DUP

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 fBdupfR or fBdup2fR, 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.

DUP