realloc in c

Answer:

The realloc function in C is used to dynamically resize the memory allocated for a previously allocated block. It takes two arguments: a pointer to the previously allocated memory block and the new size that the block should be resized to. The function returns a pointer to the resized memory block.

Here is the syntax for realloc:

void realloc(void ptr, size_t size);

The ptr argument is a pointer to the memory block that needs to be resized. If ptr is a null pointer, then realloc behaves like malloc and allocates a new memory block of the given size.

The size argument specifies the new size for the memory block. If size is 0, then realloc behaves like free and deallocates the memory block pointed to by ptr.

When realloc is called, it performs the following actions:

  1. If ptr is a null pointer, it behaves like malloc and allocates a new memory block of size size.
  2. If ptr is not a null pointer, it tries to resize the memory block pointed to by ptr to the new size size.
  3. If the new size is smaller than the current size, the extra bytes are deallocated.
  4. If the new size is larger than the current size, realloc tries to extend the memory block. If there is sufficient adjacent free space, it extends the block in-place. Otherwise, it allocates a new block of the requested size, copies the contents of the old block to the new block, and frees the old block.
  5. If the new size is equal to the current size, realloc does nothing and returns the original pointer ptr.
  6. If realloc is successful in resizing the memory block, it returns a pointer to the resized memory block. If it fails to resize the block, it returns a null pointer.

It's important to note that the original contents of the memory block may be preserved during resizing, but this is not guaranteed. Therefore, it's good practice to assign the returned pointer to the original pointer variable to avoid potential memory leaks.

Here's an example usage of realloc:

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

int main() {
    int ptr = (int )malloc(5 * sizeof(int));
    if (ptr == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }

    // Use the memory block...

    ptr = (int )realloc(ptr, 10  sizeof(int));
    if (ptr == NULL) {
        printf("Memory reallocation failed.\n");
        return 1;
    }

    // Use the resized memory block...

    free(ptr);
    return 0;
}

In this example, we initially allocate a memory block of size 5 sizeof(int) using malloc. Then, we use the memory block for some purpose. Later, we resize the memory block to a new size of 10 sizeof(int) using realloc. Finally, we free the memory block using free.