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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
SELECT(2) Linux Programmer's Manual SELECT(2)
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
#include <sys/select.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);
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
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.
File descriptor sets
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.
Arguments
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.
pselect()
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 sig‐
nal 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 sig‐
mask, avoiding the race.)
The timeout
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 unde‐
fined 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.
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 the RLIMIT_NOFILE resource limit
(see getrlimit(2)).
EINVAL 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, 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.
The self-pipe trick
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.)
Emulating usleep(3)
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.
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. How‐
ever, 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 argu‐
ment; 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 '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. See
sigprocmask(2) for a discussion of the difference between the kernel
and libc notion of the signal set.
Historical glibc details
Glibc 2.0 provided an incorrect version of pselect() that did not
take a sigmask argument.
In glibc versions 2.1 to 2.2.1, one must define _GNU_SOURCE in order
to obtain the declaration of pselect() from <sys/select.h>.
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 5.08 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/.
Linux 2020-04-11 SELECT(2)
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