asm-atoi-exit-code

My first x86-64 assembly program for Linux.

What it does

This program reads a line from standard input, tries to parse it as a decimal unsigned integer, and exits with that value as the process exit code.

Behavior:

• reads up to 63 bytes into a fixed buffer
• accepts a trailing newline
• rejects empty input
• rejects any non-digit character
• returns exit code 255 on invalid input
• prints nothing to standard output

Build

nasm -f elf64 main.asm -o main.o
ld main.o -o main

Run

printf '42\n' | ./main
echo $?

Expected exit code:

42

Invalid input example:

printf '12a\n' | ./main
echo $?

Expected exit code:

255

Notes

• The program uses raw Linux syscalls and starts at _start.
• The exit status visible in the shell is limited to the low 8 bits, so values above 255 are truncated by the OS.