ttyname(3)

✨ ttyname(3) ✨

okay bestie so this one lives in the Library Functions Manual (that's the 3 in ttyname(3), iykyk) — it's a libc situation, specifically Standard C library (that's libc, aka -lc if you're out here linking by hand like a menace). no cap. 💅

---

🌸 NAME 🌸

ttyname, ttyname_r — return name of a terminal

like, the whole vibe is "which terminal am I, actually?" 🎀

---

🦋 LIBRARY 🦋

Standard C library — libc, -lc

---

✨ SYNOPSIS ✨

#include <unistd.h>

char *ttyname(int fd);
int ttyname_r(size_t size;
              int fd, char buf[size], size_t size);

(yeah that second one has a lil semicolon situation going on in the signature. we copy it exactly. no notes, that's just how it is, periodt 💀)

---

💖 DESCRIPTION 💖

okay so! ttyname() hands you back a pointer to the null-terminated pathname of the terminal device that's open on the file descriptor fd. 🌈

or, if things go sideways, it returns NULL — for example when fd isn't even connected to a terminal. rude of it but okay. 😭

⚠️ big one bestie: the return value may point to static data — and that static data can get overwritten by the next call. so don't go holding onto that pointer like it's a forever thing. it is not. it's giving borrowed, not owned.

meanwhile ttyname_r() is the respectful one 🫶 — it takes that pathname and stores it in the buffer buf, which is size bytes big.

---

👑 RETURN VALUE 👑

• ttyname() — slays on success and returns a pointer to a pathname. on error it returns NULL and sets errno to indicate the error. 🙈
• ttyname_r() — returns 0 on success, and an error number when it's an error.

---

💀 ERRORS 💀

• EBADF — Bad file descriptor. (bestie. that fd is not fd-ing. 🫠)
• ENODEV — fd refers to a slave pseudoterminal device but the corresponding pathname could not be found (peep NOTES for the whole saga).
• ENOTTY — fd does not refer to a terminal device. nope, not a terminal, go home. 🚫
• ERANGE (only from ttyname_r()) — size was way too small to allow storing the pathname. the buffer said "no room, ma'am." 🎀

---

⭐ ATTRIBUTES ⭐

for the whole explanation of what these terms even mean, see attributes(7). the receipts:

• ttyname()
  • Attribute: Thread safety
  • Value: MT-Unsafe race:ttyname 🔥
• ttyname_r()
  • Attribute: Thread safety
  • Value: MT-Safe 👑

so yeah if you're threading, ttyname_r() is the main character and ttyname() is out here racing. pick wisely.

---

🧚 STANDARDS 🧚

POSIX.1-2008.

---

🌈 HISTORY 🌈

POSIX.1-2001, 4.2BSD. she's been around. vintage. iconic. ✨

---

🍓 NOTES 🍓

okay this part is lowkey important so listen up. 🫶

a process that keeps a file descriptor referring to a pts(4) device open while switching to another mount namespace that uses a different /dev/ptmx instance may still accidentally find that a device path of the same name exists for that file descriptor.

but bestie. that device path refers to a different device — so it can't be used to access the device that the file descriptor actually refers to. 💀 like, the name matches but it's a whole different person. catfished by a pathname.

and calling ttyname() or ttyname_r() on that file descriptor in the new mount namespace will make these functions return NULL and set errno to ENODEV. so that's why that error exists. mystery solved, we ate. 🎀

---

💜 SEE ALSO 💜

• tty(1)
• fstat(2)
• ctermid(3)
• isatty(3)
• pts(4)

---

and that's the whole page, bestie!! go read your terminals. 🌸

this lil gem comes from Linux man-pages 6.20, dated 2026-02-08 — yes queen, 2026, she's fresh. 🦋💖