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IABSD.fr/src/lib/libutil/openpty.3

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    Date : 2025-06-06 22:01:39
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  • lib/libutil/openpty.3 
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.Dd $Mdocdate: June 6 2025 $
.Dt OPENPTY 3
.Os
.Sh NAME
.Nm getptmfd ,
.Nm openpty ,
.Nm fdopenpty ,
.Nm login_tty ,
.Nm forkpty ,
.Nm fdforkpty
.Nd tty utility functions
.Sh SYNOPSIS
.Lb libutil
.In termios.h
.In util.h
.Ft int
.Fn getptmfd "void"
.Ft int
.Fn openpty "int *amaster" "int *aslave" "char *name" "const struct termios *termp" "const struct winsize *winp"
.Ft int
.Fn fdopenpty "int ptmfd" "int *amaster" "int *aslave" "char *name" "const struct termios *termp" "const struct winsize *winp"
.Ft int
.Fn login_tty "int fd"
.Ft pid_t
.Fn forkpty "int *amaster" "char *name" "const struct termios *termp" "const struct winsize *winp"
.Ft pid_t
.Fn fdforkpty "int ptmfd" "int *amaster" "char *name" "const struct termios *termp" "const struct winsize *winp"
.Sh DESCRIPTION
The
.Fn openpty ,
.Fn login_tty ,
and
.Fn forkpty
functions perform manipulations on ttys and pseudo-ttys.
.Pp
The
.Fn openpty
function finds an available pseudo-tty and returns file descriptors
for the master and slave in
.Fa amaster
and
.Fa aslave .
If
.Fa name
is non-null, the filename of the slave is returned in
.Fa name
(a string of at least 16 characters).
If
.Fa termp
is non-null, the terminal parameters of the slave will be set to the
values in
.Fa termp .
If
.Fa winp
is non-null, the window size of the slave will be set to the values in
.Fa winp .
.Pp
The
.Fn openpty
function allocates the pseudo-tty through the
.Pa /dev/ptm
device (see
.Xr pty 4
for details) which means that its ownership is changed to the UID of
the caller, permissions are set to correct values, and all earlier
uses of that device are revoked (see
.Xr revoke 2
for details).
.Pp
The
.Fn fdopenpty
and
.Fn fdforkpty
functions work like
.Fn openpty
and
.Fn forkpty
but expect a
.Pa /dev/ptm
file descriptor
.Fa ptmfd
obtained from the
.Fn getptmfd
function.
.Pp
The
.Fn login_tty
function prepares for a login on the tty
.Fa fd
(which may be a real tty device, or the slave of a pseudo-tty as
returned by
.Fn openpty )
by creating a new session, making
.Fa fd
the controlling terminal for the current process, setting
.Fa fd
to be the standard input, output, and error streams of the current
process, and closing
.Fa fd .
.Pp
The
.Fn forkpty
function combines
.Fn openpty ,
.Fn fork ,
and
.Fn login_tty
to create a new process operating in a pseudo-tty.
The file
descriptor of the master side of the pseudo-tty is returned in
.Fa amaster ,
and the filename of the slave in
.Fa name
if it is non-null.
The
.Fa termp
and
.Fa winp
parameters, if non-null, will determine the terminal attributes and
window size of the slave side of the pseudo-tty.
.Sh RETURN VALUES
If a call to
.Fn openpty ,
.Fn login_tty ,
or
.Fn forkpty
is not successful, \-1 is returned and
.Va errno
is set to indicate the error.
Otherwise,
.Fn openpty ,
.Fn login_tty ,
and the child process of
.Fn forkpty
return 0, and the parent process of
.Fn forkpty
returns the process ID of the child process.
.Sh FILES
.Bl -tag -width /dev/tty[p-zP-T][0-9a-zA-Z]x -compact
.It Pa /dev/pty[p-zP-T][0-9a-zA-Z]
master pseudo terminals
.It Pa /dev/tty[p-zP-T][0-9a-zA-Z]
slave pseudo terminals
.It Pa /dev/ptm
pseudo terminal management device
.El
.Sh ERRORS
.Fn getptmfd
may fail and set
.Va errno
for any of the errors specified for the routine
.Xr open 2 .
.Pp
.Fn openpty
and
.Fn fdopenpty
will fail if:
.Bl -tag -width Er
.It Bq Er ENOENT
There are no available ttys.
.El
.Pp
.Fn fdopenpty
and
.Fn fdforkpty
will fail if
.Fn getptmfd
fails.
.Fn forkpty
and
.Fn fdforkpty
will fail if either
.Fn openpty
or
.Fn fork
fails.
.Pp
.Fn login_tty
will fail if
.Fn ioctl
fails to set
.Fa fd
to the controlling terminal of the current process.
.Sh SEE ALSO
.Xr fork 2 ,
.Xr revoke 2 ,
.Xr pty 4