DUP2(3C) Standard C Library Functions DUP2(3C)
NAME
dup2, dup3 - duplicate an open file descriptor
SYNOPSIS
#include <unistd.h>
int dup2(
int fildes,
int fildes2);
int dup3(
int fildes,
int fildes2,
int flags);
DESCRIPTION
The
dup2() function causes the file descriptor
fildes2 to refer to
the same file as
fildes. The
fildes argument is a file descriptor
referring to an open file, and
fildes2 is a non-negative integer less
than the current value for the maximum number of open file
descriptors allowed the calling process. See
getrlimit(2). If
fildes2 already refers to an open file, not
fildes, it is closed
first. If
fildes2 refers to
fildes, or if
fildes is not a valid open
file descriptor,
fildes2 will not be closed first.
The
dup2() function is equivalent to
fcntl(
fildes,
F_DUP2FD,
fildes2). The returned file descriptor will have neither the close-
on-exec (
FD_CLOEXEC) or close-on-fork (
FD_CLOFORK) flags set on it,
regardless of what is set on
fildes.
The
dup3() function works similarly to the
dup2() function with two
exceptions. If
fildes and
fildes2 point to the same file descriptor,
-1 is returned and errno set to
EINVAL. The
flags argument allows
for control of the returned file descriptor's flags. Valid values
are the bitwise-inclusive-OR of:
O_CLOEXEC The returned file descriptor will have the close-on-exec
flag,
FD_CLOEXEC set on it. The file descriptor will be
automatically closed when the process calls the
exec(2) family of functions.
O_CLOFORK The returned file descriptor will have the close-on-fork
flag,
FD_CLOFORK set on it. The file descriptor will be
automatically closed in any child processes created with
the
fork(2) family of functions. It will remain open in
the parent.
RETURN VALUES
Upon successful completion a non-negative integer representing the
file descriptor is returned. Otherwise,
-1 is returned and
errno is
set to indicate the error.
ERRORS
The
dup2() and
dup3() functions will fail if:
EBADF The
fildes argument is not a valid open file descriptor.
EBADF The
fildes2 argument is negative or is not less than the
current resource limit returned by
getrlimit(RLIMIT_NOFILE, ...).
EINTR A signal was caught during the
dup2() call.
EMFILE The process has too many open files. See
fcntl(2).
Additionally, the
dup3() function will fail if:
EINVAL flags has a value other than 0 or the bitwise-inclusive-OR
of
O_CLOEXEC and
O_CLOFORK.
fildes and
fildes2 point to the
same file descriptor.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Standard |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
SEE ALSO
close(2),
creat(2),
exec(2),
fcntl(2),
fork(2),
getrlimit(2),
open(2),
pipe(2),
attributes(7),
standards(7) June 21, 2024 DUP2(3C)
