c++ - 这个简单的 pthread 代码的奇怪行为

Question

如果我编译并运行下面的代码

  1 #include <iostream>
  2 #include <pthread.h>
  3 #include <cstdio>
  4 #include <cstdlib>
  5
  6 #define NTHREADS 4
  7 #define N 100
  8 #define MEGEXTRA 1000000
  9
 10 using namespace std;
 11
 12 pthread_attr_t attr;
 13
 14 void *doWork (void *threadid) {
 15     double A[N][N];
 16     int tid = *(reinterpret_cast<int *>(threadid));
 17     size_t myStackSize;
 18     pthread_attr_getstacksize (&attr, &myStackSize);
 19     cout << "Thread " << tid << ": stack size = " << myStackSize << " bytes" << endl;
 20
 21     for (int i = 0; i < N; i++) {
 22         for (int j = 0; j < N; j++) {
 23             A[i][j] = ((i * j) / 3.452) + (N - 1);
 24         }
 25     }
 26
 27     pthread_exit (NULL);
 28 }
 29
 30 int main () {
 31     pthread_t threads[NTHREADS];
 32     size_t stackSize;
 33
 34     pthread_attr_init (&attr);
 35     pthread_attr_getstacksize (&attr, &stackSize);
 36
 37     cout << "Default stack size = " << static_cast<long>(stackSize) << endl;
 38
 39     stackSize = sizeof(double) * N * N + MEGEXTRA;
 40     cout << "Amount of stack needed per thread = " << static_cast<long>(stackSize) << endl;
 41
 42     pthread_attr_setstacksize (&attr, stackSize);
 43     cout << "Creating threads with stack size = " << static_cast<long>(stackSize) << endl;
 44
 45     int i[NTHREADS];
 46     for (int j = 0; j < NTHREADS; j++) {
 47         sleep(1);
 48         i[j] = j;
 49
 50         int rc = pthread_create(&threads[j], &attr, doWork, reinterpret_cast<void *>(&i[j]));
 51         if (rc) {
 52            cout << "Error Code: " << rc << endl;
 53             exit (-1);
 54         }
 55     }
 56
 57     cout << "Created " << NTHREADS << " threads" << endl;
 58     pthread_exit(NULL);
 59 }

我得到以下输出:

Default stack size = 8388608
Amount of stack needed per thread = 1080000
Creating threads with stack size = 1080000
Thread 0: stack size = 1080000 bytes
Thread 1: stack size = 1080000 bytes
Thread 2: stack size = 1080000 bytes
Created 4 threads
Thread 3: stack size = 1080000 bytes

但是如果我注释掉 sleep(1);在第 47 行，我得到以下输出

Default stack size = 8388608
Amount of stack needed per thread = 1080000
Creating threads with stack size = 1080000
Created 4 threads
Thread 3: stack size = 1080000 bytes
Thread 2: stack size = 1080000 bytes
Thread 1: stack size = 1080000 bytes
Thread 9251904: stack size = 1080000 bytes /** ERROR should be Thread 0: stack size = 1080000 /**

谁能解释一下这是怎么回事？为什么 sleep(1) 被注释掉后我会得到不正确的输出？

这是我用来编译的代码

g++ -Wall -Wextra -O2 -ggdb -pthread 5.cpp -o 5

Answer 1

那是因为创建其他线程的主线程退出了(通过 pthread_exit 的优点 - 无论如何你都在 main 的末尾)在所有创建的线程有时间运行之前。 i 数组在所有线程读取时被销毁并包含垃圾。

您必须等待 - 即 pthread_join - 在 main 退出之前为您的子线程。

sleep 为线程的执行争取了一些时间，但仍由操作系统决定谁在何时运行。 sleep(1) 可能足够，也可能不够，或者在 postman 来投递之前没有人运行。

调用pthread_join，你就安全了。