MiniLibC

This project is a custom implementation of several standard C library functions written entirely in x86-64 assembly (NASM). The functions are compiled into a dynamic shared library (.so) that can be loaded and used in C programs.

Implemented Functions

The following standard library functions have been re-implemented:

• strlen
• strchr
• strrchr
• strcmp
• strncmp
• strcasecmp
• strstr
• strpbrk
• strcspn
• memset
• memcpy
• memmove

Prerequisites

To build and test the library, you will need the following tools:

• make
• nasm (Netwide Assembler)
• gcc (GNU Compiler Collection)
• criterion (for running the unit tests)

Building the Library

You can use the provided Makefile to build the shared library libasm.so.

• Build the library:

  make

• Clean object files:

  make clean

• Clean all generated files (library and objects):

  make fclean

• Rebuild the library from scratch:

  make re

Running Tests

The repository includes a comprehensive test suite using the Criterion framework. The tests compare the behavior of each custom assembly function against its original C standard library counterpart.

• To compile and run the unit tests:

  make test_run

Usage

You can use the generated libasm.so in your own C projects by dynamically loading it.

Here is an example of how to use the custom strlen function from this library.

1. First, build the library:

   make

2. Create a C file, for example, main.c:

   #include <stdio.h>
   #include <dlfcn.h>
   #include <stdlib.h>
   
   int main(void) {
       void *handle;
       size_t (*my_strlen)(const char *);
       char *error;
   
       // Load the shared library
       handle = dlopen("./libasm.so", RTLD_LAZY);
       if (!handle) {
           fprintf(stderr, "Error loading library: %s\n", dlerror());
           exit(EXIT_FAILURE);
       }
   
       // Clear any existing error
       dlerror();
   
       // Get a pointer to the custom strlen function
       *(void **)(&my_strlen) = dlsym(handle, "strlen");
       error = dlerror();
       if (error != NULL) {
           fprintf(stderr, "Error finding function: %s\n", error);
           exit(EXIT_FAILURE);
       }
   
       // Use the function
       const char *str = "Hello from MiniLibC!";
       printf("String: \"%s\"\n", str);
       printf("Length (using my_strlen): %zu\n", my_strlen(str));
   
       // Unload the library
       dlclose(handle);
   
       return 0;
   }

3. Compile your C program, linking against the dynamic linking library (-ldl):

   gcc -o my_program main.c -ldl

4. Run your executable. Ensure libasm.so is in the same directory or in your LD_LIBRARY_PATH.

   ./my_program

   Output:

   String: "Hello from MiniLibC!"
   Length (using my_strlen): 20