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.Dd July 3, 2024
.Dt POSIX_OPENPT 3C
.Os
.Sh NAME
.Nm posix_openpt
.Nd open a pseudo-terminal manager device
.Sh SYNOPSIS
.In stdlib.h
.In fcntl.h
.Ft int
.Fo posix_openpt
.Fa "int oflag"
.Fc
.Sh DESCRIPTION
The
.Fn posix_openpt
function establishes a connection between a manager device for a
pseudo-terminal and a file descriptor.
The file descriptor is used by other I/O functions that refer to that
pseudo-terminal.
.Pp
The file status flags and file access modes of the open file description are
set according to the value of
.Fa oflag .
.Pp
Values for
.Fa oflag
are constructed by a bitwise-inclusive OR of flags from
the following list, defined in
.Xr fcntl.h 3HEAD :
.Bl -tag -width Ds
.It Dv O_RDWR
Open for reading and writing.
.It Dv O_NOCTTY
If set,
.Fn posix_openpt
does not cause the terminal device to become the controlling terminal for the
process.
.El
.Pp
The behavior of other values for the
.Fa oflag
argument is unspecified.
.Sh RETURN VALUES
The
.Fn posix_openpt
function opens a manager pseudo-terminal device and, if successful, returns a
non-negative integer representing the lowest numbered unused file descriptor ;
otherwise, the value
.Sy -1
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh EXAMPLES
.Sy Example 1
Open a pseudo-terminal.
.Pp
The following example opens a pseudo-terminal and returns the name of the
subsidiary device and a file descriptor.
.Bd -literal -offset Ds
#include <fcntl.h>
#include <stdio.h>
#include <err.h>

int managerfd, subsidiaryfd;
char *subsidiarydevice;

if ((managerfd = posix_openpt(O_RDWR|O_NOCTTY)) < 0) {
        err(1, "opening pseudo-terminal manager");
}

if (grantpt(managerfd) != 0 ||
    unlockpt(managerfd) != 0 ||
    (subsidiarydevice = ptsname(managerfd)) == NULL) {
        (void) close(managerfd);
        err(1, "locating pseudo-terminal subsidiary");
}

printf("subsidiary device is: %s\en", subsidiarydevice);

if ((subsidiaryfd = open(subsidiary, O_RDWR|O_NOCTTY)) < 0) {
        err(1, "opening pseudo-terminal subsidiary");
}
.Ed
.Sh ERRORS
The
.Fn posix_openpt
function will fail if:
.Bl -tag -width Er
.It Er EMFILE
.Brq Dv OPEN_MAX
file descriptors are currently open in the calling process.
.It Er ENFILE
The maximum allowable number of files is currently open in the system.
.El
.Pp
The
.Fn posix_openpt
function may fail if:
.Bl -tag -width Er
.It Er EINVAL
The value of
.Fa oflag
is not valid.
.It Er EAGAIN
The system has run out of pseudo-terminal resources.
.It Er ENOSR
The system has run out of STREAMS resources.
.El
.Sh USAGE
This function provides a portable method for obtaining the file descriptor of a
manager terminal device for a pseudo-terminal, as opposed to using
.Xr open 2
on the
.Xr ptm 4D
device which is system-specific.
.Pp
The
.Xr grantpt 3C
function can be used to manipulate the mode and ownership permissions
of the subsidiary device.
The
.Xr ptsname 3C
function can be used to obtain the name of the subsidiary device.
.Sh INTERFACE STABILITY
.Sy Committed
.Sh MT LEVEL
.Sy MT-Safe
.Sh SEE ALSO
.Xr open 2 ,
.Xr grantpt 3C ,
.Xr ptsname 3C ,
.Xr unlockpt 3C ,
.Xr ptm 4D ,
.Xr pts 4D ,
.Xr attributes 7 ,
.Xr standards 7