Memory sharing between threads in C

Question

can someone please explain exactly what memory is being shared between threads? I got the code from a website just so I can explain what I don't understand exactly. I want to know, if all the threads after they're created they will execute the function doSomeThing and will they share the same value for MyVariable or every thread will have separate values for it. (ignore the fact that there isn't any value assigned to the variable)

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

pthread_t tid[2];

void* doSomeThing(void *arg)
{
    unsigned long i = 0;
    int MyVariable;
    pthread_t id = pthread_self();
    if(pthread_equal(id,tid[0]))
    {
        printf("\n First thread processing\n");
    }
    else
    {
        printf("\n Second thread processing\n");
    }
    for(i=0; i<(0xFFFFFFFF);i++);
    return NULL;
}
int main(void)
{
    int i = 0;
    int err;
    while(i < 2)
    {
        err = pthread_create(&(tid[i]), NULL, &doSomeThing, NULL);
        if (err != 0)
            printf("\ncan't create thread :[%s]", strerror(err));
        else
            printf("\n Thread created successfully\n");

        i++;
    }
    return 0;
}

What @DanielA.White is trying to say is that MyVariable is a function-local variable so regardless of threads or not the value is not shared in any (useable) way. — DisappointedByUnaccountableMod, Dec 29 '19 at 21:37
These threads don't access any shared variables. Automatic variables, like MyVariable, are not shared across threads. They are all within the memory space of the process, of course, but not shared across threads. The threads *could* share access to a variable if, for example, you passed the address of the same variable from your `main` function into each thread as its starting argument. — jarmod, Dec 29 '19 at 22:00

einpoklum · Accepted Answer · 2019-12-29T22:26:45.970

You've actually asked two separate questions.

what memory is being shared between threads?

Well, all memory (on typical OSes). A main difference between threads and processes is that different processes have different memory spaces, which threads within a process have the same memory space.

Memory sharing between threads in C

1 Answers1