isatty(3)

💅 isatty(3) — the "is this a terminal or not" check ✨

okay bestie, gather round, because this little function is lowkey one of the most useful things in <unistd.h> and nobody talks about her. her name is isatty(), she lives in the Standard C library (libc, -lc), and her whole job is answering one very important question: is the thing I'm talking to an actual terminal, or is it just some file pretending? 🫶

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✨ SYNOPSIS ✨

#include <unistd.h>

int isatty(int fd);

that's it. one arg, one int. no drama. 💖

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🌸 DESCRIPTION 🌸

so the way that isatty() works is: you hand her an open file descriptor fd, and she tests whether fd is an open file descriptor that refers to a terminal. that's the whole thing. that's the assignment. she ate and left no crumbs. 👑

like — is this stdin attached to your actual keyboard-and-screen situation, or did someone pipe a file into you and now you're just ✨a file✨? isatty() knows. she always knows. 💅

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🎀 RETURN VALUE 🎀

• she returns 1 if fd is an open file descriptor referring to a terminal 🦋
• otherwise she returns 0, and errno gets set to tell you what went wrong 😭

so no, she's not doing the -1 thing. she's a 0/1 girlie. binary. iconic. periodt.

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💀 ERRORS 💀

• EBADF — fd is not a valid file descriptor. that's a you problem, bestie, she can't work with that 🙈
• ENOTTY — fd refers to a file other than a terminal. (ngl, "not a tty" is literally the error name, she's not subtle ✨)

⚠️ history lesson, no cap: on some older kernels, some types of files gave you EINVAL in this case instead. that's a violation of POSIX, which specifies the error should be ENOTTY. so if you're on ancient hardware and something's giving EINVAL where you expected ENOTTY… yeah. that's why. not your fault. 💀

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🌈 ATTRIBUTES 🌈

for the full explanation of what these terms even mean, go read attributes(7) — she explains it better than I can fr fr.

• isatty() — Thread safety: MT-Safe 🔥

so yes, she's thread-safe. multiple threads can vibe-check their file descriptors at the same time and nobody's getting hurt. we love a safe queen. 💖

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⭐ STANDARDS ⭐

POSIX.1-2008.

that's the standard she answers to. certified. official. 💅

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🧚 HISTORY 🧚

• POSIX.1-2001
• SVr4
• 4.3BSD

she's been around. she's seen things. she's a legacy icon and she's still on every system you'll ever touch 👑

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🍓 SEE ALSO 🍓

• fstat(2) — the heavier, more detailed way to inspect a file descriptor, if you want the whole lore
• ttyname(3) — if you already know it's a terminal and now you wanna know which terminal, she'll tell you the name

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so yeah. isatty() — small, fast, returns 1 or 0, sets errno when things go sideways, MT-Safe, POSIX.1-2008, with history going back to POSIX.1-2001, SVr4 and 4.3BSD. go check your file descriptors bestie, she's got you 🫶💖

this page came from Linux man-pages 6.20, dated 2025-05-17, and yeah that's the isatty(3) page — slay, bye ✨