Section (2) select
Name
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO — synchronous I/O multiplexing
Synopsis
/* According to POSIX.1-2001, POSIX.1-2008 */ #include <sys/select.h> /* According to earlier standards */ #include <sys/time.h> #include <sys/types.h> #include <unistd.h>
int
select( |
int nfds, |
fd_set *readfds, | |
fd_set *writefds, | |
fd_set *exceptfds, | |
struct timeval *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) ; |
#include <sys/select.h>
int
pselect( |
int nfds, |
fd_set *readfds, | |
fd_set *writefds, | |
fd_set *exceptfds, | |
const struct timespec *timeout, | |
const sigset_t *sigmask) ; |
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Note | ||
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|
DESCRIPTION
select
() and pselect
() allow 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) does not have this
limitation. See BUGS.
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), whilepselect
() uses a struct timespec (with seconds and nanoseconds). -
select
() may update thetimeout
argument to indicate how much time was left.pselect
() does not change this argument. -
select
() has nosigmask
argument, and behaves aspselect
() called with NULLsigmask
.
Three independent sets of file descriptors are watched.
The file descriptors listed in readfds
will be watched to see
if characters become available for reading (more precisely,
to see if a read will not block; in particular, a file
descriptor is also ready on end-of-file). The file
descriptors in writefds
will be watched to see
if space is available for write (though a large write may
still block). The file descriptors in exceptfds
will be watched for
exceptional conditions. (For examples of some exceptional
conditions, see the discussion of POLLPRI
in poll(2).)
On exit, each of the file descriptor sets is modified in
place to indicate which file descriptors actually changed
status. (Thus, if using select
() within a loop, the sets must be
reinitialized before each call.)
Each of the three file descriptor sets may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.
Four macros are provided to manipulate the sets.
FD_ZERO
() clears a set.
FD_SET
() and FD_CLR
() add and remove a given file
descriptor from a set. FD_ISSET
() tests to see if a file
descriptor is part of the set; this is useful after
select
() returns.
nfds
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).
The timeout
argument 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 NULL (no timeout),
select
() can block
indefinitely.
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.
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
The time structures involved are defined in <
sys/time.h
>
and look like
struct timeval { long tv_sec
; /* seconds */long tv_usec
; /* microseconds */};
and
struct timespec { long tv_sec
; /* seconds */long tv_nsec
; /* nanoseconds */};
(However, see below on the POSIX.1 versions.)
Some code calls select
()
with all three sets empty, nfds
zero, and a non-NULL
timeout
as a fairly
portable way to sleep with subsecond precision.
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.
RETURN VALUE
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
) which may be zero if
the timeout expires before anything interesting happens. On
error, −1 is returned, and errno
is set to indicate the error; the file
descriptor sets are unmodified, and timeout
becomes undefined.
ERRORS
- EBADF
-
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.
- EINTR
-
A signal was caught; see signal(7).
- EINVAL
-
nfds
is negative or exceeds theRLIMIT_NOFILE
resource limit (see getrlimit(2)). - EINVAL
-
The value contained within
timeout
is invalid. - ENOMEM
-
Unable to allocate memory for internal tables.
VERSIONS
pselect
() was added to Linux
in kernel 2.6.16. Prior to this, pselect
() was emulated in glibc (but see
BUGS).
CONFORMING TO
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 exit, but the BSD variant does
not.
pselect
() is defined in
POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.
NOTES
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.)
Concerning the types involved, the classical situation is
that the two fields of a timeval structure are typed as
long (as shown above), and the
structure is defined in <
sys/time.h
>
The POSIX.1 situation is
struct timeval { time_t tv_sec
; /* seconds */suseconds_t tv_usec
; /* microseconds */};
where the structure is defined in <
sys/select.h
>
and the data types time_t and suseconds_t are defined in <
sys/types.h
>
Concerning prototypes, the classical situation is that one
should include <
time.h
>
for
select
(). The POSIX.1 situation
is that one should include <
sys/select.h
>
for select
() and pselect
().
Under glibc 2.0, <
sys/select.h
>
gives the wrong prototype for
pselect
(). Under glibc 2.1 to
2.2.1, it gives pselect
() when
_GNU_SOURCE
is defined. Since
glibc 2.2.2, the requirements are as shown in the
SYNOPSIS.
Correspondence between select() and poll() notifications
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 */
Multithreaded applications
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.
C library/kernel differences
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 _zsingle_quotesz_ss_zsingle_quotesz_ */};
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.
BUGS
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) instead.
The implementation of the fd_set
arguments as
value-result arguments means that they must be reinitialized
on each call to select
(). This
design error is avoided by poll(2), which uses
separate structure fields for the input and output of the
call.
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.
Glibc 2.0 provided a version of pselect
() that did not take a sigmask
argument.
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.
Under 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 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.
EXAMPLE
#include <stdio.h> #include <stdlib.h> #include <sys/time.h> #include <sys/types.h> #include <unistd.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_zsingle_quotesz_t rely on the value of tv now! */ if (retval == −1) perror(select()); else if (retval) printf(Data is available now. ); /* FD_ISSET(0, &rfds) will be true. */ else printf(No data within five seconds. ); exit(EXIT_SUCCESS); }
SEE ALSO
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).
COLOPHON
This page is part of release 5.04 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. %%%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. |