demonstration of 'transfer of computation' (POSIX threads)

transfer_of_computation.c

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sysexits.h>
#include <errno.h>

// this code snippet demonstrates the 'transfer of computation' methodology,
// whereby two threads do not transfer data but a computation that acts upon
// this observed data

// we demonstrate this via a global value `global_v`;
// where `thread_i` awaits `global_v`'s assignment to a specific value, and provides
// `thread_ii` with a computation to perform the assignment

// compiled as `gcc transfer_of_computation.c -o main -lpthread`

/* global state, which will be toggled by the transferred computation */
int global_v;

/* initialized 'shared computation'*/
void (*shared_fn)(int x, int y) = NULL;

/* multiply assigns the global state to the awaited value*/
void multiply(int x, int y) {
  global_v = x * y;
}

/**
 * @brief Waits for the shared function, invoking it once available
 * 
 * @param arg 
 * @return void* 
 */
void* thread_i_fn(void* arg) {
  while (!shared_fn);

  printf("Shared function now available to thread %ld\n", pthread_self());
  
  shared_fn(3, 3);

  return NULL;
}

/**
 * @brief Waits for the shared, global state to change to the expected value
 * 
 * @param arg 
 * @return void* 
 */
void* thread_ii_fn(void* arg) {
  shared_fn = multiply;
  printf("Thread %ld has transferred the computation\n", pthread_self());

  while (global_v != 9);
  printf("The global state is now available as %d to thread %ld\n", global_v, pthread_self());

  return NULL;
}


int main(int argc, char* argv[]) {
  pthread_t thread_i, thread_ii;

  if (pthread_create(&thread_i, NULL, thread_i_fn, NULL) != 0 ||
    pthread_create(&thread_ii, NULL, thread_ii_fn, NULL) != 0) {
    return EX_CANTCREAT;
  }

  pthread_join(thread_i, NULL);
  pthread_join(thread_ii, NULL);
  
  return EX_OK;
}