select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO — synchronous I/O multiplexing
#include <sys/select.h>
int
select( |
int nfds, |
fd_set *restrict readfds, | |
fd_set *restrict writefds, | |
fd_set *restrict exceptfds, | |
struct timeval *restrict timeout) ; |
void
FD_CLR( |
int fd, |
fd_set *set) ; |
int
FD_ISSET( |
int fd, |
fd_set *set) ; |
void
FD_SET( |
int fd, |
fd_set *set) ; |
void
FD_ZERO( |
fd_set *set) ; |
int
pselect( |
int nfds, |
fd_set *restrict readfds, | |
fd_set *restrict writefds, | |
fd_set *restrict exceptfds, | |
const struct timespec *restrict timeout, | |
const sigset_t *restrict sigmask) ; |
Note | |||
---|---|---|---|
|
select
() allows a program to
monitor multiple file descriptors, waiting until one or more
of the file descriptors become "ready" for some class of I/O
operation (e.g., input possible). A file descriptor is
considered ready if it is possible to perform a corresponding
I/O operation (e.g., read(2), or a sufficiently
small write(2)) without
blocking.
select
() can monitor only
file descriptors numbers that are less than FD_SETSIZE
; poll(2) and epoll(7) do not have this
limitation. See BUGS.
The principal arguments of select
() are three "sets" of file
descriptors (declared with the type fd_set
), which allow the
caller to wait for three classes of events on the specified
set of file descriptors. Each of the fd_set
arguments may be
specified as NULL if no file descriptors are to be watched
for the corresponding class of events.
Note well: Upon
return, each of the file descriptor sets is modified in
place to indicate which file descriptors are currently
"ready". Thus, if using select
() within a loop, the sets
must be
reinitialized before each call. The
implementation of the fd_set
arguments as
value-result arguments is a design error that is avoided in
poll(2) and epoll(7).
The contents of a file descriptor set can be manipulated using the following macros:
FD_ZERO
()This macro clears (removes all file descriptors
from) set
. It
should be employed as the first step in initializing
a file descriptor set.
FD_SET
()This macro adds the file descriptor fd
to set
. Adding a file
descriptor that is already present in the set is a
no-op, and does not produce an error.
FD_CLR
()This macro removes the file descriptor fd
from set
. Removing a file
descriptor that is not present in the set is a no-op,
and does not produce an error.
FD_ISSET
()select
() modifies
the contents of the sets according to the rules
described below. After calling select
(), the FD_ISSET
() macro can be used to
test if a file descriptor is still present in a set.
FD_ISSET
() returns
nonzero if the file descriptor fd
is present in
set
, and zero
if it is not.
The arguments of select
()
are as follows:
readfds
The file descriptors in this set are watched to see if they are ready for reading. A file descriptor is ready for reading if a read operation will not block; in particular, a file descriptor is also ready on end-of-file.
After select
() has
returned, readfds
will be cleared
of all file descriptors except for those that are
ready for reading.
writefds
The file descriptors in this set are watched to see if they are ready for writing. A file descriptor is ready for writing if a write operation will not block. However, even if a file descriptor indicates as writable, a large write may still block.
After select
() has
returned, writefds
will be
cleared of all file descriptors except for those that
are ready for writing.
exceptfds
The file descriptors in this set are watched for
"exceptional conditions". For examples of some
exceptional conditions, see the discussion of
POLLPRI
in poll(2).
After select
() has
returned, exceptfds
will be
cleared of all file descriptors except for those for
which an exceptional condition has occurred.
nfds
This argument should be set to the highest-numbered file descriptor in any of the three sets, plus 1. The indicated file descriptors in each set are checked, up to this limit (but see BUGS).
timeout
The timeout
argument is a
timeval structure
(shown below) that specifies the interval that
select
() should block
waiting for a file descriptor to become ready. The
call will block until either:
a file descriptor becomes ready;
the call is interrupted by a signal handler; or
the timeout expires.
Note that the timeout
interval will
be rounded up to the system clock granularity, and
kernel scheduling delays mean that the blocking
interval may overrun by a small amount.
If both fields of the timeval structure are zero, then
select
() returns
immediately. (This is useful for polling.)
If timeout
is specified as NULL, select
() blocks indefinitely
waiting for a file descriptor to become ready.
The pselect
() system call
allows an application to safely wait until either a file
descriptor becomes ready or until a signal is caught.
The operation of select
()
and pselect
() is identical,
other than these three differences:
select
() uses a
timeout that is a struct
timeval (with seconds and microseconds),
while pselect
() uses a
struct
timespec (with seconds and
nanoseconds).
select
() may update
the timeout
argument to indicate how much time was left.
pselect
() does not
change this argument.
select
() has no
sigmask
argument, and behaves as pselect
() called with NULL
sigmask
.
sigmask
is a
pointer to a signal mask (see sigprocmask(2)); if it is
not NULL, then pselect
()
first replaces the current signal mask by the one pointed
to by sigmask
, then
does the "select" function, and then restores the original
signal mask. (If sigmask
is NULL, the signal
mask is not modified during the pselect
() call.)
Other than the difference in the precision of the
timeout
argument,
the following pselect
()
call:
ready = pselect(nfds, &readfds, &writefds, &exceptfds, timeout, &sigmask);
is equivalent to atomically
executing the
following calls:
sigset_t origmask; pthread_sigmask(SIG_SETMASK, &sigmask, &origmask); ready = select(nfds, &readfds, &writefds, &exceptfds, timeout); pthread_sigmask(SIG_SETMASK, &origmask, NULL);
The reason that pselect
()
is needed is that if one wants to wait for either a signal
or for a file descriptor to become ready, then an atomic
test is needed to prevent race conditions. (Suppose the
signal handler sets a global flag and returns. Then a test
of this global flag followed by a call of select
() could hang indefinitely if the
signal arrived just after the test but just before the
call. By contrast, pselect
()
allows one to first block signals, handle the signals that
have come in, then call pselect
() with the desired sigmask
, avoiding the
race.)
The timeout
argument for select
() is a
structure of the following type:
struct timeval { time_t tv_sec
; /* seconds */suseconds_t tv_usec
; /* microseconds */};
The corresponding argument for pselect
() has the following type:
struct timespec { time_t tv_sec
; /* seconds */long tv_nsec
; /* nanoseconds */};
On Linux, select
()
modifies timeout
to
reflect the amount of time not slept; most other
implementations do not do this. (POSIX.1 permits either
behavior.) This causes problems both when Linux code which
reads timeout
is
ported to other operating systems, and when code is ported
to Linux that reuses a struct
timeval for multiple select
()s in a loop without
reinitializing it. Consider timeout
to be undefined after
select
() returns.
On success, select
() and
pselect
() return the number of
file descriptors contained in the three returned descriptor
sets (that is, the total number of bits that are set in
readfds
, writefds
, exceptfds
). The return value
may be zero if the timeout expired before any file
descriptors became ready.
On error, −1 is returned, and errno
is set to indicate the error; the file
descriptor sets are unmodified, and timeout
becomes undefined.
An invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or one on which an error has occurred.) However, see BUGS.
A signal was caught; see signal(7).
nfds
is
negative or exceeds the RLIMIT_NOFILE
resource limit (see
getrlimit(2)).
The value contained within timeout
is invalid.
Unable to allocate memory for internal tables.
pselect
() was added to Linux
in kernel 2.6.16. Prior to this, pselect
() was emulated in glibc (but see
BUGS).
select
() conforms to
POSIX.1-2001, POSIX.1-2008, and 4.4BSD (select
() first appeared in 4.2BSD).
Generally portable to/from non-BSD systems supporting clones
of the BSD socket layer (including System V variants).
However, note that the System V variant typically sets the
timeout variable before returning, but the BSD variant does
not.
pselect
() is defined in
POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.
An fd_set
is a
fixed size buffer. Executing FD_CLR
() or FD_SET
() with a value of fd
that is negative or is equal
to or larger than FD_SETSIZE
will result in undefined behavior. Moreover, POSIX requires
fd
to be a valid file
descriptor.
The operation of select
()
and pselect
() is not affected
by the O_NONBLOCK
flag.
On some other UNIX systems, select
() can fail with the error
EAGAIN if the system fails to
allocate kernel-internal resources, rather than ENOMEM as Linux does. POSIX specifies this
error for poll(2), but not for
select
(). Portable programs may
wish to check for EAGAIN and
loop, just as with EINTR.
On systems that lack pselect
(), reliable (and more portable)
signal trapping can be achieved using the self-pipe trick.
In this technique, a signal handler writes a byte to a pipe
whose other end is monitored by select
() in the main program. (To avoid
possibly blocking when writing to a pipe that may be full
or reading from a pipe that may be empty, nonblocking I/O
is used when reading from and writing to the pipe.)
Before the advent of usleep(3), some code
employed a call to select
()
with all three sets empty, nfds
zero, and a non-NULL
timeout
as a fairly
portable way to sleep with subsecond precision.
Within the Linux kernel source, we find the following
definitions which show the correspondence between the
readable, writable, and exceptional condition notifications
of select
() and the event
notifications provided by poll(2) and epoll(7):
#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR) /* Ready for reading */ #define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR) /* Ready for writing */ #define POLLEX_SET (EPOLLPRI) /* Exceptional condition */
If a file descriptor being monitored by select
() is closed in another thread, the
result is unspecified. On some UNIX systems, select
() unblocks and returns, with an
indication that the file descriptor is ready (a subsequent
I/O operation will likely fail with an error, unless
another process reopens file descriptor between the time
select
() returned and the I/O
operation is performed). On Linux (and some other systems),
closing the file descriptor in another thread has no effect
on select
(). In summary, any
application that relies on a particular behavior in this
scenario must be considered buggy.
The Linux kernel allows file descriptor sets of
arbitrary size, determining the length of the sets to be
checked from the value of nfds
. However, in the glibc
implementation, the fd_set
type is fixed in
size. See also BUGS.
The pselect
() interface
described in this page is implemented by glibc. The
underlying Linux system call is named pselect6
(). This system call has somewhat
different behavior from the glibc wrapper function.
The Linux pselect6
()
system call modifies its timeout
argument. However,
the glibc wrapper function hides this behavior by using a
local variable for the timeout argument that is passed to
the system call. Thus, the glibc pselect
() function does not modify its
timeout
argument;
this is the behavior required by POSIX.1-2001.
The final argument of the pselect6
() system call is not a
sigset_t * pointer,
but is instead a structure of the form:
struct { const kernel_sigset_t * ss
; /* Pointer to signal set */size_t ss_len
; /* Size (in bytes) of object
pointed to by \(aqss\(aq */};
This allows the system call to obtain both a pointer to the signal set and its size, while allowing for the fact that most architectures support a maximum of 6 arguments to a system call. See sigprocmask(2) for a discussion of the difference between the kernel and libc notion of the signal set.
POSIX allows an implementation to define an upper limit,
advertised via the constant FD_SETSIZE
, on the range of file
descriptors that can be specified in a file descriptor set.
The Linux kernel imposes no fixed limit, but the glibc
implementation makes fd_set
a fixed-size type,
with FD_SETSIZE
defined as
1024, and the FD_*
() macros
operating according to that limit. To monitor file
descriptors greater than 1023, use poll(2) or epoll(7) instead.
According to POSIX, select
()
should check all specified file descriptors in the three file
descriptor sets, up to the limit nfds−1
. However, the
current implementation ignores any file descriptor in these
sets that is greater than the maximum file descriptor number
that the process currently has open. According to POSIX, any
such file descriptor that is specified in one of the sets
should result in the error EBADF.
Starting with version 2.1, glibc provided an emulation of
pselect
() that was implemented
using sigprocmask(2) and
select
(). This implementation
remained vulnerable to the very race condition that
pselect
() was designed to
prevent. Modern versions of glibc use the (race-free)
pselect
() system call on
kernels where it is provided.
On Linux, select
() may
report a socket file descriptor as "ready for reading", while
nevertheless a subsequent read blocks. This could for example
happen when data has arrived but upon examination has the
wrong checksum and is discarded. There may be other
circumstances in which a file descriptor is spuriously
reported as ready. Thus it may be safer to use O_NONBLOCK
on sockets that should not
block.
On Linux, select
() also
modifies timeout
if
the call is interrupted by a signal handler (i.e., the
EINTR error return). This is
not permitted by POSIX.1. The Linux pselect
() system call has the same
behavior, but the glibc wrapper hides this behavior by
internally copying the timeout
to a local variable and
passing that variable to the system call.
#include <stdio.h> #include <stdlib.h> #include <sys/select.h> int main(void) { fd_set rfds; struct timeval tv; int retval; /* Watch stdin (fd 0) to see when it has input. */ FD_ZERO(&rfds); FD_SET(0, &rfds); /* Wait up to five seconds. */ tv.tv_sec = 5; tv.tv_usec = 0; retval = select(1, &rfds, NULL, NULL, &tv); /* Don't rely on the value of tv now! */ if (retval == −1) perror("select()"); else if (retval) printf("Data is available now.\n"); /* FD_ISSET(0, &rfds) will be true. */ else printf("No data within five seconds.\n"); exit(EXIT_SUCCESS); }
accept(2), connect(2), poll(2), read(2), recv(2), restart_syscall(2), send(2), sigprocmask(2), write(2), epoll(7), time(7)
For a tutorial with discussion and examples, see select_tut(2).
This page is part of release 5.11 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
This manpage is copyright (C) 1992 Drew Eckhardt, copyright (C) 1995 Michael Shields, copyright (C) 2001 Paul Sheer, copyright (C) 2006, 2019 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified 1993-07-24 by Rik Faith <faithcs.unc.edu> Modified 1995-05-18 by Jim Van Zandt <jrvvanzandt.mv.com> Sun Feb 11 14:07:00 MET 1996 Martin Schulze <joeylinux.de> * layout slightly modified Modified Mon Oct 21 23:05:29 EDT 1996 by Eric S. Raymond <esrthyrsus.com> Modified Thu Feb 24 01:41:09 CET 2000 by aeb Modified Thu Feb 9 22:32:09 CET 2001 by bert hubert <ahuds9a.nl>, aeb Modified Mon Nov 11 14:35:00 PST 2002 by Ben Woodard <benzork.net> 2005-03-11, mtk, modified pselect() text (it is now a system call in 2.6.16. |