Originální popis anglicky:
dup, dup2 - duplicate an open file descriptor
Návod, kniha: POSIX Programmer's Manual
#include <unistd.h>
int dup(int
fildes);
int dup2(int
fildes, int
fildes2 );
The
dup() and
dup2() functions provide an alternative interface to
the service provided by
fcntl() using the F_DUPFD command. The call:
shall be equivalent to:
fid = fcntl(fildes, F_DUPFD, 0);
The call:
fid = dup2(fildes, fildes2);
shall be equivalent to:
close(fildes2);
fid = fcntl(fildes, F_DUPFD, fildes2);
except for the following:
- *
- If fildes2 is less than 0 or greater than or equal
to {OPEN_MAX}, dup2() shall return -1 with errno set to
[EBADF].
- *
- If fildes is a valid file descriptor and is equal to
fildes2, dup2() shall return fildes2 without closing
it.
- *
- If fildes is not a valid file descriptor,
dup2() shall return -1 and shall not close fildes2.
- *
- The value returned shall be equal to the value of
fildes2 upon successful completion, or -1 upon failure.
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
- The number of file descriptors in use by this process would
exceed {OPEN_MAX}.
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 following sections are informative.
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);
...
The following example redirects messages from
stderr to
stdout.
#include <unistd.h>
...
dup2(1, 2);
...
None.
The
dup() and
dup2() functions are redundant. Their services are
also provided by the
fcntl() function. They have been included in this
volume of IEEE Std 1003.1-2001 primarily for historical reasons,
since many existing applications use them.
While the brief code segment shown is very similar in behavior to
dup2(),
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.
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() ,
fcntl() ,
open() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<unistd.h>
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
.
