Question

I am learning pthread, the following is my simple code:

  1 #include <pthread.h>
  2 #include <cstdlib>
  3 #include <iostream>
  4 #include <string>
  5 #include <sstream>
  6
  7 using namespace std;
  8
  9 struct ThreadParam
 10 {
 11     int thread_id;
 12     string message;
 13 };
 14
 15 string int2string(int i)
 16 {
 17     stringstream s;
 18     s << i;
 19     return s.str();
 20 }
 21
 22 void * Hello(void * param)
 23 {
 24     ThreadParam * threadParam = (ThreadParam*)param;
 25
 26     int tid = threadParam->thread_id;
 27     string msg = threadParam->message;
 28     string output = msg + " In thread " + int2string(tid);
 29
 30     cout << output << endl;
 31     pthread_exit(NULL);
 32 }
 33
 34 int main()
 35 {
 36     const int count = 30;
 37     ThreadParam params[count];
 38
 39     pthread_t threads[count];
 40     int rc;
 41     for(int t=0; t < count; t++)
 42     {
 43         cout << "In main: creating thread " << t << endl;
 44
 45         params[t].thread_id = t;
 46         params[t].message = "Hello " + int2string(t) + "!";
 47
 48         rc = pthread_create(&threads[t], NULL, Hello, (void*)&params[t]);
 49
 50         if(rc)
 51         {
 52             cout << "Error! Return code is " << rc << endl << flush;
 53             exit(-1);
 54         }
 55     }
 56     pthread_exit(NULL);
 57 }

I got segmentation fault after creating the last thread. The output is:

In main: creating thread 0
In main: creating thread 1
In main: creating thread 2
In main: creating thread 3
In main: creating thread 4
In main: creating thread 5
In main: creating thread 6
In main: creating thread 7
In main: creating thread 8
In main: creating thread 9
In main: creating thread 10
In main: creating thread 11
Hello 0! In thread 0
Hello 1! In thread 1
Hello 2! In thread 2
Hello 3! In thread 3
Hello 4! In thread 4
In main: creating thread 12
In main: creating thread 13
In main: creating thread 14
In main: creating thread 15
In main: creating thread 16
In main: creating thread 17
In main: creating thread 18
In main: creating thread 19
In main: creating thread 20
In main: creating thread 21
In main: creating thread 22
In main: creating thread 23
In main: creating thread 24
In main: creating thread 25
In main: creating thread 26
In main: creating thread 27
In main: creating thread 28
In main: creating thread 29
Segmentation fault

I cannot figure out why, any clue?

Answer 1

Looking at your code I can tell that after you create the last thread you destroy the main one (the “creator” thread). This leads to the stack of that thread to be freed. But because you use pointers to the main thread stack memory (ThreadParam params[count]; to be precise; you pass pointers to it to pthread_create function) you then try to read the invalid data in other threads. The memory pages allocated for the main thread stack were marked as “unused” or “unallocated” after the destruction of the main thread so you get the segfault while trying to access them.

UPD: As others said, you should use pthread_join function to wait for all threads to finish and then destroy the main thread.