Multithreaded Fibonacci using POSIX threads.

fib.c

#include <pthread.h>

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// This program illustrates the use of POSIX threads.
//
// A POSIX thread has two main components: an object of type `pthread_t` which
// represents the thread and a function pointer of type `void* (*)(void*)`
// which will be the entry point of the thread.

// This function is taken from `http://git.suckless.org/dwm/file/util.c.html`.
//
// Print the error message and `perror` if the message ends in `':'`. Assumes
// `fmt` is not `NULL`.
void die(const char* fmt, ...) {
  va_list ap;

  va_start(ap, fmt);
  vfprintf(stderr, fmt, ap);
  va_end(ap);

  // Following Unix convention (see `man perror`), first check if the string is
  // not empty.
  if(fmt[0] && fmt[strlen(fmt) - 1] == ':') {
    fputc(' ', stderr);
    perror(NULL);
  } else {
    fputc('\n', stderr);
  }

  exit(0);
}

void* fib(void* arg) {
  int n = *(int*)arg;

  if(n < 2) {
    int* ret = malloc(sizeof(int));
    *ret = 1;

    return ret;
  }

  // Allocate `n - 1` and `n - 2` on the stack. Necessary because we give
  // addresses of these values to the threads.
  int n1 = n - 1;
  int n2 = n - 2;

  pthread_t t1, t2;

  // Creates new threads. The second argument is a pointer to a
  // `pthread_attr_t`, if `NULL` the thread is created with default attributes.
  if(pthread_create(&t1, NULL, fib, &n1)) die("unable to create thread");
  if(pthread_create(&t2, NULL, fib, &n2)) die("unable to create thread");

  int* ret1;
  int* ret2;

  // Wait for the threads to terminate. The return value of the thread is
  // copied into the value pointed by the second argument.
  if(pthread_join(t1, (void**)&ret1)) die("unable to join thread");
  if(pthread_join(t2, (void**)&ret2)) die("unable to join thread");

  int* ret = malloc(sizeof(int));
  *ret = *ret1 + *ret2;

  free(ret1);
  free(ret2);

  return ret;
}

int main(int argc, char** argv) {
  for(int i = 0; i < 10; ++i) {
    int* ret = fib(&i);

    printf("%d\n", *ret);

    free(ret);
  }

  return 0;
}