DUP(2)

NAME
dup, dup2, dup3 - duplicate an existing file descriptor

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
dup(int oldd);
int
dup2(int oldd, int newd);
int
dup3(int oldd, int newd, int flags);

DESCRIPTION
dup() duplicates an existing object descriptor and returns its value to the calling process (newd = dup(oldd)). The argument oldd is a small non-negative integer index in the per-process descriptor table. The value must be less than the size of the table, which is returned by getdtablesize(3). The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.

The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate references to an open file, read(2), write(2) and lseek(2) calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open(2) call. The close-on-exec flag on the new file descriptor is unset.

In dup2(), the value of the new descriptor newd is specified. If this descriptor is already in use, the descriptor is first deallocated as if a close(2) call had been done first. If newd and oldd are the same, the call has no effect.

dup3() behaves exactly like dup2() only it allows extra flags to be set on the returned file descriptor. The following flags are valid:

O_CLOEXEC
    Set the “close-on-exec” property.

O_NONBLOCK
    Sets non-blocking I/O.

RETURN VALUES
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.

ERRORS
All three functions may fail if:

[EBADF]
    oldd is not a valid active descriptor or newd is not in the range of valid file descriptors.

The dup() function may also fail if:

[EMFILE]
    Too many descriptors are active.

The dup3() function will also fail if:

[EINVAL]
    flags is other than O_NONBLOCK or O_CLOEXEC.

SEE ALSO
accept(2), close(2), fcntl(2), open(2), pipe(2), socket(2), socketpair(2), getdtablesize(3)

STANDARDS
The dup() and dup2() functions conform to p1003.1-90.

HISTORY
The dup3() function is inspired from Linux and appeared in NetBSD 6.0.