Shared a global variable into multiple threads.

c_pthread_shared_variable_example.c

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>

typedef struct {
	pthread_t thread;
	int id_thread;
	long sec_usleep;
} thread_info_t;

/** Shared variable by the threads */
static int __shared_variable = 0;
/** Mutual exclusion of the shared variable */
static pthread_mutex_t __mutex_shared_variable = (pthread_mutex_t) PTHREAD_MUTEX_INITIALIZER;

/** Encapsulation "__shared_variable" */

void set_shared_variable(int value) {
	pthread_mutex_lock(&__mutex_shared_variable);   // Start critical section
	{
		__shared_variable = value;
	}
	pthread_mutex_unlock(&__mutex_shared_variable); // End critical section
}

int get_shared_variable() {
	int value;

	pthread_mutex_lock(&__mutex_shared_variable);   // Start critical section
	{
		value = __shared_variable;
	}
	pthread_mutex_unlock(&__mutex_shared_variable); // End critical section
	return value;
}

/** thread_task_1: increment "__shared_variable" */
void * thread_task_1(void * args) {
	thread_info_t * thread_args = args;
	int local_variable;

	while (1) {
		local_variable = get_shared_variable();
		local_variable++;
		set_shared_variable(local_variable);
		printf("thread %d: set shared variable to %d \n", thread_args->id_thread, local_variable);
		usleep(thread_args->sec_usleep);
	}
}

/** thread_task_2: decrement "__shared_variable" */
void * thread_task_2(void * args) {
	thread_info_t * thread_args = args;
	int local_variable;

	while (1) {
		local_variable = get_shared_variable();
		local_variable--;
		set_shared_variable(local_variable);
		printf("thread %d: set shared variable to %d \n", thread_args->id_thread, local_variable);
		usleep(thread_args->sec_usleep);
	}
}

int main() {
	thread_info_t * thread_info_1;
	thread_info_t * thread_info_2;

	thread_info_1 = calloc(1, sizeof (thread_info_t));
	thread_info_1->id_thread = 1;
	thread_info_1->sec_usleep = 1000000; // 1 second
	pthread_create(&(thread_info_1->thread), NULL, thread_task_1, thread_info_1);

	thread_info_2 = calloc(1, sizeof (thread_info_t));
	thread_info_2->id_thread = 2;
	thread_info_2->sec_usleep = 1500000; // 1.5 seconds
	pthread_create(&(thread_info_2->thread), NULL, thread_task_2, thread_info_2);

	pthread_join(thread_info_1->thread, NULL);
	pthread_join(thread_info_2->thread, NULL);

	/*
		Example execution:
	
		thread 1: set shared variable to 1 
		thread 2: set shared variable to 0 
		thread 1: set shared variable to 1 
		thread 2: set shared variable to 0 
		thread 1: set shared variable to 1 
		thread 2: set shared variable to 0 
		thread 1: set shared variable to 1 
		thread 1: set shared variable to 2 
		thread 2: set shared variable to 1 
		thread 1: set shared variable to 2 
		thread 2: set shared variable to 1 
		thread 1: set shared variable to 2 
		thread 1: set shared variable to 3 
		thread 2: set shared variable to 2 
	*/
	
	return 0;
}