Multithreading in C - 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')\n",
eotd.id, worker->id, i);
exit(0);
}
if (strcmp(eotd.first_name, worker->first_name) != 0) {
printf("mismatching 'first_name' , %s != %s (loop '%d')\n",
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')\n",
eotd.last_name, worker->last_name, i);
exit(0);
}
if (strcmp(eotd.department, worker->department) != 0) {
printf("mismatching 'department' , %s != %s (loop '%d')\n",
eotd.department, worker->department, i);
exit(0);
}
if (eotd.room_number != worker->room_number) {
printf("mismatching 'room_number' , %d != %d (loop '%d')\n",
eotd.room_number, worker->room_number, i);
exit(0);
}
}
printf("Glory, employees contents was always consistent\n");
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.

Answer 2

Your main thread (and none of the worker threads) will execute everything within main() and then end. Both of your worker threads will execute everything within do_loop() and end once they leave the function.
This sounds a bit like you're confusing phtread_create() with fork(). pthread_create() will use the function provided as the entry point while fork() will start from the position it's been called.

Answer 3

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?
Not really, since that statement is only executed by the main thread and not by the other two threads.