README.md

Memory Allocator

Overview

This project is a custom memory allocator implemented in C for educational purposes aiming to explore operating system (OS) concepts related to memory management and the system calls needed for low-level memory handling. By recreating the functionality of malloc, calloc, realloc, and free, this library provides insight into how memory is dynamically allocated and managed at the system level.

Educational Focus

• System Calls: Demonstrates how system calls like sbrk() are used to request memory from the operating system's heap, simulating the behavior of malloc().
• Linked List Memory Management: Shows how memory blocks are tracked using a simple linked list structure, demonstrating OS concepts of memory allocation and deallocation.
• Thread Safety: Highlights the use of pthread_mutex_lock() and pthread_mutex_unlock() to ensure memory allocation is thread-safe in multi-threaded environments.

The project serves as a learning tool for studying operating systems, memory management, and low-level programming concepts.

Features

• Custom dynamic memory allocation using jmalloc.
• Memory reallocation using jrealloc
• Zero-initialized memory allocation (jcalloc), similar to the standard calloc().
• Safe memory deallocation (jfree), tracking memory blocks with a linked list.
• Thread-safe memory allocation with mutexes.

Files in the Repository

• allocator.h: Header file with function declarations.
• allocator.c: Contains the core logic for memory allocation and management.
• MemoryBlock.c: Defines the memory block structure, simulating a linked list of memory blocks.
• build.sh: Script for compiling the library and cleaning up object files.
• test.c: A sample test file demonstrating the use of custom allocator functions.

Installation

Step 1: Clone the Repository

git clone https://github.com/your_username/your_repository.git
cd your_repository

Step 2: Build the Static Library

Run the provided build.sh script to compile the library:

./build.sh

To clean up object files and the static library:

./build.sh clean

Step 3: Linking the Library

To use the custom allocator, link the static library when compiling a program. For example:

1. Write the program that uses the allocator

#include "allocator.h"   //Our Allocator
#include <stdio.h>

int main() {
    void* ptr = jmalloc(100);
    if (ptr) {
        printf("Memory allocated successfully.\n");
        jfree(ptr);
        printf("Memory freed.\n");
    }
    return 0;
}

2. Compile the program and link it with the static library:

gcc -o my_program my_program.c -L. -l:jonathanmemoryallocator.a

3. Run the program:

./my_program

System Calls and OS Concepts

sbrk(): Requesting Memory From the OS

• The sbrk() system call is used to increase the program's heap by a specified number of bytes, simulating how malloc() requests memory from the operating system when it runs out of space.
• The allocator tracks the allocated memory blocks using a linked list, similar to how the kernel manages memory for processes.

Mutexes and Thread Safety

• In multi-threaded environments, race conditions can occur if multiple threads attempt to allocate memory simultaneously.
• The library uses mutex locks (pthread_mutex_lock() and pthread_mutex_unlock()) to ensure thread-safe memory management.

Block Management Using a Linked List

• Each memory block is represented by a header that stores metadata about the block's size, whether it is free or allocated, and a pointer to the next block.
• This structure mimics how operating systems track memory allocations, forming a linked list of memory blocks.

Example Usage

#include "allocator.h"
#include <stdio.h>

int main() {
    // Allocate 100 bytes using jmalloc
    void* ptr = jmalloc(100);
    if (ptr) {
        printf("Memory allocated at address: %p\n", ptr);
    }

    // Resize the block to 200 bytes using jrealloc
    ptr = jrealloc(ptr, 200);
    if (ptr) {
        printf("Memory resized at address: %p\n", ptr);
    }

    // Free the memory block
    jfree(ptr);
    printf("Memory freed.\n");

    return 0;
}

Testing

• The provided test.c program can be used to verify the behavior of the custom memory allocator. sbrk(): Requesting Memory From the OS

Compile the test program

gcc -o test test.c -L. -l:jonathanmemoryallocator.a

Run the test

./test

Issues

For issues + contribution, make a PR request.

Thanks!