#include <stdio.h>

#include <stdlib.h>

#include <pthread.h>

#include <string.h>

typedef struct {

    char arr[8];

    unsigned long long k;

} whatever;



void *f(void *arg) {

    whatever b = *((whatever*) arg);

    printf("This thread has received a %s of size %llu\n", b.arr, b.k);

    int *ret = malloc(sizeof(int));

    *ret = 1000;

    return ret;

}



int main(int argc, char *argv[]) {

    // when creating a thread, the thread function takes a void * argument

    // void * is C's way of saying that it can be a pointer to anything

    // so for this example, we'll send a structure containing a string of maximum 8 characters and an unsigned long long, but we could send pretty much whatever we'd like - within reason

    whatever b;

    strcpy(b.arr, "banana");

    b.k = 3;



    pthread_t th;

    // pthread_create will create and start a thread

    // the thread starts by executing the given function with an argument (3rd and 4th arguments)

    pthread_create(&th, NULL, f, &b); // the thread does something like f(&b)



    // the thread function also returns void *

    // obtaining whatever the function returns can be done as follows

    // - the thread must allocate space on the heap to store its result

    // - the thread returns the pointer (address) to that heap location

    // in the main process, declare a pointer of the same type as the type returned from the thread function (in this example we return an int, but replace int with whatever data type you need, both here and in the thread function)

    // - the main process does not need to allocate space for the result, since all it need is the address returned from the thread

    // - this pointer is passed as an OUT parameter to pthread_join, as such, we must pass a reference to it (address)

    // thus, the infamous "we need the address of the pointer variable" is spawned

    int *i;

    pthread_join(th, (void **)&i);

    printf("From the thread we received: %d\n", *i);

    // finally, the main process must deallocate the memory that was allocated in the thread

    free(i);



    return 0;

}