tcgetsid(3)

💖 tcgetsid(3) — bestie edition 💖

okay so real talk, this is tcgetsid, a lil function from the Library Functions Manual and it lives in the terminal-adjacent part of your brain. it's giving "who's your session daddy" energy and honestly? iconic.

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✨ NAME ✨

tcgetsid — get session ID 🎀

📚 LIBRARY 📚

Standard C library — that's libc, link it with -lc. no cap.

🌸 SYNOPSIS 🌸

#define _XOPEN_SOURCE 500        /* See feature_test_macros(7) */
#include <termios.h>

pid_t tcgetsid(int fd);

yes bestie you literally have to #define _XOPEN_SOURCE 500 before the include, and there's a whole feature_test_macros(7) situation if you wanna know why. 💅

🦋 DESCRIPTION 🦋

so what does it do? okay. tcgetsid() hands you back the session ID of the current session — the one whose controlling terminal is the terminal attached to fd. 🔥

and the vibe check here: that terminal has to be the controlling terminal of the calling process. it's not "any terminal you happen to be holding", it's the one your process is emotionally attached to. periodt.

🫶 RETURN VALUE 🫶

when fd actually refers to the controlling terminal of your session — slay — tcgetsid() returns the session ID of that session. easy.

otherwise? it returns -1, and errno gets set to spill the tea about what went wrong. 😭

💀 ERRORS 💀

• EBADF — fd is not a valid file descriptor. girl, that's not a real fd, stop.
• ENOTTY — the calling process does not have a controlling terminal, OR it does have one but it's just not described by fd. either way, not it. 🙈

👑 ATTRIBUTES 👑 (see attributes(7) for what these words even mean)

• Interface: tcgetsid()
  • Attribute: Thread safety
  • Value: MT-Safe ✅ (go off, thread-safe king)

⭐ STANDARDS ⭐

POSIX.1-2008. it's official official.

🌈 HISTORY 🌈

• glibc 2.1. POSIX.1-2001.
• and the fun lore: this function is implemented via the TIOCGSID ioctl(2), which has been present since Linux 2.1.71. vintage but thriving. 🧚

💜 SEE ALSO 💜

• getsid(2)

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and that's the whole page, bestie 🍓. this lil number came to you courtesy of Linux man-pages 6.20, dated 2026-02-08 — shoutout to the man-pages girlies for keeping the docs fresh. now go get that session ID. 💅✨🦋