This project has NOT been created as part of the 42 curriculum by madelwau

libasm

Description

LibASM is a small library written in pure 64-bit Assembly (Intel syntax). The goal of this project is to get familiar with assembly language (x86_64), the System V AMD64 ABI calling convention, CPU registers, stack frame alignment, and direct Linux system calls (syscall).

Mandatory part

Recreation of standard C library functions:

• ft_strlen — Calculates the length of a null-terminated string (man 3 strlen).
• ft_strcpy — Copies a string from source to destination (man 3 strcpy).
• ft_strcmp — Compares two strings lexicographically (man 3 strcmp).
• ft_write — Writes data to a file descriptor and sets errno on failure (man 2 write).
• ft_read — Reads data from a file descriptor and sets errno on failure (man 2 read).
• ft_strdup — Duplicates a string using heap allocation (man 3 strdup).

Technical Notes

Calling Convention (System V AMD64 ABI)

• Integer/Pointer Arguments:
  1. RDI
  2. RSI
  3. RDX
  4. RCX
  5. R8
  6. R9
• Return Value: RAX (and RDX for 128-bit values).
• Callee-Saved Registers (must be preserved across calls): RBX, RSP, RBP, R12, R13, R14, R15.
• Caller-Saved Registers (can be overwritten by calls): RAX, RCX, RDX, RSI, RDI, R8, R9, R10, R11.
• Stack Alignment: The stack pointer (RSP) must be aligned to a 16-byte boundary before issuing any call instruction.

Error Handling & errno

Linux system calls return an error code between -1 and -4095 directly in RAX on failure (representing -errno). To match libc behavior:

1. Detect negative return values (rax < 0).
2. Negate RAX to get the positive error code.
3. Call __errno_location wrt ..plt to obtain the memory address of errno.
4. Store the error code at [rax].
5. Return -1.

Instructions

Prerequisites

• nasm (Netwide Assembler)
• gcc / clang
• make

Building the library

% make

Usage exemple

% gcc -Wall -Werror -Wextra -no-pie test.c -L. -lasm
% ./a.out

Ressources

• [Books] Art Of Intel x86 Assembly (No Starch Press)
• [YouTube] Mxy - Learn Assembly for Beginners

AI Usage

AI was used for documentation, debug and to generate tests.