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FreeBSD 11.0 - man page for grantpt (freebsd section 3)

PTSNAME(3)						   BSD Library Functions Manual 						PTSNAME(3)

grantpt, ptsname, unlockpt -- pseudo-terminal access functions
Standard C Library (libc, -lc)
#include <stdlib.h> int grantpt(int fildes); char * ptsname(int fildes); int unlockpt(int fildes);
The grantpt(), ptsname(), and unlockpt() functions allow access to pseudo-terminal devices. These three functions accept a file descriptor that references the master half of a pseudo-terminal pair. This file descriptor is created with posix_openpt(2). The grantpt() function is used to establish ownership and permissions of the slave device counterpart to the master device specified with fildes. The slave device's ownership is set to the real user ID of the calling process, and the permissions are set to user readable- writable and group writable. The group owner of the slave device is also set to the group ``tty''. The ptsname() function returns the full pathname of the slave device counterpart to the master device specified with fildes. This value can be used to subsequently open the appropriate slave after posix_openpt(2) and grantpt() have been called. The unlockpt() function clears the lock held on the pseudo-terminal pair for the master device specified with fildes.
The grantpt() and unlockpt() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. The ptsname() function returns a pointer to the name of the slave device on success; otherwise a NULL pointer is returned.
The grantpt(), ptsname() and unlockpt() functions may fail and set errno to: [EBADF] fildes is not a valid open file descriptor. [EINVAL] fildes is not a master pseudo-terminal device. In addition, the grantpt() function may set errno to: [EACCES] The slave pseudo-terminal device could not be accessed.
posix_openpt(2), pts(4), tty(4)
The ptsname() function conforms to IEEE Std 1003.1-2008 (``POSIX.1''). This implementation of grantpt() and unlockpt() does not conform to IEEE Std 1003.1-2008 (``POSIX.1''), because it depends on posix_openpt(2) to create the pseudo-terminal device with proper permissions in place. It only validates whether fildes is a valid pseudo-terminal master device. Future revisions of the specification will likely allow this behaviour, as stated by the Austin Group.
The grantpt(), ptsname() and unlockpt() functions appeared in FreeBSD 5.0.
August 20, 2008 BSD