Difference between Compile Time and Run Time Memory Allocation in C

Memory allocation is a crucial part of any C program. Whether you’re declaring an integer variable, creating an array, or dynamically allocating space for a data structure, you’re working with memory. In C, memory allocation happens in two major ways:

• Compile-Time Memory Allocation (also known as Static Memory Allocation)
• Run-Time Memory Allocation (also known as Dynamic Memory Allocation)

What is Compile-Time (Static) Memory Allocation?

Compile-time memory allocation, also known as static memory allocation, occurs when memory for variables is allocated during the compilation of the program. This means that the size and type of variables must be known beforehand.​

Key Characteristics:

• You must know the size of variables before compiling the program.
• The program sets the memory location and doesn’t change it.
• It runs faster because it allocates memory during compilation.
• It doesn’t offer flexibility for dynamic memory allocation.
• After allocating memory, the program can’t change it while running.

Example:

#include <stdio.h>
 
int main() {
    int x = 10; // Allocated during compile time
    printf("%d", x);
    return 0;
}

This program creates an int variable x with the value 10. The memory is set at compile time, so it’s fast. It then prints the value using printf.

What is Run-Time (Dynamic) Memory Allocation?

Run-time memory allocation, or dynamic memory allocation, happens during the execution of the program. It allows for allocating memory when the size of the data structure cannot be determined at compile-time.

Key Characteristics:

• Allocation happens while the program is running.
• Memory size can be set dynamically, based on user input or other conditions.
• Uses the heap area of memory.
• Requires manual memory management (you must free memory yourself).
• Uses functions like malloc(), calloc(), realloc(), and free() from <stdlib.h>.

Example:

#include <stdio.h>
#include <stdlib.h>
 
int main() {
    int *x;
    x = (int*)malloc(sizeof(int)); // Allocated during runtime
    if (x != NULL) {
        *x = 10;
        printf("%d", *x);
        free(x); // Free allocated memory
    }
    return 0;
}

This program uses malloc to allocate memory for an integer while the program is running. It checks if the allocation worked, sets the value to 10, and prints it. Then, it frees the memory using free to prevent memory leaks.

Differences Between Compile-Time and Run-Time Memory Allocation

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