Created: 2023-01-15 18:38
Status: #concept
Subject: Programming
Tags: C Memory Memory Address Functions Garbage stdlib.h Linked List

Dynamic Memory Allocation

Description

Dynamic Memory Allocation is the process of allocating Byte Memory from the Memory Heap in order to be used by the program during runtime.

C

This allow us to return variable length Strings or Arrays from Functions.
The functions related to Memory Allocation are found in the <stdlib.h> Library.
The Memory Allocation functions return a void * which is a Pointer that can be assigned to ANY C Data Type.

What does malloc do? What is the syntax?

void *malloc(size_t size)

It allocates a block of memory, with a specified size, in Bytes, but doesn’t initialize it.
Returns a void Pointer void * which can be assigned to Pointer Variables, IF SUCCESSFUL.
Otherwise, returns a NULL pointer with a Memory Address of 00000000, IF FAILURE.

void calloc(size_t nitems, size_t size)

This function allocates a block of memory, and clears it (Initializes each Memory Address with 0.

What function lets us deallocate memory? Why is it important to do so?

We have to deallocate unused Memory (known as Garbage in order to prevent Memory Leaks or NULL Return Values from malloc or calloc.

void free(void *ptr);

Deallocates the memory block pointed by *ptr

p = malloc(...);
q = malloc(...);
free(p); // frees or 'collects' the memory block pointed by p
p = q;

C free() Illustration.png

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

char *concat(const char *str1, const char *str2)
{
    char *result = malloc(strlen(str1) + strlen(str2) + 1);

    if (result == NULL) // no memory error handler
    {
        printf("Error: malloc failed.");
        exit(EXIT_FAILURE);
    }

    strcpy(result, str1);
    strcat(result, str2);
    
    return result;
}

int main(void)
{
    char *str1 = "poggers ", *str2 = "champ!";

    puts(concat(str1, str2));
    // poggers champ!
}

Idiom: Conditional Comparison with `NULL`

Since NULL is 00000000 and any non-null Pointer evaluates to non-zero, NULL = False non-null Address = True.
This will be important in traversing through Linked Lists.

if (p == NULL) ...
if (!p) ... // equivalent

if (p != NULL) ...
if (p) ... // equivalent

// It's better to do explicit comparison with NULL due to compatibility

References

C Programming, A Modern Approach, 2nd Edition, Chapter 17.1

Dynamic Memory Allocation

C

Idiom: Conditional Comparison with NULL

References

Idiom: Conditional Comparison with `NULL`