### 17.1 Identifying Terminals

The functions described in this chapter only work on files that correspond to terminal devices. You can find out whether a file descriptor is associated with a terminal by using the `isatty` function.

Prototypes for the functions in this section are declared in the header file `unistd.h`.

— Function: int isatty (int filedes)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> This function returns `1` if filedes is a file descriptor associated with an open terminal device, and 0 otherwise.

If a file descriptor is associated with a terminal, you can get its associated file name using the `ttyname` function. See also the `ctermid` function, described in Identifying the Terminal.

— Function: char * ttyname (int filedes)

> Preliminary: | MT-Unsafe race:ttyname | AS-Unsafe heap lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

> If the file descriptor filedes is associated with a terminal device, the `ttyname` function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file. The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined.

— Function: int ttyname_r (int filedes, char *buf, size_t len)

> Preliminary: | MT-Safe | AS-Unsafe heap | AC-Unsafe mem fd | See POSIX Safety Concepts.

> The `ttyname_r` function is similar to the `ttyname` function except that it places its result into the user-specified buffer starting at buf with length len.

> The normal return value from `ttyname_r` is 0. Otherwise an error number is returned to indicate the error. The following `errno` error conditions are defined for this function:

> `EBADF`
>   The filedes argument is not a valid file descriptor.
> `ENOTTY`
>   The filedes is not associated with a terminal.
> `ERANGE`
>   The buffer length len is too small to store the string to be returned.