Minimal implementation of malloc and free using sbrk() and brk() system calls.

Context:

How does a process actually request the kernel to allocate/deallocate memory blocks in runtime?

Whenever we create an object or list, we allocate memory in heap. Unlike stack, memory allocation in heap happens in runtime. No matter what programming language you choose, the fundamentals remain the same.

In Clang, there is malloc() to create memory in heap and free() to release the memory. If you don't know Clang, you can relate malloc() call as your creating object from Class. But there are no free() equivalent functions available in other high level-programming languages as they have automatic garbage collection. Probably garbage collector has some function like free().

But malloc() and free() is library (glibc) functions, which means anyone can implement those function or tweak those function. Then the question is how malloc() or free() allocate or deallocate memory.

There is a few system call available to do that. brk, sbrk and mmap, munmap. Please check the man page to know more about the functions.

Here I've tried to implement malloc and free using brk, sbrk system calls.

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Local setup

> cd <project dir>
> mkdir bin obj
> make build
> ./bin/mem_test

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Gist

!! Try to read only the comments.

my_malloc() implementation using brk and sbrk.

my_free() implementation using brk and sbrk.

Reference:

• https://blog.holbertonschool.com/hack-the-virtual-memory-malloc-the-heap-the-program-break/
• The Linux Programming Interface by Michael Kerrisk
• Man page https://man7.org/linux/man-pages/man2/sbrk.2.html
• https://stackoverflow.com/questions/6988487/what-does-the-brk-system-call-do (https://stackoverflow.com/a/31082353)

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