The simple thread example you saw in the last lesson was useful in showing how threads are created and how they can run in different orders depending on how the scheduler decides to run them. What it doesn’t show you, though, is how threads interact when they access shared data.

Interaction of threads

Let’s imagine a simple example where two threads wish to update a global shared variable. The code you’ll study is in the coding widget below.

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

#include "common.h"
#include "common_threads.h"

int max;
volatile int counter = 0; // shared global variable

void *mythread(void *arg) {
    char *letter = arg;
    int i; // stack (private per thread) 
    printf("%s: begin [addr of i: %p]\n", letter, &i);
    for (i = 0; i < max; i++) {
	counter = counter + 1; // shared: only one
    }
    printf("%s: done\n", letter);
    return NULL;
}
                                                                             
int main(int argc, char *argv[]) {                    
    if (argc != 2) {
	fprintf(stderr, "usage: main-first <loopcount>\n");
	exit(1);
    }
    max = atoi(argv[1]);

    pthread_t p1, p2;
    printf("main: begin [counter = %d] [%x]\n", counter, 
	   (unsigned int) &counter);
    Pthread_create(&p1, NULL, mythread, "A"); 
    Pthread_create(&p2, NULL, mythread, "B");
    // join waits for the threads to finish
    Pthread_join(p1, NULL); 
    Pthread_join(p2, NULL); 
    printf("main: done\n [counter: %d]\n [should: %d]\n", 
	   counter, max*2);
    return 0;
}

Why It Gets Worse: Shared Data

Interaction of threads