Dascălu Ștefan-Nicolae 321CA

Mini-libc

Objectives

• Learn about the structure and functionalities provided by the standard C library.
• Acquaint with the syscall interface in Linux.
• Gain a better understanding of strings and memory management functions.
• Learn how the standard C library supports low-level input/output operations.

Statement

Build a minimalistic standard C library implementation for Linux systems (named mini-libc) that can be used as a replacement for the system libc (glibc in Linux). The goal is to have a minimally functional libc with features such as string management, basic memory support, and POSIX file I/O.

The implementation of mini-libc will be freestanding, meaning it will not use any outside library calls. It will be built on top of the system call interface provided by Linux on an x86_64 architecture. Any function you require that is typically part of libc must be implemented by you. Functions that you implement can be reused in other parts of the mini-libc.

If using a macOS device with ARM64/Aarch64, an x86_64 virtual machine must be installed.

Support Code

The support code consists of three directories:

• libc/: The skeleton mini-libc implementation where you will implement the missing parts marked as TODO items.
• samples/: Contains use cases and tests for mini-libc.
• tests/: Includes states used to validate (and grade) the assignment.

System call invocation is done via the syscall() function defined in libc/syscall.c, which calls the architecture-specific function in libc/internal/arch/x86_64/syscall_arch.h, thus depending on the x86_64 architecture.

API and Implementation Tasks

The application programming interface (API) of the C standard library is declared in several header files. For your minimal implementation, the following header files are of interest:

• <string.h>: Defines string-handling functions. Implement the following functions:

  • strcpy(), strcat(), strlen(), strncpy(), strncat(), strcmp(), strncmp(), strstr(), strrstr(), memcpy(), memset(), memmove(), memcmp().

• <stdio.h>: Defines printing and I/O functions. Implement:

  • puts().

• <unistd.h>, <sys/fcntl.h>, and <sys/stat.h>: Define I/O primitives. Implement:

  • open(), close(), lseek(), stat(), fstat(), truncate(), ftruncate(), nanosleep(), and sleep().

• <stdlib.h> and <sys/mman.h>: Define memory allocation functions. Implement:

  • malloc(), free(), calloc(), realloc(), realloc_array(), mmap(), mremap(), and munmap().

• <errno.h> and errno.c: Declare and define the integer variable errno, which is set by system calls and some library functions to indicate errors.

Some tests may not build; this is intentional. You will need to add the missing features to compile those tests, including:

• The time.h header.
• The declaration and implementation of puts(), nanosleep(), and sleep().
• Updating the libc Makefile to build the source code files for puts(), nanosleep(), and sleep().

Building mini-libc

To build mini-libc, run make in the libc/ directory:

student@so:~/.../assignments/mini-libc$ cd libc/
student@so:~/.../assignments/mini-libc/libc$ make