FD_SET (3) - Linux Manuals
NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing
SYNOPSIS
#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
DESCRIPTION
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,
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.)
struct timeval {
The corresponding argument for
pselect()
has the following type:
struct timespec {
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 error, -1 is returned, and
errno
is set to indicate the error;
the file descriptor sets are unmodified,
and
timeout
becomes undefined.
pselect()
is defined in POSIX.1g, and in
POSIX.1-2001 and POSIX.1-2008.
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.
#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
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 {
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.
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>.
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.
int
main(void)
{
The timeout
The
timeout
argument for
select()
is a structure of the following type:
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).
The return value may be zero if the timeout expired before any
file descriptors became ready.
ERRORS
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 returning, but the BSD variant does not.
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 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):
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.
Historical glibc details
Glibc 2.0 provided an incorrect version of
pselect()
that did not take a
sigmask
argument.
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)
or
epoll(7)
instead.
EXAMPLES
#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>