|
PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
DUP(3P) POSIX Programmer's Manual DUP(3P)
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.
dup, dup2 — duplicate an open file descriptor
#include <unistd.h>
int dup(int fildes);
int dup2(int fildes, int fildes2);
The dup() function provides an alternative interface to the service
provided by fcntl() using the F_DUPFD command. The call dup(fildes)
shall be equivalent to:
fcntl(fildes, F_DUPFD, 0);
The dup2() function shall cause the file descriptor fildes2 to refer
to the same open file description as the file descriptor fildes and
to share any locks, and shall return fildes2. If fildes2 is already
a valid open file descriptor, it shall be closed first, unless fildes
is equal to fildes2 in which case dup2() shall return fildes2 without
closing it. If the close operation fails to close fildes2, dup2()
shall return −1 without changing the open file description to which
fildes2 refers. If fildes is not a valid file descriptor, dup2()
shall return −1 and shall not close fildes2. If fildes2 is less than
0 or greater than or equal to {OPEN_MAX}, dup2() shall return −1 with
errno set to [EBADF].
Upon successful completion, if fildes is not equal to fildes2, the
FD_CLOEXEC flag associated with fildes2 shall be cleared. If fildes
is equal to fildes2, the FD_CLOEXEC flag associated with fildes2
shall not be changed.
If fildes refers to a typed memory object, the result of the dup2()
function is unspecified.
Upon successful completion a non-negative integer, namely the file
descriptor, shall be returned; otherwise, −1 shall be returned and
errno set to indicate the error.
The dup() function shall fail if:
EBADF The fildes argument is not a valid open file descriptor.
EMFILE All file descriptors available to the process are currently
open.
The dup2() function shall fail if:
EBADF The fildes argument is not a valid open file descriptor or the
argument fildes2 is negative or greater than or equal to
{OPEN_MAX}.
EINTR The dup2() function was interrupted by a signal.
The dup2() function may fail if:
EIO An I/O error occurred while attempting to close fildes2.
The following sections are informative.
Redirecting Standard Output to a File S
The following example closes standard output for the current
processes, re-assigns standard output to go to the file referenced by
pfd, and closes the original file descriptor to clean up.
#include <unistd.h>
...
int pfd;
...
close(1);
dup(pfd);
close(pfd);
...
Redirecting Error Messages
The following example redirects messages from stderr to stdout.
#include <unistd.h>
...
dup2(1, 2);
...
Implementations may use file descriptors that must be inherited into
child processes for the child process to remain conforming, such as
for message catalog or tracing purposes. Therefore, an application
that calls dup2() with an arbitrary integer for fildes2 risks non-
conforming behavior, and dup2() can only portably be used to
overwrite file descriptor values that the application has obtained
through explicit actions, or for the three file descriptors
corresponding to the standard file streams. In order to avoid a race
condition of leaking an unintended file descriptor into a child
process, an application should consider opening all file descriptors
with the FD_CLOEXEC bit set unless the file descriptor is intended to
be inherited across exec.
The dup() function is redundant. Its services are also provided by
the fcntl() function. It has been included in this volume of
POSIX.1‐2008 primarily for historical reasons, since many existing
applications use it. On the other hand, the dup2() function provides
unique services, as no other interface is able to atomically replace
an existing file descriptor.
The dup2() function is not marked obsolescent because it presents a
type-safe version of functionality provided in a type-unsafe version
by fcntl(). It is used in the POSIX Ada binding.
The dup2() function is not intended for use in critical regions as a
synchronization mechanism.
In the description of [EBADF], the case of fildes being out of range
is covered by the given case of fildes not being valid. The
descriptions for fildes and fildes2 are different because the only
kind of invalidity that is relevant for fildes2 is whether it is out
of range; that is, it does not matter whether fildes2 refers to an
open file when the dup2() call is made.
None.
close(3p), fcntl(3p), open(3p)
The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 DUP(3P)
Pages that refer to this page: unistd.h(0p), sh(1p), fstatvfs(3p), open(3p), posix_spawn(3p), posix_spawn_file_actions_addclose(3p), posix_spawn_file_actions_adddup2(3p), posix_typed_mem_open(3p), shm_open(3p), write(3p)