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cmultithreadingpthreads
John Vn
John Vn

Reputation: 91

Thread doesn't get executed in C

I am trying to create a program in C to calculate the sum and the product via threads.It's my exercise for university.The problem I face is that when I run the program the thread doesn't seem to execute.I am stuck and I don't know how to proceed.

  #include <pthread.h>
  #include <stdio.h>
  #include <stdlib.h>
  int T1; //store the sum
  int *array; //global varriable

 void *Sum(void *N){ //function used by thread to calculate sum
     printf("I am the 1st thread i ll calculate the sum for you");
     long S=(long) N;
     int sum=0;
     int i;

     for (i=0; i<S; ++i){
       sum = sum +array[i];
     }  
     T1 = sum;
     pthread_exit(NULL);
}


int main(int argc, char *argv[])
{
  long N;
  printf("%d\n",T1);
  printf("give the size of the array please\n");
  scanf("%ld", &N);
  int *array= (int*) malloc((N)*sizeof(int)); // dynamic array
  int i;
  for (i=0; i<N; ++i)
  {
     array[i]=rand() % 301 + (-150);// creates random numbers from -150     to 150
  }
  for (i=0; i<N; ++i) {
    printf(" %d\n", array[i]);
  }

  pthread_t Th;
  pthread_create(&Th, NULL,Sum, (void*)N); //creates thread

  printf("%d\n",T1);
  return (0);
 }`

I tried changing pthread_exit(NULL); to return(0) or return(T1) but it didn't work. I changed the Sum function to:

    void *Sum(void *N){ //function used by thread to calculate sum
      printf("I am the 1st thread i ll calculate the sum for you");
      pthread_exit(NULL);
   }

It didn't work either.the output I get anyway is :

0
give the size of the array please
2
 117
 113
0

Note that I get no compile errors or warnings.

Upvotes: 3

Views: 3257

Answers (2)

P.P
P.P

Reputation: 121427

1) Your main() thread is not waiting for the thread to complete. So your thread Sum may not get executed at all. Call pthread_join():

pthread_create(&Th, NULL,Sum, &N); 
pthread_join(Th, 0); // waits for the thread "Sum"

2) You are declaring and assigning array again in main(). So the allocation is only for the array in main() since it shadows the global variable. This leads to undefined behaviour as the array you write into in Sum thread wasn't allocated any memory. So 

  int *array= (int*) malloc((N)*sizeof(int)); // dynamic array

should be

  array= malloc((N)*sizeof(int));

A void* can be converted to any other object pointer automatically. So the cast is unnecessary and potentially dangerous.

3) Integer to pointer conversion has implementation defined behaviour. So I would avoid it. Since the thread only reads N, you can pass the address of it:

pthread_create(&Th, NULL,Sum, &N); //creates thread

and in the thread function do:

  long S=*((long*) N);

Upvotes: 4

FrozenHawk
FrozenHawk

Reputation: 150

pthread_create(&Th, NULL,Sum, (void*)N); //creates thread

After this line you need to to either join or detach the thread. In this case you'll want to join the thread or your program will end right away.

pthread_join(Th, NULL);

Upvotes: 2

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