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.Dd August 6, 2025 .Dt POSIX_OPENPT 3 .Os .Sh NAME .Nm posix_openpt .Nd open a pseudo-terminal device .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdlib.h n fcntl.h .Ft int .Fn posix_openpt "int oflag" .Sh DESCRIPTION The .Fn posix_openpt function searches for an unused master pseudo-terminal device, opens it, and returns a file descriptor associated with the now used pseudo-terminal device. The .Fa oflag argument has the same meaning as in the .Xr open 2 call. .Sh RETURN VALUES If successful, .Fn posix_openpt returns a non-negative integer, which corresponds to a file descriptor pointing to the master pseudo-terminal device. Otherwise, a value of -1 is returned and .Va errno is set to indicate the error.
p Note that unlike implementations on some other operating systems, .Fn posix_openpt does not return .Er EINVAL if a flag supplied in .Fa oflag would be deemed invalid, instead it is simply ignored. This means it is not possible to dynamically test which .Xr open 2 flags are possible to set, and apply a fallback if .Er EINVAL is received. However, this is unlikely to be a concern in practice, as flags such as .Dv O_NONBLOCK , .Dv O_CLOEXEC , and .Dv O_CLOFORK are supported. .Sh SEE ALSO .Xr ioctl 2 , .Xr open 2 , .Xr grantpt 3 , .Xr ptsname 3 , .Xr unlockpt 3 .Sh RATIONALE The standards committee did not want to directly expose the cloning device, thus decided to wrap the functionality in this function. The equivalent code would be: d -literal int posix_openpt(int oflag) { return open("/dev/ptmx", oflag); } .Ed .Sh STANDARDS The .Fn posix_openpt function conforms to .St -p1003.1-2001 . .Sh HISTORY This function first appeared in .Nx 3 . Support for non-standard flags appeared in .Nx 10 .