minix/lib/libc/sys/dup.2

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.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
.\"
.Dd January 23, 2012
.Dt DUP 2
.Os
.Sh NAME
.Nm dup ,
.Nm dup2 ,
.Nm dup3
.Nd duplicate an existing file descriptor
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn dup "int oldd"
.Ft int
.Fn dup2 "int oldd" "int newd"
.Ft int
.Fn dup3 "int oldd" "int newd" "int flags"
.Sh DESCRIPTION
.Fn dup
duplicates an existing object descriptor and returns its value to
the calling process
.Fa ( newd
=
.Fn dup oldd ) .
The argument
.Fa oldd
is a small non-negative integer index in
the per-process descriptor table.
The value must be less than the size of the table, which is returned by
.Xr getdtablesize 3 .
The new descriptor returned by the call
is the lowest numbered descriptor currently not in use by the process.
.Pp
The object referenced by the descriptor does not distinguish
between
.Fa oldd
and
.Fa newd
in any way.
Thus if
.Fa newd
and
.Fa oldd
are duplicate references to an open
file,
.Xr read 2 ,
.Xr write 2
and
.Xr lseek 2
calls all move a single pointer into the file,
and append mode, non-blocking I/O and asynchronous I/O options
are shared between the references.
If a separate pointer into the file is desired, a different
object reference to the file must be obtained by issuing an
additional
.Xr open 2
call.
The close-on-exec flag on the new file descriptor is unset.
.Pp
In
.Fn dup2 ,
the value of the new descriptor
.Fa newd
is specified.
If this descriptor is already
in use, the descriptor is first deallocated as if a
.Xr close 2
call had been done first.
If
.Fa newd
and
.Fa oldd
are the same, the call has no effect.
.Pp
.Fn dup3
behaves exactly like
.Fn dup2
only it allows extra
.Fa flags
to be set on the returned file descriptor.
The following flags are valid:
.Bl -tag -width O_NONBLOCK -offset indent
.It Dv O_CLOEXEC
Set the
.Dq close-on-exec
property.
.It Dv O_NONBLOCK
Sets non-blocking I/O.
.It Dv O_NOSIGPIPE
Return
.Er EPIPE
instead of raising
.Dv SIGPIPE .
.El
.Sh RETURN VALUES
The value \-1 is returned if an error occurs in either call.
The external variable
.Va errno
indicates the cause of the error.
.Sh ERRORS
All three functions may fail if:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa oldd
is not a valid active descriptor
or
.Fa newd
is not in the range of valid file descriptors.
.El
.Pp
The
.Fn dup
function may also fail if:
.Bl -tag -width Er
.It Bq Er EMFILE
Too many descriptors are active.
.El
.Pp
The
.Fn dup3
function will also fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa flags
is other than
.Dv O_NONBLOCK
or
.Dv O_CLOEXEC .
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr getdtablesize 3
.Sh STANDARDS
The
.Fn dup
and
.Fn dup2
functions conform to
.St -p1003.1-90 .
.Sh HISTORY
The
.Fn dup3
function is inspired from Linux and appeared in
.Nx 6.0 .