Octal integer literalLiteral

Explanation

A base-8 integer literal, prefixed with 0o, as in 0o755.

Note the letter o, not a digit 0 — unlike C's ambiguous leading-zero octal notation (0755), Rust requires the explicit 0o prefix, so a literal like 0755 is just decimal 755, never accidentally misinterpreted as octal.

Usage examples

Assigning a file permission mode

let mode = 0o644; // <- `0o` prefix marks a base-8 (octal) integer literal

Restriction: only digits 0–7 may appear after the 0o prefix — 0o8 is a compile error (underscores and a type suffix like 0o644_u16 are still allowed).

Bit manipulation and flags

Unix-style file permission bits are conventionally written and read in octal, since each digit maps exactly onto one rwx triplet.

let mode: u32 = 0o755; // <- octal literal: owner rwx, group rx, other rx — matches `ls -l` directly

let owner_bits = (mode >> 6) & 0o7;
let group_bits = (mode >> 3) & 0o7;
let other_bits = mode & 0o7;

assert_eq!((owner_bits, group_bits, other_bits), (0o7, 0o5, 0o5));

Unix permission tooling (chmod 755, ls -l) is itself built around octal because each digit is one 3-bit rwx group — a mapping that's lost if the same mode is written in hex or decimal; for masks that aren't naturally 3-bit-grouped, hex is the more common convention — see integer-hexadecimal.

Explanation

Embedded support: Full

Octal literals mean exactly the same thing under #![no_std], but honestly there's little embedded-specific to say beyond that: register addresses and bitmasks are conventionally written in hex or binary (see integer-hexadecimal and integer-binary), and bare-metal firmware has no Unix-style file-permission model for octal's one real everyday use case to attach to. Where 0o does show up in genuine embedded Rust is on the "embedded Linux" side of the ecosystem — a program running on an SBC-class device (a Raspberry Pi controlling a production line, say) under embedded Linux still has a real filesystem, so file and sysfs device permissions look exactly like they would in any other Rust program built for Linux. On a genuinely bare-metal, no-filesystem target, octal is legal but essentially unused.

Usage examples

Setting sysfs GPIO export permissions on an embedded Linux board

use std::fs;
use std::os::unix::fs::PermissionsExt;

fn set_gpio_permissions(path: &str) -> std::io::Result<()> {
    let perms = fs::Permissions::from_mode(0o660); // <- octal literal: same Unix permission convention as any hosted Linux program
    fs::set_permissions(path, perms)
}