DUP(2) System Calls Manual DUP(2)

dup, dup2, dup3 — duplicate an existing file descriptor

#include <unistd.h>

int
dup(int fildes);

int
dup2(int fildes, int fildes2);

int
dup3(int fildes, int fildes2, int flags);

() duplicates an existing object descriptor and returns its value to the calling process (fildes2 = dup(fildes)). The argument fildes 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(2). 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 fildes and fildes2 in any way. Thus if fildes2 and fildes 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 and close-on-fork flags on the new file descriptor are unset.

In (), the value of the new descriptor fildes2 is specified. If fildes and fildes2 are equal, then dup2() just returns fildes2; no other changes are made to the existing descriptor. Otherwise, if descriptor fildes2 is already in use, it is first deallocated as if a close(2) call had been done first.

The () interface behaves the same as dup2(), except that if fildes and fildes2 are the same value, then dup3() will fail with EINVAL. The flags argument allows control over the initial attributes of the new file descriptor. Values for flags are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

Set the close-on-exec flag on the new descriptor.
Set the close-on-fork flag on the new descriptor.

Upon successful completion, the new file descriptor is returned. Otherwise, a value of -1 is returned and the global integer variable errno is set to indicate the error.

The dup(), and dup2() and dup3() system calls will fail if:

[]
fildes is not an active, valid file descriptor.
[]
Execution is interrupted by a signal.
[]
Too many file descriptors are active.

The dup2() and dup3() system calls will fail if:

[]
fildes2 is negative or greater than the maximum allowable number (see getdtablesize(2)).

The dup3() system call will fail if:

[]
The flags argument is invalid.
[]
The fildes and fildes2 arguments are equal.

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

dup() and dup2() are expected to conform to IEEE Std 1003.1-1988 (“POSIX.1”).

December 1, 2010 BSD 4