C++ malloc()

The malloc() function dynamically allocates a specified number of bytes at runtime and returns a pointer to the beginning of the allocated memory. It is defined in the <cstdlib> header and allocates memory from the heap.

• Allocates uninitialized memory during program execution.
• Returns a void* pointer that can be cast to the required type.

Syntax

The malloc() function allocates a block of memory of the specified size in bytes.

pointer = (cast_type*) malloc(size);

Where:

• pointer stores the address of the allocated memory.
• cast_type specifies the desired pointer type.
• size is the number of bytes to allocate.

Return Value

The return value of malloc() depends on whether memory allocation succeeds.

• Returns a pointer to the allocated memory on success.
• Returns nullptr (or NULL in older code) if memory allocation fails.

Note: The allocated memory is not initialized. It contains indeterminate values until explicitly initialized.

Allocating Memory Using malloc()

The malloc() function allocates a block of memory of the specified size and returns a pointer to the beginning of that memory block. Since the allocated memory is uninitialized, its values must be assigned before use.

Example: The following program allocates memory for a single integer using malloc(), initializes it, and then prints its value.

#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
    int* ptr = (int*)malloc(sizeof(int));

    if (ptr == nullptr) {
        cout << "Memory allocation failed";
        return 0;
    }

    *ptr = 25;

    cout << "Value: " << *ptr << endl;

    free(ptr);

    return 0;
}

Value: 25

Explanation

• malloc(sizeof(int)) allocates memory for one integer.
• The returned void* pointer is cast to int*.
• The allocated memory is initialized manually.
• free() releases the allocated memory.

Allocating Arrays Using malloc()

malloc() can also allocate memory for arrays whose size is determined at runtime.

Example: The following program dynamically allocates an integer array.

#include <cstdlib>
#include <iostream>
using namespace std;

int main()
{
    int n = 5;

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

    if (arr == nullptr) {
        cout << "Memory allocation failed";
        return 0;
    }

    for (int i = 0; i < n; i++)
        arr[i] = (i + 1) * 10;

    cout << "Array elements: ";

    for (int i = 0; i < n; i++)
        cout << arr[i] << " ";

    free(arr);

    return 0;
}

Array elements: 10 20 30 40 50 

Explanation

• Memory is allocated for n integers.
• Each element is initialized after allocation.
• The allocated memory is released using free().

Releasing Memory Using free()

Memory allocated using malloc() must be released using free() when it is no longer required.

• Prevents memory leaks.
• Should only be used with memory allocated by malloc(), calloc(), or realloc().

Syntax

free(ptr);

Common Uses of malloc()

malloc() is commonly used when memory requirements are determined during program execution.

• Allocating memory whose size is known only at runtime.
• Implementing dynamic data structures such as linked lists, stacks, queues, and trees.
• Managing heap memory manually in low-level programs.

Related Article: malloc() vs new