c - pthread_join和void **-错误理解

Question

我写了一个简单的任务，如下所示。它打印一个字符串，增加全局变量glob，然后通过pthread_exit将其值返回给pthread_join。

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

int glob = 0;

void *task()
{
    printf("I am a simple thread.\n");
    glob++;
    pthread_exit((void*)&glob);
}

int main()
{
pthread_t   tid;
int         create = 1;
void        **ppvglob;


    create = pthread_create(&tid, NULL, task, NULL);
    if (create != 0)    exit(EXIT_FAILURE);
    
    pthread_join(tid, ppvglob);
    
    int **ppv = (int**)ppvglob;
    printf("Variabile globale restituita alla terminazione del thread: %d\n", **ppv);                   
    
    return(0);
}

编译器给我错误:

main.c: In function ‘main’:
main.c:29:2: warning: ‘ppvglob’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   29 |  pthread_join(tid, ppvglob);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~

你能告诉我原因吗？

Answer 1

进行时:

pthread_join(tid, ppvglob);

因为您从未初始化过ppvglob，所以编译器通常会提出抗议，但实际上您必须替换:

void        **ppvglob;
....
pthread_join(tid, ppvglob);

通过:

void        *pvglob;
....
pthread_join(tid, &pvglob);

然后，当然:

int **ppv = (int**)ppvglob;
printf("Variabile globale restituita alla terminazione del thread: %d\n", **ppv);                   

通过:

int *pv = (int*)pvglob;
printf("Variabile globale restituita alla terminazione del thread: %d\n", *pv);

因此具有:

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

int glob = 0;

void *task()
{
    printf("I am a simple thread.\n");
    glob++;
    pthread_exit((void*)&glob);
}

int main()
{
  pthread_t   tid;
  int         create = 1;
  void        *pvglob;


  create = pthread_create(&tid, NULL, task, NULL);
  if (create != 0)    exit(EXIT_FAILURE);
    
  pthread_join(tid, &pvglob);
    
  int *pv = (int*)pvglob;
  printf("Variabile globale restituita alla terminazione del thread: %d\n", *pv);                   
    
  return(0);
}

编译与执行:

% gcc -Wall c.c -lpthread
% ./a.out
I am a simple thread.
Variabile globale restituita alla terminazione del thread: 1

打印1，因为这是glob的值