t even_mux = PTHREAD_MUTEX_INITIALIZER; 
pthread_mutex_t odd_mux = PTHREAD_MUTEX_INITIALIZER; 
pthread_mutex_t main_mux = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t even = PTHREAD_COND_INITIALIZER; 
pthread_cond_t odd = PTHREAD_COND_INITIALIZER; 
pthread_cond_t ready = PTHREAD_COND_INITIALIZER; /* to signal main that the 
                threads are created */ 
void print_even() 
{ 
    pthread_mutex_lock (&even_mux); 

    pthread_mutex_lock (&main_mux); 
    pthread_cond_signal (&ready); 
    pthread_mutex_unlock (&main_mux); 

    int i = 0; 
    do { 
     pthread_cond_wait (&even, &even_mux); 
     i += 2; 
     printf ("%d ", i); 
     pthread_mutex_lock (&odd_mux); 
     pthread_cond_signal (&odd); 
     pthread_mutex_unlock (&odd_mux); 
    } while (i < 100); 

    pthread_mutex_unlock (&even_mux); 
} 

void print_odd() 
{ 
    pthread_mutex_lock (&odd_mux); 

    pthread_mutex_lock (&main_mux); 
    pthread_cond_signal (&ready); 
    pthread_mutex_unlock (&main_mux); 

    int i = 1; 
    do { 
     pthread_cond_wait (&odd, &odd_mux); 
     i += 2; 
     printf ("%d ", i); 
     pthread_mutex_lock (&even_mux); 
     pthread_cond_signal (&even); 
     pthread_mutex_unlock (&even_mux); 
    } while (i < 100); 

    pthread_mutex_unlock (&odd_mux); 
} 

int main (void) 
{ 
    pthread_t podd, peven; 
    pthread_mutex_init (&odd_mux, NULL); 
    pthread_mutex_init (&even_mux, NULL); 
    pthread_mutex_init (&main_mux, NULL); 

    printf ("The values are:\n"); 

    pthread_mutex_lock (&main_mux); 
    if (pthread_create (&podd, NULL, (void *) &print_odd, NULL) != 0) { 
     exit (1); 
    } 
    pthread_cond_wait (&ready, &main_mux); 

    if (pthread_create (&peven, NULL, (void *) &print_even, NULL) != 0) { 
     exit (1); 
    } 
    pthread_cond_wait (&ready, &main_mux); 

    pthread_mutex_unlock (&main_mux); 
    pthread_mutex_lock (&even_mux); 
    pthread_cond_signal (&even); 
    pthread_mutex_unlock (&even_mux); 
    pthread_join (podd, NULL); 
    pthread_join (peven, NULL); 
    printf ("\ndone\n"); 

    return 0; 
}