C/Threads, return value

Question

I created a thread that should return a sqrt of integer sent to it, it works fine while returning int value, but when i want to return double or float value it returns some crazy numbers, how to change that?

Here's the code that works fine:

int* function(int* x) {

printf("My argument: %d \n", *x);
int *y = malloc(sizeof(int));
*y=sqrt(*x);
return y;
}

int main(int argc, char* argv[])
{
pthread_t thread;
int arg = 123;
int *retVal;


pthread_create(&thread, NULL, (void * ( * ) (void *))function, &arg);

pthread_join(thread, (void **) &retVal);
printf("Sqrt of our argument: %d\n", * retVal);
free(retVal);
return 0;

}

But when I change it into:

double* function(int* x) {


double *y = malloc(sizeof(double));
*y=sqrt(*x);
printf("My argument: %d \n", *x);
return y;
}

int main(int argc, char* argv[])
{
pthread_t thread;
int arg = 123;
double *retVal;


pthread_create(&thread, NULL, (void * ( * ) (void *))function, &arg);

pthread_join(thread, (void **) &retVal);
printf("Sqrt of our argument: %d\n", * retVal);
free(retVal);
return 0;
}

It returns 1076244058

consider `double *y = malloc(sizeof(*y));` with `double *retVal;` ...? — Myst, May 03 '17 at 10:13
where's the code that doesn't work? we can't tell you what is wrong if we can't see the problems — Chris Turner, May 03 '17 at 10:14
It's much easier to make the return value a parameter, i.e. use the thread argument (`arg`) to hold the argument but let the thread overwrite it with the result. — unwind, May 03 '17 at 10:14
Casting the function is UB. Don't do that. Use a wrapper function. — David Schwartz, May 03 '17 at 10:29
Your title doesn't describe the question to anybody. Please improve it. — Lightness Races in Orbit, May 03 '17 at 11:32

score 3 · Answer 1 · edited May 23 '17 at 12:09

Your change is wrong

printf("Sqrt of our argument: %d\n", * retVal);

must be

printf("Sqrt of our argument: %f\n", * retVal);

I guess your compiler is telling you something like

warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘double *’ [-Wformat=]

BTW your implementation invokes undefined behavior casting function: take a look at this SO answer

As already suggested you can use arg to pass the value back to main instead of return it from task function.

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

void* function(void* x)
{
    double *y = x;

    *y = sqrt(*y);

    return x;
}

int main(void)
{
    pthread_t thread;
    double arg = 123;
    void *retVal = NULL;

    pthread_create(&thread, NULL, function, &arg);

    pthread_join(thread, &retVal);
    printf("Sqrt of our argument using arg   : %f\n", arg);

    if (retVal != NULL)
    {
        printf("Sqrt of our argument using retVal: %f\n", *((double *)retVal));
    }

    return 0;
}

C/Threads, return value

1 Answers1