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Bacardi34

passing data to thread functions?

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Hi, im having trouble trying to figure out how to pass data to a function that is run by a thread. I figured i could pass in an array with all the data in it, but the problem is not all the data is of a uniform type, there are ints, chars, and strings. So i tried to create an array of void pointers that point to the data and pass the void pointer to the function, but im getting seg faults when i try and access the data. can anyone point me in the right direction? here is my code:
#include <stdio.h>
#include <pthread.h>

void * work( void * a )
{
  
  int i;
  for( i = 0; i < 11; i++ )
    {
      fprintf( stdout, "%d %d\n", a[1], i );
      usleep( 1 );
    }

  pthread_exit(0);
  return;
}


int main()
{
  pthread_t t1, t2;
  char string[5] = { 'h', 'e', 'l', 'l', 'o' };
  
  void * b[2];
  int x = 5;
  b[0] = string;
  b[1] = &x;

  pthread_create(&t1, 0, work, (void *)&b);
  pthread_create(&t2, 0, work, (void *)&b);

  printf( "HI\n" );
  pthread_join(t1, 0);
  pthread_join(t2, 0);
  printf( "END\n" );
  return 0;
}

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Create a struct to hold your data and pass a pointer to the struct as a parameter.


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


struct myData
{
char *string;
int x;
};

void * work( void * a )
{
myData *threadData = (myData *)a;
int i;
for( i = 0; i < 11; i++ )
{
fprintf( stdout, "%d %d\n", threadData->x,threadData->_string );
usleep( 1 );
}

pthread_exit(0);
return;
}


int main()
{
pthread_t t1, t2;

myData *threadData = new myData;
threadData->string = "hello";
threadData->x = 2;

pthread_create(&t1, 0, work, (void *)threadData);
pthread_create(&t2, 0, work, (void *)threadData);

printf( "HI\n" );
pthread_join(t1, 0);
pthread_join(t2, 0);
printf( "END\n" );
return 0;
}
[/soure]

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In the original code, methinks the problem is that you were looping over 11 array elements (0-10 inclusive), but only passing in an array of size 2... ^^

In the general case, though, passing a struct is a good idea, yeah.

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Actually, the problem with the original code is pointer confusion(which is why it is best to use a struct, it is so much cleaner)

An array of pointers is passed, but the function accepts a void* so you would have to use casting to access the actual elements. so,

fprintf( stdout, "%d %d\n", a[1], i );

becomes:

fprintf( stdout, "%d %d\n", *(((int**)a)[1]), i );

this will cast the a to a int** or an array of int pointers.

But anyways, use the struct. :)

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