/*

Simple example demonstrating ensuring safe access to a global variable
from threads using a semaphore.

    $ gcc -O3 -pthread bank_account_semphore.c -o bank_account_semphore
    $ ./bank_account_semphore
    Andrew's bank account has $200000
    $

*/

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>

int bank_account = 0;

sem_t bank_account_semaphore;

// add $1 to Andrew's bank account 100,000 times
void *add_100000(void *argument) {
    for (int i = 0; i < 100000; i++) {
        // decrement bank_account_semaphore if > 0
        // otherwise wait until > 0
        sem_wait(&bank_account_semaphore);

        // only one thread can execute this section of code at any time
        // because  bank_account_semaphore was initialized to 1

        bank_account = bank_account + 1;

        // increment bank_account_semaphore
        sem_post(&bank_account_semaphore);
    }

    return NULL;
}

int main(void) {
    // initialize bank_account_semaphore to 1
    sem_init(&bank_account_semaphore, 0, 1);

    // create two threads performing  the same task

    pthread_t thread_id1;
    pthread_create(&thread_id1, NULL, add_100000, NULL);

    pthread_t thread_id2;
    pthread_create(&thread_id2, NULL, add_100000, NULL);

    // wait for the 2 threads to finish
    pthread_join(thread_id1, NULL);
    pthread_join(thread_id2, NULL);

    // will always be $200000
    printf("Andrew's bank account has $%d\n", bank_account);

    sem_destroy(&bank_account_semaphore);
    return 0;
}