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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 Controlling 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:


The filedes argument is not a valid file descriptor.


The filedes is not associated with a terminal.


The buffer length len is too small to store the string to be returned.


The filedes is associated with a terminal device that is a slave pseudo-terminal, but the file name associated with that device could not be determined. This is a GNU extension.

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