A brief note on how malloc() works in C

A brief note on how malloc() works in C


4 min read

malloc() is a C Standard Library function that is defined in stdlib.h header file.

The name malloc stands for Memory Allocation.

Definition :

malloc() is a function that is used to dynamically allocate a block of memory with the specified size at run time. It returns a pointer of type void which can then be typecasted into any type of pointer.

Syntax of malloc() :

void * malloc(int);


  • int refers to an integer value of the memory size to be created dynamically.

  • void * will return the pointer to the 1st byte of the memory block that was created.

Example :

int *p = (int *) malloc(2);

To Create a platform-independent code, it's always recommended to use the sizeof() operator.

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

That's all about the basic definition and syntax. Now let's understand how malloc() works internally.

In Depth :

Since malloc() is a library function, it will run in the user mode, and we all know that in user mode, we do not get the powers to modify the system as much.

So malloc() has to call the system call to be able to get the uninitialized memory.

In UNIX, sbrk(n) is a system call that helps in creating dynamic memory allocation.

malloc() will call the system call internally (in unix its sbrk()) to get the memory block which is greater in size than requested. Later if the user needs more memory then malloc() will give from the extra chunk that it got.

malloc() maintains all the locations of the memory block that Operating System provides to it.

Point to Remember: Operating System will not always allocate a continuous memory block to malloc for every request.

So to manage these memory blocks, malloc makes use of the Linked List data structure. malloc() will create a Linked List to store all the memory blocks and their addresses.

malloc() uses the first fit technique to store the data in those memory blocks because it is very efficient than the best fit technique.

Program Example :

// Program to calculate the sum of n numbers entered by the user

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

int main()
    int n, i, *ptr, sum = 0;

    printf("Enter number of elements: ");
    scanf("%d", &n);

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

    // if memory cannot be allocated
    if(ptr == NULL)                     
        printf("Error! memory not allocated.");

    printf("Enter elements: ");
    for(i = 0; i < n; ++i)
        scanf("%d", ptr + i);
        sum += *(ptr + i);

    printf("Sum = %d", sum);

    // deallocating the memory

    return 0;