what's going wrong here with realloc()?

Question

I am writing a code which uses realloc(). The following is a simplified version of the problem. Though the code looks obvious yet it doesn't seem to work.

// Program for implementing variable length integer array.

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

void add(int* ptr,int len,int ele){
    ptr = (int*)realloc(ptr,len);
    *(ptr+len-1) = ele;
}

void main(){
    
    int max_len = 10;
    
    int* arr = (int*)malloc(sizeof(int));
    
    for(int i=0;i<max_len;i++)
        add(arr,i+1,i+1);
    
    printf("The elements are...\n");
    
    for(int i=0;i<max_len;i++)
        printf("%d\n",*(arr+i));
}

The program runs for max_len=8 or low but not beyond it. Why is this happening? Thanks in advance.

Function arguments in C are *copies* of what are passed, so modifying them will not affect what are passed. You should pass *pointers to what should be modified* when you want functions to modify caller's local things. — MikeCAT, Mar 22 '21 at 16:03
`arr` is no longer valid, and you have reallocated memory that cannot be `free`d. Another solution is to change the function definition and `return` the new pointer value, so that you call as `arr = add(arr, i+1, i+1);` — Weather Vane, Mar 22 '21 at 16:05
The linked question did not state that C always passes by value, and it did not provide any solution. It was not a duplicate. There are surely duplicates, but that wasn't it. — ikegami, Mar 22 '21 at 16:06
Does this answer your question? [How do I modify a pointer that has been passed into a function in C?](https://stackoverflow.com/questions/766893/how-do-i-modify-a-pointer-that-has-been-passed-into-a-function-in-c) — Ian Abbott, Mar 22 '21 at 16:25

score 0 · Accepted Answer · answered Mar 22 '21 at 16:24

A few things:

First, you should pass your array by reference with &. You pass your pointer by value which has no effect.
Second, you forgot to make your realloc with sizeof(int), equivalent to 4.
Third, you have to assign your value the following way: *(*(ptr) + len - 1) = ele;

See the corrected code below:

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

void add(int** ptr,int len,int ele){
    *ptr = (int*)realloc(*ptr,len*sizeof(int));
    *(*(ptr) + len - 1) = ele;
}

void main(){

    int max_len = 10;

    int* arr = (int*)malloc(sizeof(int));

    for(int i=0;i<max_len;i++)
        add(&arr,i+1,i);

    printf("The elements are...\n");

    for(int i=0;i<max_len;i++)
        printf("%d\n",arr[i]);
}

output:

what's going wrong here with realloc()?

1 Answers1