#include <stdio.h>     //printf, perror
#include <stdlib.h>    //EXIT_FAILURE, EXIT_SUCCESS
#include <string.h>    //sprintf
#include <unistd.h>    //open, close, read, write
#include <fcntl.h>     //flags: O_CREAT, O_EXCL
#include <semaphore.h> //sem_open, sem_wait, sem_post, sem_close

#define SEMAPHORE_NAME "/global_counter" //name of semaphore
sem_t *semaphore = NULL;
//pointer to semaphore
const int PERM = 0600;
//Permission to the semaphore

void create_semaphore()
{
    semaphore = sem_open(SEMAPHORE_NAME, O_CREAT, PERM, 1);
    if (semaphore == SEM_FAILED)
    {
        perror("Error when creating the semaphore ...\n");
        exit(EXIT_FAILURE);
    }
}

void delete_semaphore()
{
    if (sem_close(semaphore) == -1)
    {
        perror("Error can't close semaphore ...\n");
        exit(EXIT_FAILURE);
    }
    if (sem_unlink(SEMAPHORE_NAME) == -1)
    {
        perror("Error can't delete (unlink) semaphore ...\n");
        exit(EXIT_FAILURE);
    }
}

int main()
{
    //TODO: Create or use the existing semaphore.
    create_semaphore();

    //Main task: Loop 2000000 times and add 1 to the counter inside the loop
    for (int i = 0; i < 200000; ++i)
    {
        //TODO: You have to place the P/V operations here...
        //      But remember, the Linux API does not provide P/V directly.
        sem_wait(semaphore);

        //Open the file
        int file = open("counter", O_RDWR);
        if (file == -1)
        {
            printf("Could not open file, exiting!\n");
            exit(EXIT_FAILURE);
        }

        //Read the number
        const int MAX_LEN = 64;
        char number[MAX_LEN];
        read(file, &number, sizeof(number));

        //Convert the string into an integer
        int counter = atoi(number);
        counter++;

        //Write the new number into the counter
        sprintf(number, "%d\n", counter);
        lseek(file, 0, 0);
        write(file, &number, strlen(number) + 1);

        //Close the file
        close(file);

        sem_post(semaphore);
    }

    delete_semaphore();

    printf("Finished!\n");

    return EXIT_SUCCESS;
}