| DUP(2) | System Calls Manual | DUP(2) |
dup, dup2 —
duplicate an existing file descriptor
Standard C Library (libc, -lc)
#include
<unistd.h>
int
dup(int
oldd);
int
dup2(int
oldd, int
newd);
The
dup()
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.
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.
The dup() system call fails if:
EBADF]EMFILE]The dup2() system call fails if:
EBADF]accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3)
The dup() and
dup2() system calls are expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”).
The dup() function appeared in
Version 3 AT&T UNIX. The
dup2() function appeared in
Version 7 AT&T UNIX.
| December 1, 2017 | Debian |