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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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.1‐2017, unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 DUP(3P)
Pages that refer to this page: unistd.h(0p), sh(1p), close(3p), 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)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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