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DUP(2) | System Calls Manual | DUP(2) |
NAME¶
dup
, dup2
—
duplicate an existing file descriptor
LIBRARY¶
Standard C Library (libc, -lc)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¶
Thedup
() system call fails if:
- [
EBADF
] - The oldd argument is not a valid active descriptor
- [
EMFILE
] - Too many descriptors are active.
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¶
Thedup
() and
dup2
() system calls are expected to conform
to IEEE Std 1003.1-1990
(“POSIX.1”).
HISTORY¶
Thedup
() and
dup2
() functions appeared in
Version 7 AT&T UNIX.June 1, 2013 | Debian |