NAME
dup, dup2 —
duplicate an existing file
descriptor
SYNOPSIS
#include
<unistd.h>
int
dup(int
oldd);
int
dup2(int
oldd, int
newd);
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, it is first deallocated as if a
close(2) call had been done first. When newd
equals oldd, dup2() just
returns without affecting the close-on-exec flag.
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
dup() will fail if:
- [
EBADF] - oldd is not a valid active descriptor.
- [
EMFILE] - Too many descriptors are active.
dup2() will fail if:
- [
EBADF] - oldd is not a valid active descriptor or
newd is negative or greater than or equal to the
process's
RLIMIT_NOFILElimit. - [
EINTR] - An interrupt was received.
- [
EIO] - An I/O error occurred while writing to the file system.
SEE ALSO
accept(2), close(2), fcntl(2), getrlimit(2), open(2), pipe(2), socket(2), socketpair(2), getdtablesize(3)
STANDARDS
dup() and dup2()
are expected to conform to IEEE Std 1003.1-2008
(“POSIX.1”).
HISTORY
The dup() system call first appeared in
Version 3 AT&T UNIX and
dup2() in Version 7 AT&T
UNIX.