多线程程序中的分段错误

Question

I'm trying to multiply two matrices using multithreading. But the code is giving segmentation fault. I am passing the row number and column number using a structure. The matrices a and b are made global. This is not entirely correct way to do it, but I'm just trying to understand how multithreading stuff works.

#include <pthread.h>    
#include <unistd.h>
#include <iostream>
using namespace std;
int a[3][3]={{1,2,3},{4,5,6},{7,8,9}};
int b[3][2]={{1,2},{3,4},{5,6}};
int c[3][2];
int k =3;
struct thread_data{
int m;
int n;
};
void* do_loop(void* threadarg)
{
int p,q;
struct thread_data *my_data;

my_data = (struct thread_data *) threadarg;
int i=my_data->m;
int j=my_data->n;
c[i][j]=0;

 for(q=0;q<k;q++)
 {
     c[i][j]=c[i][j]+a[i][q]*b[q][j];
 }

pthread_exit(NULL);
}
int main(int argc, char* argv[])
{
    int i,j,k;
struct thread_data td[6];

int        thr_id;       
pthread_t  p_thread[6];       
int count=0;
for(i=0;i<3;i++)
 for(j=0;j<2;j++)
 {
     td[count].m=i;
     td[count].n=j;

thr_id = pthread_create(&p_thread[count], NULL, do_loop, (void*)&td[count]);
//  pthread_join(p_thread[count],NULL);
count++;
 }
return 0;
}

How can I fix the segmentation fault?

Answer 1

First thing, you need to wait for all the threads to finish (in main ):

for (i = 0; i < count; ++i) {
    pthread_join(p_thread[i],NULL);
}

Failure to do so will crash your app as the thread continue to work why the application is being destroyed. You need to call pthread_join after you create all the threads.

If you create a thread and immediately call pthread_join you execution is serial as one thread is active at any given time.

Explanation:

"join" means: "wait for thread to finish execution". A thread finishes execution when either it returns from it's entry point function (function passed to pthread_create ) or it calls pthread_exit .