42 | Unix project

malloc

This project is about implementing a dynamic memory allocation mechanism. Subject

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Overview

This repository contains the source code of my thread-safe memory management library libft_malloc.so.
The supplied shell script allows to run any program with the shared library:

./loader.sh <cmd>

APIs

The library provides an implementation of the libc functions malloc, free, calloc and realloc, and the libbsd function reallocf:

void *malloc(size_t size);
void free(void *ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *ptr, size_t size);
void *reallocf(void *ptr, size_t size);

It also offers two functions that allow visual on the state of the allocated memory:

void show_alloc_mem();
void show_alloc_mem_free();

Implementation

Every pointeurs are 16 bytes aligned. Inspired by ptmalloc2, this malloc handles 3 types of allocations:

• TINY allocations, from 16 to 128 bytes, are stored in larger arenas to avoid requesting pages too often.
• SMALL allocations contain between 129 and 4096 bytes and are stored in arenas for the same reason.
• LARGE allocations, over 4096 bytes, are directly mmap()'ed as needed.

Each allocation is associated with an overhead to form a chunk. The sizes of the chunk and of the previous chunk are always accessible and allow to move from chunk to chunk in both direction. If a chunk is free()'d, it also possesses pointers to its next and to its previous free chunks. Given that the pointers are relevant only in the case of a free chunk, the data of the end-user in an allocated chunk begins right where these pointers begin. Large chunks being not contiguous, their two pointers are however necessary.

Tiny free chunks are store in 8 distinct lists, one for each size (16 bytes step). This makes the transition from allocated to free - and inversly - really fast.
Small free chunks have a similar functioning but are inserted in one doubly linked and circular list, in which they are sorted by increasing size. Large free chunks are munmap()'ed.

Environment variables

The following environment variables changes the behaviour of the allocation related functions:

MALLOC_VERBOSE
Ouptut information about what happens in the functions. If set to `all`, display all level of messages.

MALLOC_LOG_FILE
Append verbosity messages to the specified file.

MALLOC_ERROR_ABORT
Make the program abort() in case of an error in the memory management.

MALLOC_CORRUPTION_ABORT
Same as above, but only in case of memory corruption (a failed malloc() won't abort the program).

MALLOC_SCRIBBLE
Fill newly allocated memory with 0xaa bytes, and fill newly released memory with 0x55 bytes. This makes sure a program doesn't make an assumption about the content of the memory, nor try to access data it just free'd.