_newselect.2.gz
SELECT
NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO -
synchronous I/O multiplexing
SYNOPSIS
/* 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); I void FD_CLR(int fd , fd_set * set );
I int FD_ISSET(int fd , fd_set * set );
I void FD_SET(int fd , fd_set * set );
I void FD_ZERO(fd_set * set ); #define _XOPEN_SOURCE 600 #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);
DESCRIPTION
R select ()
and
R 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
R select ()
and
R pselect ()
is identical, with three differences:
(i)
R select ()
uses a timeout that is a
struct timeval
(with seconds and microseconds), while
R pselect ()
uses a
struct timespec
(with seconds and nanoseconds).
(ii)
R select ()
may update the
timeout
argument to indicate how much time was left.
R pselect ()
does not change this argument.
(iii)
R select ()
has no
sigmask
argument, and behaves as
R pselect ()
called with NULL
R 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.
R FD_ZERO ()
clears a set.
R FD_SET ()
and
R FD_CLR ()
respectively add and remove a given file descriptor from a set.
R FD_ISSET ()
tests to see if a file descriptor is part of the set;
this is useful after
R select ()
returns.
nfds
is the highest-numbered file descriptor in any of the three sets, plus 1.
timeout
is an upper bound on the amount of time elapsed before
R select ()
returns.
It may be zero, causing
R select ()
to return immediately.
(This is useful for polling.)
If
timeout
is NULL (no timeout),
R select ()
can block indefinitely.
sigmask
is a pointer to a signal mask (see
sigprocmask(2));
if it is not NULL, then
R pselect ()
first replaces the current signal mask by the one pointed to by
R 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
R pselect ()
call:
ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);
is equivalent to
atomically
executing the following calls:
sigset_t origmask;
sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);
The reason that
R 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
R select ()
could hang indefinitely if the signal arrived just after the test
but just before the call.
By contrast,
R pselect ()
allows one to first block signals, handle the signals that have come in,
then call
R pselect ()
with the desired
R sigmask ,
avoiding the race.)
The timeout
The time structures involved are defined in <sys/time.h> and look likestruct 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
R 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,
R 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
R select ()s
in a loop without reinitializing it.
Consider
timeout
to be undefined after
R select ()
returns.
RETURN VALUE
On success,
R select ()
and
R 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
R readfds ,
R writefds ,
R 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.
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.)
EINTR
A signal was caught.
EINVAL
nfds
is negative or the value contained within
timeout
is invalid.
ENOMEM
unable to allocate memory for internal tables.
VERSIONS
R pselect ()
was added to Linux in kernel 2.6.16.
Prior to this,
R pselect ()
was emulated in glibc (but see BUGS).
CONFORMING TO
R 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.
R pselect ()
is defined in POSIX.1g, and in
POSIX.1-2001.
NOTES
An
fd_set
is a fixed size buffer.
Executing
R FD_CLR ()
or
R 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 longs (as shown above),
and the structure is defined in
R <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
R <sys/types.h> .
Concerning prototypes, the classical situation is that one should
include
<time.h>
for
R select ().
The POSIX.1-2001 situation is that one should include
<sys/select.h>
for
R select ()
and
R 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
R pselect (),
under glibc 2.1-2.2.1 it gives
R pselect ()
when
_GNU_SOURCE
is defined, under glibc 2.2.2-2.2.4 it gives it when
_XOPEN_SOURCE
is defined and has a value of 600 or larger.
No doubt, since POSIX.1-2001, it should give the prototype by default.
Linux Notes
The Linux R pselect () 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 R pselect () function does not modify its timeout argument; this is the behavior required by POSIX.1-2001.BUGS
Glibc 2.0 provided a version of
R pselect ()
that did not take a
sigmask
argument.
Since version 2.1, glibc has provided an emulation of
R pselect ()
that is implemented using
sigprocmask(2)
and
R select ().
This implementation remains vulnerable to the very race condition that
R pselect ()
was designed to prevent.
On systems that lack
R pselect ()
reliable (and more portable) signal trapping can be achieved
using the self-pipe trick
(where a signal handler writes a byte to a pipe whose other end
is monitored by
R select ()
in the main program.)
Under Linux,
R 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,
R 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(2)
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 <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);
}
SEE ALSO
For a tutorial with discussion and examples, see
select_tut(2).
For vaguely related stuff, see
accept(2),
connect(2),
poll(2),
read(2),
recv(2),
send(2),
sigprocmask(2),
write(2),
epoll(7),
feature_test_macros(7)
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