Question

有人可以指出为什么我的代码会抛出分段错误吗？我很可能在致电pthread_create()时犯了一个错误，但我没有理解错误。有人可以指出我哪里出错了吗？

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

int x = 0;
int counter =
    0;  // This is the global variable that all the threads will increment.
pthread_mutex_t lock;

void* addcounter(void* threadid) {
  pthread_mutex_lock(&lock);
  int* id = (int*)threadid + 1;
  int* y = (int*)x;
  int total = (*id) * (*y);
  int i = 0;
  for (i = 0; i < total; i++) {
    counter += 1;
  }
  pthread_mutex_unlock(&lock);
}

int main(int argc, char* argv[]) {
  int thread_count = 0;
  int loop_count = 0;
  int expected_value = -1;

  if (argc != 3) {
    printf("Usage: threads <no_of_threads> <no_of_loops>\n");
    exit(1);
  }

  thread_count = atoi(argv[1]);
  loop_count = atoi(argv[2]);
  x = loop_count;

  // Start your implementation after this comment.
  pthread_t* threads;
  threads = malloc(sizeof(pthread_t) * thread_count);
  int t;
  int flag = 0;
  for (t = 0; t < thread_count; t++) {
    int* z = malloc(sizeof(int));
    *z = t;
    flag = pthread_create(&threads[t], NULL, addcounter, (void*)z);
  }
  if (flag != 0) {
    printf("error in creating threads");
    exit(0);
  }

  for (t = 0; t < thread_count; t++) pthread_join(threads[t], NULL);
  // Do not change the code below this comment
  return 0;
}

此代码的输出为：

Segmentation fault (core dumped)

我该如何解决此错误？

Answer 1

首先，您应该使用mutex初始化PTHREAD_MUTEX_INITIALIZER变量。

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

或者您可以从主线程中调用pthread_mutex_init(&lock,0)来初始化lock。

第二件事，替换下面的陈述

 int* y = (int*) x;

带

 int* y = (int*) &x;

因为y应该有valid address，否则你以后就无法执行*y

我希望它有所帮助。

Answer 2

中的多个指针问题

void* addcounter(void* threadid) { 
  pthread_mutex_lock(&lock);
  int* id = (int*)threadid + 1;    // <== A
  int* y = (int*)x;                // <== B
  int total = (*id) * (*y);        // <== C
  int i = 0;
  for (i = 0; i < total; i++) {
    counter += 1;
  }
  pthread_mutex_unlock(&lock);
}

说明

A：可能是int id = *(int *)threadid + 1;
B：int *y = (int *)x将y指针设置为x ... int危险！可能是int y = x;（？）
C：然后修复总计int total = id + y;（不需要以下循环，counter += total;就够了

pthread分段错误（核心转储）错误

2 个答案: