pthread program crashes while I fetch buffer size in C

Question

I'm trying to build a lift simulator in which a thread adds lift requests to the buffer and three threads representing lifts take the requests from the buffer and stimulate them simultaneously. I'm using pthread.h and semaphore.h to help me with the multithreaded nature of the program. As the requests need to be served in the order they are produced, I thought it would be best to use a queue as the buffer.

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

#define THREAD_NUM 4

sem_t semEmpty;
sem_t semFull;
pthread_mutex_t mutexBuffer;
pthread_cond_t cond;
pthread_cond_t condII;
// file pointer assignment statements.
FILE *input;
FILE *output;

int count = 0;
int totalRequests = 0;
int totalMovement = 0;

// A structure to store requests.
struct Request
{
    int reqNo;
    int from;
    int to;
};

// A structure to store lift info.
struct Lift
{
    int liftNo;
    int c_pos;
    int movement;
    int tMovement;
    int tRequests;
};

// Declaring and defining queue functions.
struct queue
{
    int size;
    int f;
    int r;
    struct Request *arr;
};

int isEmpty(struct queue *q)
{
    if (q->r == q->f)
    {
        return 1;
    }
    return 0;
};

int isFull(struct queue *q)
{
    if (q->r == q->size - 1)
    {
        return 1;
    }
    return 0;
};

void enqueue(struct queue *q, struct Request req)
{
    if (isFull(q))
    {
        printf("This Queue is full\n");
    }
    else
    {
        q->r++;
        q->arr[q->r] = req;
        printf("Enqued element.\n");
    }
};

struct Request dequeue(struct queue *q)
{
    struct Request r;
    if (isEmpty(q))
    {
        printf("This Queue is empty.\n");
    }
    else
    {
        q->f++;
        r = q->arr[q->f];
        printf("Dequed element.\n");
    }
    return r;
};

// Declaring Buffer.
struct queue Buffer;

void *request(void *args)
{

    struct Request r1;

    while (!feof(input))
    {
        fscanf(input, "%d %d %d", &r1.reqNo, &r1.from, &r1.to);

        fprintf(output, "--------------------------------------------\n");
        fprintf(output, "Request No: %d\n", r1.reqNo);
        fprintf(output, "From Floor %d to Floor %d\n", r1.from, r1.to);
        fprintf(output, "--------------------------------------------\n");

        
        while (isFull(&Buffer) == 1)
        {
            pthread_cond_wait(&condII, NULL);
        }

        sem_wait(&semEmpty);
        pthread_mutex_lock(&mutexBuffer);
        enqueue(&Buffer, r1);
        count++;
        pthread_mutex_unlock(&mutexBuffer);
        sem_post(&semFull);
    }
}

void *lift(void *args)
{
    pthread_mutex_lock(&mutexBuffer);
    struct Lift currentLift = *(struct Lift *)args;
    struct Request r1;
    pthread_mutex_unlock(&mutexBuffer);

    while (count > 0)
    {
        while(isEmpty(&Buffer) == 0){
            sem_wait(&semFull);
            pthread_mutex_lock(&mutexBuffer);

            r1 = dequeue(&Buffer);
            count--;
            pthread_cond_broadcast(&condII);

            currentLift.movement = 0; // Resetting movement.
            currentLift.tRequests++;  // Incrementing total requests for lift.
            totalRequests++;          // Incrementing total requests.

            fprintf(output, "Lift %d Operations\nPrevious Position: %d Floor\nRequest: number %d from Floor %d to Floor %d\n", currentLift.liftNo, currentLift.c_pos, r1.reqNo, r1.from, r1.to);
            fprintf(output, "Detail Operations:\n\tGo from Floor %d to Floor %d\n", currentLift.c_pos, r1.from);

            // Stimulating lift movement.
            sleep(1);
            currentLift.movement += abs(currentLift.c_pos - r1.from);
            currentLift.c_pos = r1.from;

            fprintf(output, "\tGo from Floor %d to Floor %d\n", r1.from, r1.to);

            sleep(1);
            currentLift.movement += abs(r1.to - r1.from);
            currentLift.c_pos = r1.to;

            currentLift.tMovement += currentLift.movement; // Adding to total movement of lift.

            fprintf(output, "\t#movement for this request: %d\n", currentLift.movement);
            fprintf(output, "\t#Requests: %d\n", currentLift.tRequests);
            fprintf(output, "\tTotal #movement: %d\n", currentLift.tMovement);
            fprintf(output, "Current Position: %d Floor\n", r1.to);

            pthread_mutex_unlock(&mutexBuffer);
            sem_post(&semEmpty);

            pthread_mutex_lock(&mutexBuffer);
            totalMovement += currentLift.tMovement;
            pthread_mutex_unlock(&mutexBuffer);
        }
    }
}

int main(int argc, char *argv[])
{
    // Creating thread array, mutexes, semaphores, and lifts.
    pthread_t threads[THREAD_NUM];
    pthread_mutex_init(&mutexBuffer, NULL);
    sem_init(&semEmpty, 0, 5);
    sem_init(&semFull, 0, 0);
    pthread_cond_init(&cond, NULL);
    pthread_cond_init(&condII, NULL);
    Buffer.size = 5;
    Buffer.f = -1;
    Buffer.r = -1;
    Buffer.arr = malloc(sizeof(struct Request) * Buffer.size);
    struct Lift l1, l2, l3;

    // Setting values for lifts.
    l1.liftNo = 1;
    l2.liftNo = 2;
    l3.liftNo = 3;
    l1.c_pos = 1;
    l2.c_pos = 1;
    l3.c_pos = 1;
    l1.tMovement = 0;
    l2.tMovement = 0;
    l3.tMovement = 0;
    l1.tRequests = 0;
    l2.tRequests = 0;
    l3.tRequests = 0;
    l1.movement = 0;
    l2.movement = 0;
    l3.movement = 0;

    // Dynamically allocating lifts for use in threads.
    struct Lift *lift1 = malloc(sizeof(struct Lift));
    *lift1 = l1;
    struct Lift *lift2 = malloc(sizeof(struct Lift));
    *lift2 = l3;
    struct Lift *lift3 = malloc(sizeof(struct Lift));
    *lift3 = l2;

    // assigning files to file pointers.
    input = fopen("sim_input.txt", "r");
    if (input == NULL)
    {
        printf("No file read.\n\n");
        return -1;
    }
    else
    {
        fseek(input, 0, SEEK_SET);
        printf("File read successfully.\n\n");
    }
    output = fopen("sim_out.txt", "w");
    if (output == NULL)
    {
        printf("No file read.\n\n");
        return -1;
    }
    else
    {
        printf("File read successfully.\n\n");
    }

    // Creating producer thread and joining.
    if (pthread_create(&threads[0], NULL, &request, NULL) != 0)
    {
        perror("Failed to create thread");
    }

    if (pthread_join(threads[0], NULL) != 0)
    {
        perror("Failed to join thread");
    }

    // Creating threads and passing lift structures.
    if (pthread_create(&threads[1], NULL, &lift, lift1) != 0)
    {
        perror("Failed to create thread");
    }
    if (pthread_create(&threads[2], NULL, &lift, lift2) != 0)
    {
        perror("Failed to create thread");
    }
    if (pthread_create(&threads[3], NULL, &lift, lift3) != 0)
    {
        perror("Failed to create thread");
    }

    // Joining all lifts using loop structure.
    for (int i = 1; i < THREAD_NUM; i++)
    {
        if (pthread_join(threads[i], NULL) != 0)
        {
            perror("Failed to join thread");
        }
    }

    // Printing total requests and total movements.
    fprintf(output, "Total number of requests: %d\n", totalRequests);
    fprintf(output, "Total number of movements: %d\n", totalMovement);

    // Freeing memory.
    if (count < 1)
    {
        sem_destroy(&semEmpty);
        sem_destroy(&semFull);
        pthread_cond_destroy(&cond);
        pthread_cond_destroy(&condII);
        pthread_mutex_destroy(&mutexBuffer);
        fclose(input);
        fclose(output);
        free(lift1);
        free(lift2);
        free(lift3);
    }
    return 0;
}

Here's the complete code for the program. It requires sim_input.txt with the following contents:

1 1 5
2 7 2
3 3 8
4 4 11
5 12 15
6 2 9
7 11 7
8 8 15
9 12 19
10 20 7 

I assume the program crashes at while(isEmpty(&Buffer) == 0) where I try to fetch the size of the Buffer(Queue) but there might be some other reason too, can you please help?

Answer 1

I assume the program crashes at

You shouldn't assume. Run your program under debugger and observe where it crashes.

Your question lacks MCVE, because your program requires input file sim_input.txt which you didn't provide.

There are obvious bugs though -- you are accessing Buffer from multiple threads, some but not all of these accesses are guarded by a mutex.

For a mutex to be effective, all accesses to variables shared between multiple threads must be guarded, or you will have a data race, which is undefined behavior.

Update:

The actual crash is happening here:

            pthread_cond_wait(&condII, NULL);

This call makes absolutely no sense: you must provide a mutex to pthread_cond_wait, and it must be the same mutex as the one used in pthread_cond_signal or similar. Here it should be mutexBuffer, and the code should look something like:

pthread_mutex_lock(&mutexBuffer);
while (isFull(&Buffer)) {
  pthread_cond_wait(&condII, &mutexBuffer);
}
// sem_wait()   <<- this is bogus/unnecessary
enqueue(&Buffer, r1);
count++;
pthread_mutex_unlock(&mutexBuffer);

There are many other bugs as well, e.g. the code in lift() assumes that it will run after request has been enqueued, and will exit immediately if that hasn't happened.

In my runs, all 3 lift threads exit before a single request is processed.