pthread_create not working. passing argument 3 warning - c

Question

I am trying to create a thread and from what I remember this should be the right way to do it:
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 5
int SharedVariable =0;
void SimpleThread(int which)
{
int num,val;
for(num=0; num<20; num++){
if(random() > RAND_MAX / 2)
usleep(10);
val = SharedVariable;
printf("*** thread %d sees value %d\n", which, val);
SharedVariable = val+1;
}
val=SharedVariable;
printf("Thread %d sees final value %d\n", which, val);
}
int main (int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
for(t=0; t< NUM_THREADS; t++){
printf("In main: creating thread %ld\n", t);
rc = pthread_create(&threads[t], NULL, SimpleThread, (void* )t);
if (rc){
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
/* Last thing that main() should do */
pthread_exit(NULL);
}
And the error that I'm getting is this one:
test.c: In function ‘main’: test.c:28: warning: passing argument 3 of
‘pthread_create’ from incompatible pointer type
/usr/include/pthread.h:227: note: expected ‘void * (*)(void *)’ but
argument is of type ‘void (*)(int)’
I cannot change the SimpleThread function so changing the type of the parameter is not an option even though I already tried and it didn't work either.
What am I doing wrong?

Answer 1

SimpleThread should be declared as
void* SimpleThread(void *args) {
}
When you pass parameters to your thread, it is best to define a struct for them, pass a pointer to that struct as void*, and cast back to the right type inside the function.

Answer 2

Here's a compiling and "working" version of your program, although I have to admit to not knowing exactly what it's doing. For the critics in the audience, I apologize in advance for the pthread_join loop at the end.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define NUM_THREADS 5
struct my_thread_info {
long which;
};
int SharedVariable = 0;
void *SimpleThread(void *data)
{
int num, val;
struct my_thread_info *info = data;
for (num = 0; num < 20; num++) {
if (random() > RAND_MAX / 2)
usleep(10);
val = SharedVariable;
printf("*** thread %ld sees value %d\n", info->which, val);
SharedVariable = val + 1;
}
val = SharedVariable;
printf("Thread %ld sees final value %d\n", info->which, val);
free(info);
return NULL;
}
int main(int argc, char *argv[])
{
pthread_t threads[NUM_THREADS];
int rc;
long t;
struct my_thread_info *info;
for (t = 0; t < NUM_THREADS; t++) {
printf("In main: creating thread %ld\n", t);
info = malloc(sizeof(struct my_thread_info));
info->which = t;
rc = pthread_create(&threads[t], NULL, SimpleThread, info);
if (rc) {
printf("ERROR; return code from pthread_create() is %d\n", rc);
exit(-1);
}
}
for (t = 0; t < NUM_THREADS; t++) {
pthread_join(threads[t], NULL);
}
}