DUP(2) | System Calls Manual | DUP(2) |
dup
, dup2
,
dup3
—
#include <unistd.h>
int
dup
(int
oldfd);
int
dup2
(int
oldfd, int
newfd);
int
dup3
(int
oldfd, int newfd,
int flags);
dup
() family of calls duplicates an existing file
descriptor oldfd. A new file descriptor is produced; it
is a new reference to the same underlying system object. The object in
question does not distinguish between the descriptors referencing it in any
way. Thus for files, read(2),
write(2) and
lseek(2) calls all move a single
shared seek position. Similarly, all object modes, settings, properties, and
behavior other than the close-on-exec flag are shared between references. This
includes the setting of append mode, non-blocking I/O actions, asynchronous
I/O operations in progress, socket options, and so forth. The close-on-exec
flag, however, is a property of the descriptor rather than the object and can
be set independently for each reference.
To get an independent handle with its own seek position and settings, an additional open(2) call must be issued. (This is not generally possible for pipes and sockets.)
The dup
() call chooses the new descriptor:
it is the lowest-numbered descriptor not currently in use. The
dup2
() and dup3
() calls
allow the caller to choose the new descriptor by passing
newfd, which must be within the range of valid
descriptors. If newfd is the same as
oldfd, the call has no effect. Otherwise, if
newfd is already in use, it is closed as if
close(2) had been called.
File descriptors are small non-negative integers that index into
the per-process file table. Values 0, 1, and 2 have the special property
that they are treated as standard input, standard output, and standard error
respectively. (The constants STDIN_FILENO
,
STDOUT_FILENO
, and
STDERR_FILENO
are provided as symbolic forms for
these values.) The maximum value for a file descriptor is one less than the
file table size. The file table size can be interrogated with
getdtablesize(3) and
can to some extent be adjusted with
setrlimit(2).
The dup3
() call includes an additional
flags argument supporting a subset of the
open(2) flags:
O_CLOEXEC
O_NONBLOCK
O_NOSIGPIPE
SIGPIPE
when a
write is made to a broken pipe. Instead, the write will fail with
EPIPE
.dup3
() operation.
In the case of dup
() and
dup2
() the close-on-exec flag on the new file
descriptor is always left unset and all the modes and settings of the
underlying object are left unchanged.
Functionality similar to dup
() with
slightly different semantics is also available via
fcntl(2).
dup2
() and dup3
() this is
always the same as newfd. If an error occurs, the value
-1 is returned and errno is set to indicate what
happened.
#include <unistd.h> int fds[2]; pid_t pid; pipe(fds); pid = fork(); if (pid == 0) { /* child; use read end of pipe to stdin */ dup2(fds[0], STDIN_FILENO); close(fds[0]); close(fds[1]); execv("/some/program", args); } /* parent process; return write end of pipe */ close(fds[0]); return fds[1];
dup
() and dup2
() functions
conform to IEEE Std 1003.1-1990
(“POSIX.1”).
dup3
() function originated in Linux and appeared in
NetBSD 6.0.
December 24, 2013 | NetBSD 9.2 |