Multithreading in C

Question

I am new to multithreading, and any answers will be greatly appreciated. I am running an example from a tutorial which uses 3 threads; two created by the user, and one for main itself. Here's the code:

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

#define NUM_EMPLOYEES 2

/* global mutex for our program. assignment initializes it */
pthread_mutex_t a_mutex = PTHREAD_MUTEX_INITIALIZER;

struct employee {
    int number;
    int id;
    char first_name[20];
    char last_name[30];
    char department[30];
    int room_number;
};

/* global variable - our employees array, with 2 employees */
struct employee employees[] = {
        {1, 12345678, "danny", "cohen", "Accounting", 101},
        {2, 87654321, "moshe", "levy", "Programmers", 202}
};

/* global variable - employee of the day */
struct employee employee_of_the_day;

void copy_employee(struct employee *from, struct employee *to) {
    int rc;     /* contain mutex lock/unlock results */

    /*lock the mutex, to assure exclusive access to 'a' and 'b' */
    rc = pthread_mutex_lock(&a_mutex);

    to->number = from->number;
    to->id = from->id;
    strcpy(to->first_name, from->first_name);
    strcpy(to->last_name, from->last_name);
    strcpy(to->department, from->department);
    to->room_number = from->room_number;

    /* unlock mutex */
    rc = pthread_mutex_unlock(&a_mutex);
}

/* function to be executed by the variable setting threads thread */
void *do_loop(void *data) {
    int my_num = *((int*)data);

    while(1) {
        /* set employee of the day to be the one with number 'my_num' */
        copy_employee(&employees[my_num-1], &employee_of_the_day);
    }
}

/* program's execution begins in main */

int main(int argc, char *argv[]) {
    int i;
    int thr_id1;
    int thr_id2;
    pthread_t p_thread1;
    pthread_t p_thread2;
    int num1 = 1;
    int num2 = 2;
    struct employee eotd;
    struct employee *worker;

    /* initialize employee of the day to first 1 */
    copy_employee(&employees[0], &employee_of_the_day);

    /* create a new thread that will execute 'do_loop()' with '1' */
    thr_id1 = pthread_create(&p_thread1, NULL, do_loop, (void*)&num1);

    /* create a new thread that will execute 'do_loop()' with '2' */
    thr_id2 = pthread_create(&p_thread2, NULL, do_loop, (void*)&num2);

    /* run a loop that verifies integrity of 'employee of the day' many */
    /* many times.... */
    for (i = 0; i < 600000; i++) {
        /* save contents of 'employee of the day' to local 'worker' */
        copy_employee(&employee_of_the_day, &eotd);
        worker = &employees[eotd.number-1];

        /* compare employees */
        if (eotd.id != worker->id) {
            printf("mismatching 'id', %d != %d (loop '%d')
",
                    eotd.id, worker->id, i);
            exit(0);
        }
        if (strcmp(eotd.first_name, worker->first_name) != 0) {
            printf("mismatching 'first_name' , %s != %s (loop '%d')
",
                    eotd.first_name, worker->first_name, i);
            exit(0);
        }
        if (strcmp(eotd.last_name, worker->last_name) != 0) {
            printf("mismatching 'last_name' , %s != %s (loop '%d')
",
                    eotd.last_name, worker->last_name, i);
            exit(0);
        }
        if (strcmp(eotd.department, worker->department) != 0) {
            printf("mismatching 'department' , %s != %s (loop '%d')
",
                    eotd.department, worker->department, i);
            exit(0);
        }
        if (eotd.room_number != worker->room_number) {
            printf("mismatching 'room_number' , %d != %d (loop '%d')
",
                    eotd.room_number, worker->room_number, i);
            exit(0);
        }
    }

    printf("Glory, employees contents was always consistent
");
    return 0;
}

I basically want to confirm that in the for loop in main, the following statement

copy_employee(&employee_of_the_day, &eotd);

could be executed by ANY of the 3 threads; am I right? The fact that the subsequent comparisons are obviously not atomic raises some confusions. Any clarifications/corrections to this will be greatly helpful.

Incidentally, any good recommendations for tutorials on multithreading in C?

Thanks a lot!

Answer 1

No, the code in main is executed by only one thread.

The atomicity is ensured in copy_employee functions using mutexes.