DUP(2)DUP(2)NAME
dup, dup2 - duplicate a descriptor
SYNOPSIS
#include <unistd.h>
int dup(int oldd)
int dup2(int oldd, int newd)
DESCRIPTION
Dup duplicates an existing descriptor. The argument oldd is a small
non-negative integer index in the per-process descriptor table. The
value must be less than OPEN_MAX, the size of the table. The new
descriptor returned by the call, let's name it newd, is the lowest num‐
bered descriptor that is not currently in use by the process.
The object referenced by the descriptor does not distinguish between
references using 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 refer‐
ences. 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 the second form of the call, the value of newd desired is specified.
If this descriptor is already in use, the descriptor is first deallo‐
cated as if a close(2) call had been done first. Newd is not closed if
it equals oldd.
RETURN VALUE
The value -1 is returned if an error occurs in either call. The exter‐
nal variable errno indicates the cause of the error.
ERRORS
Dup and dup2 fail if:
[EBADF] Oldd or newd is not a valid active descriptor
[EMFILE] Too many descriptors are active.
NOTES
Dup and dup2 are now implemented using the F_DUPFD function of
fcntl(2), although the old system call interfaces still exist to sup‐
port old programs.
SEE ALSOopen(2), close(2), fcntl(2), pipe(2).
4th Berkeley Distribution May 13, 1986 DUP(2)
