TTYNAME(3) Library Functions Manual TTYNAME(3)

ttyname, ttyname_r - return name of a terminal

Standard C library (libc, -lc)

#include <unistd.h>
char *ttyname(int fd);
int ttyname_r(int fd, char *buf, size_t buflen);

The function ttyname() returns a pointer to the null-terminated pathname of the terminal device that is open on the file descriptor fd, or NULL on error (for example, if fd is not connected to a terminal). The return value may point to static data, possibly overwritten by the next call. The function ttyname_r() stores this pathname in the buffer buf of length buflen.

The function ttyname() returns a pointer to a pathname on success. On error, NULL is returned, and errno is set to indicate the error. The function ttyname_r() returns 0 on success, and an error number upon error.

Bad file descriptor.
fd refers to a slave pseudoterminal device but the corresponding pathname could not be found (see NOTES).
fd does not refer to a terminal device.
(ttyname_r()) buflen was too small to allow storing the pathname.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
ttyname () Thread safety MT-Unsafe race:ttyname
ttyname_r () Thread safety MT-Safe

POSIX.1-2001, POSIX.1-2008, 4.2BSD.

A process that keeps a file descriptor that refers to a pts(4) device open when switching to another mount namespace that uses a different /dev/ptmx instance may still accidentally find that a device path of the same name for that file descriptor exists. However, this device path refers to a different device and thus can't be used to access the device that the file descriptor refers to. Calling ttyname() or ttyname_r() on the file descriptor in the new mount namespace will cause these functions to return NULL and set errno to ENODEV.

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

2022-10-09 Linux man-pages 6.01