These functions operate on the system file descriptors for terminal type devices. These descriptors are not related to the standard I/O
FILE typedef, but refer to the special device files found in
/dev and named
/dev/ttyxx and for which an entry exists in the initialization file
/etc/ttys (see
ttys(5)), or for pseudo-terminal devices created in ptyfs and named
/dev/pts/n.
The
isatty() function determines if the file descriptor
fd refers to a valid terminal type device.
The
ttyname() function gets the related device name of a file descriptor for which
isatty() is true. The
ttyname_r() is the reentrant version of the above, and it places the results in
buf. If there is not enough space to place the results (indicated by
len), then it returns an error.
The
ttyslot() function fetches the current process' control terminal number from the
ttys(5) file entry. If the terminal is a pseudo-terminal, and there is no special entry in the
ttys(5) file for it, the slot number returned is 1 + (last slot number) + minor(tty). This will return a consistent and unique number for each pseudo-terminal device without requiring one to enumerate all of them in
ttys(5).