unlockpt —
unlock the slave pseudo-terminal device
Standard C Library (libc, -lc)
#include <stdlib.h>
int
  
  unlockpt(int
    fildes);
The unlockpt() unlocks access to the pseudo-terminal
  device corresponding to the master pseudo-terminal device associated with
  fildes. Conforming applications must call this function
  before opening the slave pseudo-terminal device.
If successful, unlockpt() returns 0; otherwise a value
  of -1 is returned and errno is set to indicate the
  error.
The unlockpt() function will fail if:
  - [EACCESS]
- the corresponding pseudo-terminal device could not be accessed.
- [EBADF]
- fildes is not a valid descriptor.
- [EINVAL]
- fildes is not associated with a master
      pseudo-terminal device.
In NetBSDunlockpt() does
  nothing.
The unlockpt() function conforms to
  IEEE Std 1003.1-2001 (“POSIX.1”). Its
  first release was in X/Open Portability Guide Issue 4,
  Version 2 (“XPG4.2”).