CODEWITHSUNDEEP
cmultithreadingsegmentation-faultpthreads
Arijit
Arijit

Reputation: 1516

C pthread_join() generates Segmentation Fault

The below code gives seg fault for the second case but for the first part it is working fine . but both of them are doing the same thing . Here the pthread_join() call is not generating any error , but while printing the response from the pthread_join() it generates Segmentation fault . What the first one is doing is different from the second one ? and Where the second one is actually wrong ?

Code

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

void * thread_ops(void *arg){
  printf("Thread Start!\n");
  int val=((int *)arg)[0];
  printf("value at thread -> %d\n",val);
  int * c=(int *)malloc(sizeof(int *));
  *c=val;
  pthread_exit((void*)c);
}
int main(){
  pthread_t pt,pt2;
  pthread_attr_t ptatr;
  int ar[1]={1};
  // working
  pthread_attr_init(&ptatr);
  int status;
  status=pthread_create(&pt,&ptatr,&thread_ops,ar);
  printf("Thread Stats : %d\n",status);
  int *result;
  pthread_join(pt,(void **)&result);
  printf("Result is %d\n",*result);
  // why does the next set of lines generate seg fault ??
  void **result2;
  status=pthread_create(&pt,&ptatr,&thread_ops,ar);
  pthread_join(pt,result2);
  int ** res2=(int **)result2;
  printf("Result is %d\n",**res2);
  return 0;
}

Output

Thread Stats : 0
Thread Start!
value at thread -> 1
Result is 1
Thread Start!
value at thread -> 1
Segmentation fault (core dumped)

Upvotes: 0

Views: 110

Answers (1)

alk
alk

Reputation: 70883

Instead of 

void **result2;
pthread_join(pt,result2);

use

void *pvresult;
pthread_join(pt, &pvresult);

and as you expect an int add the following:

int result = *((int*) pvresult);

and print it this way:

printf("Result is %d\n", result);

Also replace this

int * c=(int *)malloc(sizeof(int *));

by this

int *c = malloc(sizeof *c);

Upvotes: 1

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