Mem_alloc

This is a very basic implementation of a memory allocator in C made for learning purposes so there are probably better ways to do this like mmap

What's dynamic allocation?

Dynamic allocation allows more flexibilty when we're working with memory, it involves allocating memory from a region called the heap wich is larger and can save more data

+-------------------+  High Memory
|       Stack       |  (grows downwards)
|                   |
|                   |
|                   |
|                   |
+-------------------+
|       Heap        |  (grows upwards)
+-------------------+
|       Data        |  (initialized variables)
+-------------------+
|       Text        |  (instructions)
+-------------------+

Explanation

• Stack: Grows downwards from higher addresses to lower ones, typically used for local functions calls, local variables etc.
• Heap: Grows upwards from lower adresses to higher ones, contain the dynamic allocated data mem_alloc().
• Data: This is where all initialized global and static variables are stored.
• Text: The part that contains the binary instructions to be executed by the processor.

How it works?

The memory blocks are linked each other by a pointer that refers to the memory address of the header of the next memory block, doing this we can keep track of all the memory blocks and know which ones are free and which ones not. When a user calls the mem_alloc() function to allocate data the program will increment the program break to the size requested and return a pointer to the memory address where the heap starts. In case of freeing memory free() it'll only remove the block if it's at the end of the heap, otherwise it will mark it as free.

Heap Start
|
V
+------------------------------------------------------------------------------------------+
|                                   Memory Block 1                                        |
| +----------------+     +--------------------------------------------------------------+ |
| |     Header     | --> | next (points to Block 2)                                   | |
| +----------------+     +--------------------------------------------------------------+ |
| |     Data       |                                                              |      |
| |  (requested    |                                                              |      |
| |   memory)      |                                                              |      |
| +----------------+                                                              |      |
+------------------------------------------------------------------------------------------+
|                                   Memory Block 2                                        |
| +----------------+     +--------------------------------------------------------------+ |
| |     Header     | --> | next (points to Block 3)                                   | |
| +----------------+     +--------------------------------------------------------------+ |
| |     Data       |                                                              |      |
| |  (requested    |                                                              |      |
| |   memory)      |                                                              |      |
| +----------------+                                                              |      |
+------------------------------------------------------------------------------------------+
|                                   Memory Block 3                                        |
| +----------------+     +--------------------------------------------------------------+ |
| |     Header     | --> | next = NULL (end of heap)                                 | |
| +----------------+     +--------------------------------------------------------------+ |
| |     Data       |                                                              |      |
| |  (requested    |                                                              |      |
| |   memory)      |                                                              |      |
| +----------------+                                                              |      |
+------------------------------------------------------------------------------------------+
|                                 End of Heap (sbrk(0))                                  |
+------------------------------------------------------------------------------------------+

To Do

• Replace sbrk() with mmap()

Resources

• https://arjunsreedharan.org/post/148675821737/memory-allocators-101-write-a-simple-memory
• http://wiki.osdev.org/Memory_Allocation