man-pages/man3p/dup.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "DUP" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" dup 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
dup, dup2 \- duplicate an open file descriptor
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int dup(int\fP \fIfildes\fP\fB);
.br
int dup2(int\fP \fIfildes\fP\fB, int\fP \fIfildes2\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIdup\fP() and \fIdup2\fP() functions provide an alternative
interface to the service provided by \fIfcntl\fP() using the F_DUPFD
command. The call:
.sp
.RS
.nf

\fBfid = dup(\fP\fIfildes\fP\fB);
\fP
.fi
.RE
.LP
shall be equivalent to:
.sp
.RS
.nf

\fBfid = fcntl(\fP\fIfildes\fP\fB, F_DUPFD, 0);
\fP
.fi
.RE
.LP
The call:
.sp
.RS
.nf

\fBfid = dup2(\fP\fIfildes\fP\fB,\fP \fIfildes2\fP\fB);
\fP
.fi
.RE
.LP
shall be equivalent to:
.sp
.RS
.nf

\fBclose(\fP\fIfildes2\fP\fB);
fid = fcntl(\fP\fIfildes\fP\fB, F_DUPFD,\fP \fIfildes2\fP\fB);
\fP
.fi
.RE
.LP
except for the following:
.IP " *" 3
If \fIfildes2\fP is less than 0 or greater than or equal to {OPEN_MAX},
\fIdup2\fP() shall return -1 with \fIerrno\fP set to
[EBADF].
.LP
.IP " *" 3
If \fIfildes\fP is a valid file descriptor and is equal to \fIfildes2\fP,
\fIdup2\fP() shall return \fIfildes2\fP without
closing it.
.LP
.IP " *" 3
If \fIfildes\fP is not a valid file descriptor, \fIdup2\fP() shall
return -1 and shall not close \fIfildes2\fP.
.LP
.IP " *" 3
The value returned shall be equal to the value of \fIfildes2\fP upon
successful completion, or -1 upon failure.
.LP
.SH RETURN VALUE
.LP
Upon successful completion a non-negative integer, namely the file
descriptor, shall be returned; otherwise, -1 shall be
returned and \fIerrno\fP set to indicate the error.
.SH ERRORS
.LP
The \fIdup\fP() function shall fail if:
.TP 7
.B EBADF
The \fIfildes\fP argument is not a valid open file descriptor.
.TP 7
.B EMFILE
The number of file descriptors in use by this process would exceed
{OPEN_MAX}.
.sp
.LP
The \fIdup2\fP() function shall fail if:
.TP 7
.B EBADF
The \fIfildes\fP argument is not a valid open file descriptor or the
argument \fIfildes2\fP is negative or greater than or
equal to {OPEN_MAX}.
.TP 7
.B EINTR
The \fIdup2\fP() function was interrupted by a signal.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Redirecting Standard Output to a File
.LP
The following example closes standard output for the current processes,
re-assigns standard output to go to the file referenced
by \fIpfd\fP, and closes the original file descriptor to clean up.
.sp
.RS
.nf

\fB#include <unistd.h>
\&...
int pfd;
\&...
close(1);
dup(pfd);
close(pfd);
\&...
\fP
.fi
.RE
.SS Redirecting Error Messages
.LP
The following example redirects messages from \fIstderr\fP to \fIstdout\fP.
.sp
.RS
.nf

\fB#include <unistd.h>
\&...
dup2(1, 2);
\&...
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The \fIdup\fP() and \fIdup2\fP() functions are redundant. Their services
are also provided by the \fIfcntl\fP() function. They have been included
in this volume of IEEE\ Std\ 1003.1-2001
primarily for historical reasons, since many existing applications
use them.
.LP
While the brief code segment shown is very similar in behavior to
\fIdup2\fP(), a conforming implementation based on other
functions defined in this volume of IEEE\ Std\ 1003.1-2001 is significantly
more complex. Least obvious is the possible
effect of a signal-catching function that could be invoked between
steps and allocate or deallocate file descriptors. This could be
avoided by blocking signals.
.LP
The \fIdup2\fP() function is not marked obsolescent because it presents
a type-safe version of functionality provided in a
type-unsafe version by \fIfcntl\fP(). It is used in the POSIX Ada
binding.
.LP
The \fIdup2\fP() function is not intended for use in critical regions
as a synchronization mechanism.
.LP
In the description of [EBADF], the case of \fIfildes\fP being out
of range is covered by the given case of \fIfildes\fP not
being valid. The descriptions for \fIfildes\fP and \fIfildes2\fP are
different because the only kind of invalidity that is
relevant for \fIfildes2\fP is whether it is out of range; that is,
it does not matter whether \fIfildes2\fP refers to an open
file when the \fIdup2\fP() call is made.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIclose\fP(), \fIfcntl\fP(), \fIopen\fP(), the
Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .