passing struct to pthread as an argument - c

Question

Ok I am trying to pass pair of numbers through struct to pthread_create function in pthread. But the numbers i am passing and numbers i am getting when the function is called are different and random
Here is the struct
struct Pairs {
long i,j;
};
And inside main
void main()
{
long thread_cmp_count = (long)n*(n-1)/2;
long t,index = 0;
struct Pairs *pair;
pair = malloc(sizeof(struct Pairs));
cmp_thread = malloc(thread_cmp_count*sizeof(pthread_t));
for(thread = 0;(thread < thread_cmp_count); thread++){
for(t = thread+1; t < n; t++){
(*pair).i = thread;
(*pair).j = t;
pthread_create(&cmp_thread[index++], NULL, Compare, (void*) pair);
}
}
for(thread= 0;(thread<thread_cmp_count); thread++){
pthread_join(cmp_thread[thread], NULL);
}
free(cmp_thread);
}
And function Compare
void* Compare(void* pair){
struct Pairs *my_pair = (struct Pairs*)pair;
printf("\nThread %ld, %ld", (*my_pair).i, (*my_pair).j);
return NULL;
}
Number I am getting and it is also random.
Thread 0,2
Thread 1,2
Thread 2,3
Thread 2,3
Thread 2,3
Thread 2,3
am i passing the struct wrong ?

Answer 1

That is because you are passing the same pointer to all pthreads.
When you invoke pthread_create(..., (void*) pair) you are passing the pointer to the new thread, but in the next iteration you are overwriting that memory (potentially before the new thread has extracted those values).
long thread_cmp_count = (long)n*(n-1)/2;
long t,index = 0;
struct Pairs *pair;
cmp_thread = malloc(thread_cmp_count*sizeof(pthread_t));
for(thread = 0;(thread < thread_cmp_count); thread++){
for(t = thread+1; t < n; t++){
// allocate a separate pair for each thread
pair = malloc(sizeof(struct Pairs));
(*pair).i = thread;
(*pair).j = t;
pthread_create(&cmp_thread[index++], NULL, Compare, (void*) pair);
}
}
for(thread= 0;(thread<thread_cmp_count); thread++){
pthread_join(cmp_thread[thread], NULL);
}
free(cmp_thread);
.
void* Compare(void* pair){
struct Pairs *my_pair = (struct Pairs*)pair;
printf("\nThread %ld, %ld", (*my_pair).i, (*my_pair).j);
// free that memory after it has been used
free (pair);
return NULL;
}

Answer 2

Problem solved. Problem was with overlap. using pointer as an array of type struct Pairs it is solved
Here is the correct code
long thread_cmp_count = (long)n*(n-1)/2;
long t,index = 0;
Pair * pair;
pair = malloc(thread_cmp_count*sizeof(Pair));
free(thread_handles);
thread_handles = malloc(thread_cmp_count*sizeof(pthread_t));
for(thread = 0;(thread < n-1); thread++){
for(t = thread+1; t < n; t++){
(pair+index)->i = thread;
(pair+index)->j = t;
pthread_create(&thread_handles[index], NULL, Compare, (void*) (pair+index));
index++;
}
}
for(thread= 0;(thread<thread_cmp_count); thread++){
pthread_join(thread_handles[thread], NULL);
}
free(thread_handles);
And the function Compare
void* Compare(void* pair){
long t,i,j;
Pair *my_pair = (Pair*)pair;
i = my_pair->i;
j = my_pair->j;
printf("\n..................................................................");
if((x_array[i] < x_array[j])&&(x_array[i] != x_array[j])){
w_array[i] = 0;
printf(
"\nThread T(%ld,%ld)"
" compares x[%ld] = %ld and x[%ld] = %ld,"
" and writes 0 to w[%ld]", i, j,
i,x_array[i],
j,x_array[j],
i);
}
else if((x_array[i] > x_array[j])&&(x_array[i] != x_array[j])){
w_array[j] = 0;
printf(
"\nThread T(%ld,%ld)"
" compares x[%ld] = %ld and x[%ld] = %ld,"
" and writes 0 to w[%ld]", i, j,
i,x_array[i],
j,x_array[j],
j);
}
else
return NULL;
return NULL;
}