What is the difference between `ptr = (int *) realloc(ptr,sizeof(int));` and `ptr = (int *) realloc(ptr,sizeof(int)*m);`?

Question

This is the code with extra m no of memory reallocation:

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

int main(){
    int n,m,*ptr;
    printf("Enter the number of varriables you want to add in array : ");
    scanf("%d",&n);
    ptr = (int *) malloc(n*sizeof(int));
    for(int i=0;i<n;i++)
    {
        printf("\nEnter the value of element %d : ", i+1);
        scanf("%d",&ptr[i]);
    }
    printf("\nYour array is : ");
    for (int i = 0; i < n; i++)
    {
        printf("%d  ",ptr[i]);
    }
    printf("\nEnter the number of varriables you want to add in array : ");
    scanf("%d",&m);
    **ptr = (int *) realloc(ptr,sizeof(int)*m);**
    for(int i=0;i<m;i++)
    {
        printf("\nEnter the value of element %d : ", i+1);
        scanf("%d",&ptr[n+i]);
    }
    printf("\nYour array is : ");
    for (int i = 0; i < (n+m); i++)
    {
        printf("%d  ",ptr[i]);
    }
    free(ptr);

    return 0;
}

and the next code is without particular no of memory allocation:

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

int main(){
    int n,m,*ptr;
    printf("Enter the number of varriables you want to add in array : ");
    scanf("%d",&n);
    ptr = (int *) malloc(n*sizeof(int));
    for(int i=0;i<n;i++)
    {
        printf("\nEnter the value of element %d : ", i+1);
        scanf("%d",&ptr[i]);
    }
    printf("\nYour array is : ");
    for (int i = 0; i < n; i++)
    {
        printf("%d  ",ptr[i]);
    }
    printf("\nEnter the number of varriables you want to add in array : ");
    scanf("%d",&m);
    **ptr = (int *) realloc(ptr,sizeof(int));**
    for(int i=0;i<m;i++)
    {
        printf("\nEnter the value of element %d : ", i+1);
        scanf("%d",&ptr[n+i]);
    }
    printf("\nYour array is : ");
    for (int i = 0; i < (n+m); i++)
    {
        printf("%d  ",ptr[i]);
    }
    free(ptr);

    return 0;
}

These two code is giving me same output in vs code, with out any runtime error nor any problem. so my question is,

1. why should I specify ptr = (int *) realloc(ptr,sizeof(int)*m); in spite of ptr = (int *) realloc(ptr,sizeof(int)); ?

2. and how much memory exactly the second one allocating?

Answer 1

You want

ptr = (int *) realloc(ptr,sizeof(int)*(n+m));

since you want enough space for n+m int values.