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.
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.
- 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.
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 '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.
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).