The function ptsname returns the name of the slave
pseudo-terminal device associated with a master pseudo-terminal device.
fildes is a file descriptor returned from a successful open
of the master device.
ptsname returns a pointer to a
string containing the null-terminated path name of the
slave device of the form /dev/pts/N, where N is an integer between
0 and 255.
Return values
Upon successful completion, the function ptsname returns a
pointer to a string which is the name of the pseudo-terminal slave device.
This value points to a static data area that is
overwritten by each call to ptsname.
Upon failure, ptsname returns NULL.
This could occur if fildes is an invalid file descriptor or if
the slave device name does not exist in the file system.