dup(2) -- Linux man page
NAME
dup, dup2 - duplicate a file descriptor
SYNOPSIS
#include <unistd.h>
int dup(int oldfd);
int dup2(int oldfd, int newfd);
DESCRIPTION
dup and dup2
create a copy of the file descriptor
oldfd.
After successful return of dup or dup2,
the old and new descriptors may be used interchangeably. They share
locks, file position pointers and flags; for example, if the file
position is modified by using
lseek
on one of the descriptors, the position is also changed for the other.
The two descriptors do not share the close-on-exec flag, however.
dup
uses the lowest-numbered unused descriptor for the new descriptor.
dup2
makes newfd be the copy of oldfd, closing newfd
first if necessary.
RETURN VALUE
dup and dup2
return the new descriptor, or -1 if an error occurred (in which case,
errno
is set appropriately).
ERRORS
- EBADF
-
oldfd
isn't an open file descriptor, or
newfd
is out of the allowed range for file descriptors.
- EMFILE
-
The process already has the maximum number of file
descriptors open and tried to open a new one.
- EINTR
-
The
dup2
call was interrupted by a signal.
- EBUSY
-
(Linux only) This may be returned by
dup2
during a race condition with open() and dup().
WARNING
The error returned by
dup2
is different from that returned by
fcntl(..., F_DUPFD, ...)
when
newfd
is out of range. On some systems
dup2
also sometimes returns
EINVAL
like
F_DUPFD.
BUGS
If
newfd
was open, any errors that would have been reported at
close()
time, are lost. A careful programmer will not use
dup2
without closing
newfd
first.
CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. SVr4 documents additional
EINTR and ENOLINK error conditions. POSIX.1 adds EINTR.
The EBUSY return is Linux-specific.
SEE ALSO
fcntl(2),
open(2),
close(2)
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