DUP(2) Linux Programmer's Manual DUP(2)
NAME
dup, dup2 - duplicate a file(1,n) 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(1,n) descriptor oldfd.
After successful return of dup or dup2, the old and new descriptors may
be used interchangeably. They share locks, file(1,n) position pointers and
flags; for example, if(3,n) the file(1,n) 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(3,n) neces-
sary.
RETURN VALUE
dup and dup2 return the new descriptor, or -1 if(3,n) an error(8,n) occurred (in(1,8)
which case, errno is set(7,n,1 builtins) appropriately).
ERRORS
EBADF oldfd isn't an open(2,3,n) file(1,n) descriptor, or newfd is out of the
allowed range for file(1,n) descriptors.
EBUSY (Linux only) This may be returned by dup2 during a race condi-
tion with open(2,3,n)() and dup().
EINTR The dup2 call was interrupted by a signal.
EMFILE The process already has the maximum number of file(1,n) descriptors
open(2,3,n) and tried to open(2,3,n) a new one.
WARNING
The error(8,n) 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(2,3,n), any errors that would have been reported at close(2,7,n)()
time(1,2,n), 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(8,n) conditions. POSIX.1 adds EINTR. The EBUSY return is
Linux-specific.
SEE ALSO
close(2,7,n)(2), fcntl(2), open(2,3,n)(2)
Linux 1.1.46 1994-08-21 DUP(2)