Pass argument to multiple threads - c

Question

New to C, I was reading here about how to properly pass an argument to a thread. What about if this argument needed to be passed to multiple threads? Where/how should I use free()? Say:
void *foo(void *i) {
int a = *((int *) i);
while(1){
printf("foo running \n");
sleep(1);
}
}
void *bar(void *i) {
int a = *((int *) i);
while(1){
printf("bar running \n");
sleep(1);
}
}
int main() {
pthread_t threads[2];
int i;
for (i = 0; i < 2; i++ ) {
int *arg = malloc(sizeof(*arg));
if ( arg == NULL ) {
fprintf(stderr, "Couldn't allocate memory for thread arg.\n");
exit(1);
}
*arg = i;
pthread_create(&threads[0], NULL, foo, arg);
pthread_create(&threads[1], NULL, bar, arg);
}
for (i = 0; i < 2; i++){
pthread_join(threads[i],NULL);
}
return 0;
}
Is calling free(arg); in main after spawning the threads the same thing / safe?

Answer 1

If all threads needs the exact same argument, and they are not modifying the argument, there's no need to allocate it dynamically at all, instead just declare it as a variable at function scope in the main function. If there's no dynamic allocation, there's no need to free it.
On the other hand if you need separate arguments in a loop like you do, you need to keep track of all arguments, for example using an array:
// Rest of program...
#define NUMBER_OF_ITERATIONS 2
int main(void)
{
int args[NUMBER_OF_ITERATIONS];
pthread_t threads[NUMBER_OF_ITERATIONS][2];
// Create threads
for (unsigned i = 0; i < NUMBER_OF_ITERATIONS; ++i)
{
args[i] = i;
pthread_create(&threads[i][0], NULL, foo, &args[i]);
pthread_create(&threads[i][1], NULL, bar, &args[i]);
}
// Wait for threads to finish
for (unsigned i = 0; i < NUMBER_OF_ITERATIONS; ++i)
{
pthread_join(threads[i][0]);
pthread_join(threads[i][1]);
}
return 0;
}
The program above also solves another problem you have, when you create a total of four threads but only join two.

Answer 2

You need to make sure both threads have finished before calling free(arg).
That means you can do it after you call pthread_join on both threads.
for (i = 0; i < 2; i++){
pthread_join(threads[i],NULL);
}
free(args);