freverse

freverse is a low-level utility written in x86-64 assembly for Linux that reverses the contents of a file in place.
It was developed as a university assignment at the University of Warsaw (MIMUW).

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Overview

The program takes one argument: a file path.
It reverses the file’s content efficiently using memory mapping (mmap) and direct memory manipulation.

Behavior

• If the file is smaller than 2 bytes, it is left unchanged.
• The program works correctly with very large files, including those larger than 4 GiB.
• All system resources (file descriptors, mapped memory) are explicitly released before exit.
• On error, the program terminates with exit code 1. On success, it exits with 0.
• Nothing is printed to standard output or error.

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Usage

./freverse file

• file must be a valid, accessible file.
• If the argument count is invalid, or a system call fails, the program exits with code 1.

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Build Instructions

The program builds with NASM and ld. A Makefile is provided.

make
make run ARGS=path/to/file
make clean

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Implementation Details

• Written in x86-64 assembly using the Linux syscall interface.
• Relies on:
  • open, fstat, mmap, msync, munmap, close, exit
• Efficient reversal algorithm:
  • Uses qword-by-qword swapping with bswap where possible.
  • Falls back to byte-by-byte swapping for the tail section.
• Memory safety and cleanup are prioritized:
  • File descriptors and memory mappings are tracked in registers and closed/unmapped explicitly.

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Notes

• This program was created as part of coursework.
• It is provided as an educational example of systems programming in assembly.