off_t(3type)

💖 off_t — file sizes, bestie 💖

okay so this is the off_t page from section 3type of the Linux manual — like, the type-definitions corner of the man pages, iykyk. we got three little friends here: off_t, off64_t, and loff_t, and they're all about how big a file is. let's get into it 🎀

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✨ LIBRARY ✨

Standard C library (libc) 🫶

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🌸 SYNOPSIS 🌸

#include <sys/types.h>

typedef /* ... */  off_t;

#define _LARGEFILE64_SOURCE
#include <sys/types.h>

typedef /* ... */  off64_t [[deprecated]];

#define _GNU_SOURCE
#include <sys/types.h>

typedef /* ... */  loff_t;

okay so notice how off64_t needs that _LARGEFILE64_SOURCE macro and loff_t needs _GNU_SOURCE — and also 👀 off64_t is literally tagged [[deprecated]] in the source. the compiler is giving you the side-eye. not me pointing that out or anything 🙈

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💅 DESCRIPTION 💅

off_t is the one you use for describing file sizes. it's a signed integer type. periodt.

off64_t is a 64-bit version of the type, and it's the one used in glibc. 💖

loff_t is also a 64-bit version, but this one was introduced by the Linux kernel. king behavior tbh 👑

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⭐ STANDARDS ⭐

• off_t — POSIX.1-2024. iconic, standardized, we love to see it ✨
• off64_t — None. literally nobody claimed it. it's giving rogue 🫠
• loff_t — Linux. homegrown Linux energy 🐧

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🦋 HISTORY 🦋

• off_t — POSIX.1-1988. an elder. a legend. been here since the 80s 💫
• off64_t — came from the Large File Summit (LFS). yes that's a real thing that really happened 🏔️
• alsoooo <aio.h> and <stdio.h> have defined off_t since POSIX.1-2008. so like, the glow-up was gradual 🌈

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🎀 NOTES 🎀

on some architectures, the width of off_t can be controlled with the feature test macro _FILE_OFFSET_BITS. so like, it's not always the same size everywhere — vibe check your target arch first, bestie.

and these headers also hand you off_t:

• <aio.h>
• <fcntl.h>
• <stdio.h>
• <sys/mman.h>
• <sys/stat.h>
• <unistd.h>

so if you're wondering where it even came from, surprise, it's everywhere 🫶

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🔥 SEE ALSO 🔥

copy_file_range(2), lseek(2), mmap(2), posix_fadvise(2), pread(2), readahead(2), sync_file_range(2), truncate(2), fseeko(3), lockf(3), posix_fallocate(3), feature_test_macros(7)

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💀 reminder bestie: off64_t is deprecated, so don't go building your whole personality around it. and always check the header before assuming you got the type for free ✨

okay bye ily, this was Linux man-pages 6.20, dated 2026-06-13 — go read feature_test_macros(7) next, it's lowkey a bop 💖🌸🦋