Threads - problem

[Hydraulik2K12]

Hi,
I've run into a wall while working on a pretty basic C program that uses threads. I've already written a very similar one using basically the same method, only that the previous one used one recursive call (and worked perfectly fine), and this one requires two. I keep getting "Segmentation fault (core dumped)" message while trying to run it. What could be the matter? Because, quite frankly, I've run out of options...



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

typedef struct Newton
{
unsigned n, k;
} Newton;

int alg(Newton s)
{
int value;
Newton s1, s2;

if(s.k == 0 || s.k == s.n)
{
value = 1;
}
else
{
s1.n = s.n - 1;
s1.k = s.k - 1;

s2.n = s.n - 1;
s2.k = s.k;

value = alg(s1) + alg(s2);
}

return value;
pthread_exit((void*)value);
}

int main(void)
{
int x;
Newton data;

pthread_t thread1;

printf("(n choose k) = ?\n");
printf(" n: ");
scanf("%d", &data.n);
printf(" k: ");
scanf("%d", &data.k);

if(pthread_create(&thread1, NULL, (void*)&alg, (void*)&data) != 0)
printf("Error! (creating a thread)\n");

if(pthread_join(thread1, (void**)&x) != 0)
printf("Error! (exiting a thread)\n");

printf("(%d choose %d) = %d\n\n", data.n, data.k, x);

return(0);
}

[kbw]

My comments are in the code.

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

typedef struct Newton
{
	unsigned n, k;
} Newton;

int alg(Newton s)  // kbw: thread functions should return void*
{
	int value;
	Newton s1, s2;

	if (s.k == 0 || s.k == s.n)
	{
		value = 1;
	}
	else
	{
		s1.n = s.n - 1;
		s1.k = s.k - 1;

		s2.n = s.n - 1;
		s2.k = s.k;

		value = alg(s1) + alg(s2);
	}

	return value;  // kbw: value is interpreted as a pointer, the derefence gives you unpredictable results
	pthread_exit((void*)value);  // kbw: never runs this code
}

int main(void)
{
	int x;
	Newton data;

	pthread_t thread1;

	printf("(n choose k) = ?\n");
	printf(" n: ");
	scanf("%d", &data.n);
	printf(" k: ");
	scanf("%d", &data.k);

	// kbw: cast overrides the compiler's refusal to accept alg it stands
	if (pthread_create(&thread1, NULL, (void*)&alg, (void*)&data) != 0)
		printf("Error! (creating a thread)\n");

	if (pthread_join(thread1, (void**)&x) != 0)  // kbw: x has the wrong type
		printf("Error! (exiting a thread)\n");

	printf("(%d choose %d) = %d\n\n", data.n, data.k, x); // kbw: crash happens here, x dereferences possibly bad pointer

	return(0);
}

[Hydraulik2K12]

Hi, thanks for the help. I've changed the code according to your feedback, but it still doesn't seem to work. Now the result is always 0, no matter what numbers I put.

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

typedef struct Newton
{
	unsigned n, k;
} Newton;

void* alg(Newton s)
{
	int value;
	Newton s1, s2;

	if (s.k == 0 || s.k == s.n)
	{
		value = 1;
	}
	else
	{
		s1.n = s.n - 1;
		s1.k = s.k - 1;

		s2.n = s.n - 1;
		s2.k = s.k;

		value = (int)alg(s1) + (int)alg(s2);
	}

	pthread_exit((void*)value);
    return((void*)value);
}

int main(void)
{
	int x;
	Newton data;
	pthread_t thread1;

	printf("(n choose k) = ?\n");
	printf(" n: ");
	scanf("%d", &data.n);
	printf(" k: ");
	scanf("%d", &data.k);

	if(pthread_create(&thread1, NULL, alg, (void*)&data) != 0)
		printf("Error! (creating a thread)\n");

	if(pthread_join(thread1, (void*)x) != 0)
		printf("Error! (exiting a thread)\n");

	printf("(%d choose %d) = %d\n\n", data.n, data.k, x);

	return(0);
}