minilibc

A minimal C standard library implementation written entirely in x86_64 assembly language using NASM.

Overview

This project implements 15 core C library functions in x86_64 assembly, providing a deep dive into low-level system programming and processor architecture. All functions maintain compatibility with the standard C library interface while being optimized for performance at the assembly level.

Implemented Functions

Memory Operations

• memcpy - Copy memory area
• memmove - Copy memory area (handles overlapping regions)
• memset - Fill memory with a constant byte
• memfrob - Frobnicate (XOR) memory area

String Operations

• strlen - Calculate string length
• strchr - Locate character in string
• strrchr - Locate last occurrence of character in string
• strstr - Locate substring
• strcmp - Compare two strings
• strncmp - Compare first n characters of two strings
• strcasecmp - Case-insensitive string comparison
• strcspn - Get length of complementary substring
• strpbrk - Search string for any of a set of characters
• strfry - Randomize string contents

Bit Operations

• ffs - Find first set bit

Architecture & Technical Details

• Target: x86_64 (64-bit) with NASM assembler
• Calling Convention: System V AMD64 ABI (rdi, rsi, rdx, rcx for first 4 args)
• Register Management: Preserves callee-saved registers (rbx, rbp, r12-r15)
• Output Format: ELF64 shared library with position-independent code
• Optimization: Hand-tuned assembly with efficient memory access patterns

Building

make          # Build libasm.so
make clean    # Remove object files
make fclean   # Remove all build artifacts
make re       # Rebuild from scratch

Usage Example

#include <stdio.h>
#include <dlfcn.h>

int main() {
    // Dynamic loading example
    void *lib = dlopen("./libasm.so", RTLD_LAZY);
    if (!lib) {
        fprintf(stderr, "Cannot load library: %s\n", dlerror());
        return 1;
    }
    
    // Get function pointers
    size_t (*my_strlen)(const char*) = dlsym(lib, "strlen");
    int (*my_strcmp)(const char*, const char*) = dlsym(lib, "strcmp");
    
    // Use the functions
    char *str = "Hello, World!";
    printf("Length: %zu\n", my_strlen(str));
    printf("Comparison: %d\n", my_strcmp(str, "Hello, World!"));
    
    dlclose(lib);
    return 0;
}

Or link directly:

gcc your_program.c -L. -lasm -o your_program

Project Structure

minilibc/
├── Makefile
├── README.md
└── src/
    ├── ffs.asm, memcpy.asm, memfrob.asm, memmove.asm, memset.asm
    ├── strcasecmp.asm, strchr.asm, strcmp.asm, strcspn.asm
    ├── strfry.asm, strlen.asm, strncmp.asm, strpbrk.asm
    ├── strrchr.asm, strstr.asm

Requirements

• NASM (Netwide Assembler)
• GNU ld (linker)
• x86_64 processor
• Linux/Unix environment

Author

Guillaume HEIN - @Thyodas | hein.guillaume@gmail.com