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SELECT(2) Linux Programmer's Manual SELECT(2)
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
/* According to POSIX.1-2001 */
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
pselect(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
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 the
corresponding I/O operation (e.g., read(2)) without blocking.
The operation of select() and pselect() is identical, other than
these three differences:
(i) select() uses a timeout that is a struct timeval (with seconds
and microseconds), while pselect() uses a struct timespec
(with seconds and nanoseconds).
(ii) select() may update the timeout argument to indicate how much
time was left. pselect() does not change this argument.
(iii) select() has no sigmask argument, and behaves as pselect()
called with NULL sigmask.
Three independent sets of file descriptors are watched. Those 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), those in
writefds will be watched to see if a write will not block, and those
in exceptfds will be watched for exceptions. On exit, the sets are
modified in place to indicate which file descriptors actually changed
status. 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() respectively 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 is the highest-numbered file descriptor in any of the three
sets, plus 1.
The timeout argument specifies the minimum interval that select()
should block waiting for a file descriptor to become ready. (This
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 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.1-2001 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.1-2001
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) which may be zero if the timeout expires before anything
interesting happens. On error, -1 is returned, and errno is set
appropriately; the sets and timeout become undefined, so do not rely
on their contents after an error.
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.)
EINTR A signal was caught; see signal(7).
EINVAL nfds is negative or the value contained within timeout is
invalid.
ENOMEM 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 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.
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.
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.1-2001
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.1-2001 situation is that
one should include <sys/select.h> for select() and pselect().
Libc4 and libc5 do not have a <sys/select.h> header; under glibc 2.0
and later this header exists. Under glibc 2.0 it unconditionally
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.
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 the file descriptor reopened between the time select()
returned and the I/O operations was 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 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 sigset_t *ss; /* Pointer to signal set */
size_t ss_len; /* Size (in bytes) of object pointed
to by 'ss' */
};
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.
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.
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.)
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.1-2001. 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/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'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), 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 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-08-17 SELECT(2)
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