unlockpt — unlock a pseudoterminal master/slave pair
#define _XOPEN_SOURCE /* See feature_test_macros(7) */ #include <stdlib.h>
| int
            unlockpt( | int fd ); | 
The unlockpt() function
      unlocks the slave pseudoterminal device corresponding to the
      master pseudoterminal referred to by fd.
unlockpt() should be called
      before opening the slave side of a pseudoterminal.
When successful, unlockpt()
      returns 0. Otherwise, it returns −1 and sets
      errno appropriately.