NAME
dup, dup2 —
duplicate an existing file
descriptor
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
dup(int
oldd);
int
dup2(int
oldd, int
newd);
DESCRIPTION
Thedup()
system call 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 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) system 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 and oldd ≠
newd, the descriptor is first deallocated as if the
close(2) system call had been used. If oldd is
not a valid descriptor, then newd is not closed. If
oldd == newd and
oldd is a valid descriptor, then
dup2() is successful, and does nothing.
RETURN VALUES
These calls return the new file descriptor if successful; otherwise the value -1 is returned and the external variable errno is set to indicate the cause of the error.
ERRORS
The dup() system call fails if:
- [
EBADF] - The oldd argument is not a valid active descriptor
- [
EMFILE] - Too many descriptors are active.
The dup2() system call fails if:
- [
EBADF] - The oldd argument is not a valid active descriptor or the newd argument is negative or exceeds the maximum allowable descriptor number
SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3)
STANDARDS
The dup() and
dup2() system calls are expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”).
HISTORY
The dup() function appeared in
Version 3 AT&T UNIX. The
dup2() function appeared in
Version 7 AT&T UNIX.