simple multithreading error in C? - c

Question

I am fairly new to the C language. I have written a piece of code which creates two thread to calculate two different results. the code actually works but the compiling is showing errors and I want to know what is actually the error. can anyone help me? I am new to C, so I guess it might be a very silly mistake...
Errors:
q3.c: In function ‘main’:
q3.c:63:2: warning: passing argument 3 of ‘pthread_create’ from incompatible pointer type [enabled by default]
In file included from q3.c:5:0:
/usr/include/pthread.h:225:12: note: expected ‘void * (*)(void *)’ but argument is of type ‘double * (*)(void *)’
q3.c:64:9: warning: passing argument 3 of ‘pthread_create’ from incompatible pointer type [enabled by default]
In file included from q3.c:5:0:
/usr/include/pthread.h:225:12: note: expected ‘void * (*)(void *)’ but argument is of type ‘double * (*)(void *)’
The code:
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
#include<stdio.h>
#include<math.h>
#define PI 3.1415
int i, x, n;
double *sin_x(void* dimension);
double *cos_x(void* dimension);
int fact(int);
struct data
{
int ang_deg;
int no_of_terms;
};
int fact(int num)
{
int f = 0;
if (num == 1)
return 1;
else
f = num * fact(num - 1);
return f;
}
int main(int argc, char argv[])
{
printf("\nEnter x and n:\t");
scanf("%d %d", &x, &n);
struct data
{
int ang_rad;
int no_of_terms;
};
struct data dimension; // automatic allocation, all fields placed on stack
dimension.ang_rad = x;
dimension.no_of_terms = n;
pthread_t thrd1, thrd2;
int thret1, thret2;
thret1 = pthread_create(&thrd1, NULL, sin_x, (void *) &dimension);
thret2 = pthread_create(&thrd2, NULL, cos_x, (void *) &dimension);
pthread_join(thrd1, NULL );
pthread_join(thrd2, NULL );
//printf("\nthret1 = %d\n", thret1);
//printf("thret2 = %d\n", thret2);
sleep(5);
printf("Parent Thread exiting...\n");
exit(1);
return 0;
}
double *sin_x(void* dimension)
{
struct data* dim = (struct data*) dimension;
int ang_deg = dim->ang_deg;
int no_of_terms = dim->no_of_terms;
//int ang_deg, int no_of_terms
int term, j;
double value = 0.0, ang_rad = 0.0;
ang_rad = (double) ang_deg * PI / 180;
for (term = 1, j = 2; term < no_of_terms * 2; term += 2, j++)
{
value += (double) pow(-1.0, j) * pow(ang_rad, term) / fact(term);
}
printf("\nSin(%d) = %f", ang_deg, value);
double *a = &value;
return a;
}
double *cos_x(void* dimension)
{
struct data* dim = (struct data*) dimension;
int ang_deg = dim->ang_deg;
int no_of_terms = dim->no_of_terms;
int term, j;
double value = 1.0, ang_rad = 0.0;
ang_rad = (double) ang_deg * PI / 180;
for (term = 2, j = 1; term <= no_of_terms; term += 2, j++)
{
value += (double) pow(-1.0, j) * pow(ang_rad, term) / fact(term);
}
printf("\nCos(%d) = %f", ang_deg, value);
double *a = &value;
return a;
}

Answer 1

It's not errors, but warnings.
Your functions are :
double *sin_x (void *dimension);
double *cos_x (void *dimension);
Since a function's name is a pointer to the function, the type of sin_x is : double * (*) (void *) (literally : a pointer to a function taking a void* as argument and returning a double *), same for cos_x.
But pthread_create is waiting for the type void * (*) (void *) (a pointer to a function taking void* as argument and returning void*).
Since double* and void* are both pointers, they have the same size (8 bytes on a 64b for example), so the compiler just warn you.

Answer 2

Changes:
Replaced sleep() with pthread_exit(). Sleep is a poor idea for synchronization.
Corrected main()'s parameters. It should be int main(void) or int main(int argc, char**argv) or equivalent.
Removed unnecessary struct inside main().
Changed the return type of fact() to size_t, which would allow you to calculate more factorials than possible with int type.
Rewrote the factorial function to use memoization. This is more efficient way of calculating factorial since you don't need to recalculate the previous factorials that have been computed already. The array size is 64K which you can change. As it currently stands, it can calculate
first 64K factorials.
You should add eror checking for pthread_ functions.
Removed pthread_join() calls since main() thread exits. The process will exit naturally once the last thread exits.
Removed the thread values since you are not using them. If you do want to know computed values, you can add another member to struct dimension and store the result in that. This method would allow to return complex value from thread functions.
--
Here's a fully functional code with all the above changes done. For more info, read pthread_create() and manuals of other functions.
#include <stdlib.h>
#include <pthread.h>
#include <string.h>
#include<stdio.h>
#include<math.h>
#define PI 3.1415
int i, x, n;
void *sin_x( void* dimension );
void *cos_x( void* dimension );
size_t fact( int );
size_t tab[64*1024] = {1};
struct data
{
int ang_deg;
int no_of_terms;
};
size_t fact( int num )
{
size_t f = 0;
if ( tab[num]) return tab[num];
f = num * fact( num - 1 );
tab[num] = f;
return f;
}
int main (int argc, char **argv)
{
printf( "\nEnter x and n:\t" );
scanf( "%d %d", &x, &n );
struct data dimension; // automatic allocation, all fields placed on stack
dimension.ang_deg = x;
dimension.no_of_terms= n;
pthread_t thrd1, thrd2;
int thret1, thret2;
thret1 = pthread_create(&thrd1, NULL, sin_x, &dimension);
thret2 = pthread_create(&thrd2, NULL, cos_x, &dimension);
printf("Parent Thread exiting...\n");
pthread_exit(0);
}
void *sin_x( void* dimension )
{
struct data* dim = dimension;
int ang_deg = dim->ang_deg;
int no_of_terms = dim->no_of_terms;
int term, j;
double value = 0.0, ang_rad = 0.0;
ang_rad = ( double ) ang_deg * PI / 180;
for ( term = 1, j = 2;term < no_of_terms*2;term += 2, j++ )
{
value += ( double ) pow( -1.0, j ) * pow( ang_rad, term ) / fact( term );
}
printf("\nSin(%d) = %f", ang_deg, value);
return 0;
}
void *cos_x( void* dimension )
{
struct data* dim = dimension;
int ang_deg = dim->ang_deg;
int no_of_terms = dim->no_of_terms;
int term, j;
double value = 1.0, ang_rad = 0.0;
ang_rad = ( double ) ang_deg * PI / 180;
for ( term = 2, j = 1;term <= no_of_terms;term += 2, j++ )
{
value += ( double ) pow( -1.0, j ) * pow( ang_rad, term ) / fact( term );
}
printf("\nCos(%d) = %f", ang_deg, value);
double *a = &value;
return 0;
}