Global counters and concurrent threads. How to check counter progress?

I am trying to write a simple program to understand threads. I want each thread to increment a global variable 'counter' to 4 million. Each thread only counts to 2 million. I placed a print statement at the end of each function to see how many iterations and where the global counter is at upon completion of the function. But the global counter in thread1Func is always very high, like 3.8 - 3.9 million, and then in thread2Func the counter is always 4 mil (as expected).

Am I doing this correctly? Is there a reason thread1Func is always printing such a high value for 'counter'? I would imagine it should be somewhere between 2 mil - 4 mil more evenly. Any advice would be greatly appreciated!

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


#define MAX 2000000UL
pthread_mutex_t lock;
//pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

struct shared_data
{
    int value;     /* shared variable to store result*/
    
};

struct shared_data  *counter;


void* thread1Func(void * tid){
    uint32_t i = 0;
    while(i < MAX){
        
        if(pthread_mutex_trylock(&lock) == 0){
            counter->value++;
            pthread_mutex_unlock(&lock);
            i++;
        }
    }
    printf("I am thread 1. I counted %d times. Global counter = %d\n", i, counter->value);
    return NULL;
}

void* thread2Func(void * tid){
    
    uint32_t i = 0;
    while(i < MAX){
        
        if(pthread_mutex_trylock(&lock) == 0){
            counter->value++;
            pthread_mutex_unlock(&lock);
            i++;
        }

    }
    printf("I am thread 2. I counted %d times. Global counter = %d\n", i, counter->value);
    return NULL;
}


int main() {
    
    counter = (struct shared_data *) malloc(sizeof(struct shared_data));
    printf("Initial Counter Value: %d\n", counter->value);
    
    pthread_t thread1;
    pthread_t thread2;
    pthread_mutex_init(&lock, NULL);
    
    pthread_create(&thread1, NULL, thread1Func, NULL);
    pthread_create(&thread2, NULL, thread2Func, NULL);
    
    
    pthread_join(thread1,NULL);
    pthread_join(thread2,NULL);
    
    
    printf("Final Counter: %d\n", counter->value);
    return 0;
}

> the global counter in thread1Func is always very high, like 3.8 - 3.9 million, and then in thread2Func the counter is always 4 mil

That is no surprise. If both threads are working at roughly the same speed, then when the first thread finishes, the second thread should be very nearly finished too. The first thread at that point has added its two million to the global counter, and then second thread already will have added almost two million of its own. You should totally expect the global counter to be almost four million when the first thread finishes.

The only way the first thread could print two million is if the first thread is finished before the second thread has begun to work.

Fun example, good work!

Try changing the thread priorities and see what happens to see different counts (see code below).

Maybe consider adding a semaphore to ping pong the count between the 2 threads so they execute equally and report 3999999 and 4000000 as the final counts.

I am sure you have other ideas too, thanks for posting.

gcc main.c -o main
./main
Initial Counter Value: 0
I am thread 2. I counted 2000000 times. Global counter = 3880728
I am thread 1. I counted 2000000 times. Global counter = 4000000
Final Counter: 4000000

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


#define MAX 2000000UL
pthread_mutex_t lock;
//pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;

struct shared_data
{
    int value;     /* shared variable to store result*/
    
};

struct shared_data  *counter;


void* thread1Func(void * tid){
    uint32_t i = 0;
    while(i < MAX){
        
        if(pthread_mutex_trylock(&lock) == 0){
            counter->value++;
            pthread_mutex_unlock(&lock);
            i++;
        }
    }
    printf("I am thread 1. I counted %d times. Global counter = %d\n", i, counter->value);
    return NULL;
}

void* thread2Func(void * tid){
    
    uint32_t i = 0;
    while(i < MAX){
        
        if(pthread_mutex_trylock(&lock) == 0){
            counter->value++;
            pthread_mutex_unlock(&lock);
            i++;
        }

    }
    printf("I am thread 2. I counted %d times. Global counter = %d\n", i, counter->value);
    return NULL;
}


int main() {
    
    counter = (struct shared_data *) malloc(sizeof(struct shared_data));
    printf("Initial Counter Value: %d\n", counter->value);
    
    pthread_t thread1;
    pthread_t thread2;
    pthread_mutex_init(&lock, NULL);

    pthread_attr_t attr;
    struct sched_param sch_params;
    pthread_attr_init(&attr);
    pthread_attr_getschedparam(&attr, &sch_params);
    sch_params.sched_priority = 99;
    //pthread_attr_setschedpolicy(&attr, SCHED_RR);
    pthread_attr_setschedparam(&attr, &sch_params);
    pthread_create(&thread1, &attr, thread1Func, NULL);

    sch_params.sched_priority = 1;
    pthread_create(&thread2, &attr, thread2Func, NULL);
    
    
    pthread_join(thread1,NULL);
    pthread_join(thread2,NULL);
    
    
    printf("Final Counter: %d\n", counter->value);
    return 0;
}