Sorry to put this as an answer. (I can't comment.) your programs with a little bit of changes works fine for me.

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>

// Semaphore initialization for writer and reader 
sem_t wrt;
sem_t rd;

// Mutex 1 blocks other readers, mutex 2 blocks other writers
pthread_mutex_t mutex1;
pthread_mutex_t mutex2;

// Value the writer is changing, we are simply multiplying this value by 2
int cnt = 2;

int numreader = 0;
int numwriter = 0;

void *writer(void *wno)
{   
    pthread_mutex_lock(&mutex2);
    numwriter++;
    if (numwriter == 1) {
        sem_wait(&rd);      
    }
    pthread_mutex_unlock(&mutex2);
    sem_wait(&wrt); 

    // Writing Section
    cnt = cnt * 2;
    printf("Writer %d modified cnt to %d\n", *((int *)wno), cnt);

    sem_post(&wrt);
    pthread_mutex_lock(&mutex2);
    numwriter--;
    if (numwriter == 0) {
        sem_post(&rd);
    }
    pthread_mutex_unlock(&mutex2); 

    return NULL; // Correctly return a void pointer
}

void *reader(void *rno)
{   
    sem_wait(&rd);
    pthread_mutex_lock(&mutex1);
    numreader++;
    if (numreader == 1) {
        sem_wait(&wrt);
    }
    pthread_mutex_unlock(&mutex1);
    sem_post(&rd); 

    // Reading Section
    printf("Reader %d: read cnt as %d\n", *((int *)rno), cnt);

    pthread_mutex_lock(&mutex1);
    numreader--;
    if (numreader == 0) {
        sem_post(&wrt);
    }
    pthread_mutex_unlock(&mutex1); 

    return NULL; // Correctly return a void pointer
}

int main()
{   
    pthread_t read[10], write[5];
    pthread_mutex_init(&mutex1, NULL);
    pthread_mutex_init(&mutex2, NULL);
    sem_init(&wrt, 0, 1);
    sem_init(&rd, 0, 1);

    int a[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; // Just used for numbering the writer and reader

    for (int i = 0; i < 5; i++) {
        pthread_create(&write[i], NULL, writer, &a[i]); // No explicit casting
    }

    for (int i = 0; i < 10; i++) {
        pthread_create(&read[i], NULL, reader, &a[i]); // No explicit casting
    }

    for (int i = 0; i < 5; i++) {
        pthread_join(write[i], NULL);
    }

    for (int i = 0; i < 10; i++) {
        pthread_join(read[i], NULL);
    }

    pthread_mutex_destroy(&mutex1);
    pthread_mutex_destroy(&mutex2);
    sem_destroy(&wrt);
    sem_destroy(&rd);

    return 0;
}

here is the proof: