select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - syn
chronous I/O multiplexing
SYNOPSIS
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int n, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
int pselect(int n, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, const struct timespec *timeout,
sigset_t * sigmask);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
DESCRIPTION
The functions select and pselect wait for a number of file
descriptors to change status.
Their function is identical, with three differences:
(i) The select function uses a timeout that is a struct
timeval (with seconds and microseconds), while pse
lect uses a struct timespec (with seconds and
nanoseconds).
(ii) The select function may update the timeout parame
ter to indicate how much time was left. The pselect
function does not change this parameter.
(iii) The select function has no sigmask parameter, and
behaves as pselect called with NULL sigmask.
Three independent sets of 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 descriptors
actually changed status.
Four macros are provided to manipulate the sets. FD_ZERO
will clear a set. FD_SET and FD_CLR add or remove a given
descriptor from a set. FD_ISSET tests to see if a
descriptor is part of the set; this is useful after select
returns.
sets, plus 1.
timeout is an upper bound on the amount of time elapsed
before select returns. It may be zero, causing select to
return immediately. (This is useful for polling.) If time
out is NULL (no timeout), select can block indefinitely.
sigmask is a pointer to a signal mask (see sigproc
mask(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 again.
The idea of pselect is that if one wants to wait for an
event, either a signal or something on a file descriptor,
an atomic test is needed to prevent race conditions. (Sup
pose 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. On the other
hand, pselect allows one to first block signals, handle
the signals that have come in, then call pselect() with
the desired sigmask, avoiding the race.) Since Linux
today does not have a pselect() system call, the current
glibc2 routine still contains this race.
RETURN VALUE
On success, select and pselect return the number of
descriptors contained in the descriptor sets, 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.
EINTR A non blocked signal was caught.
EINVAL n is negative.
ENOMEM select was unable to allocate memory for internal
tables.
NOTES
Some code calls select with all three sets empty, n zero,
and a non-null timeout as a fairly portable way to sleep
with subsecond precision.
On Linux, timeout is modified to reflect the amount of
time not slept; most other implementations do not do this.
This causes problems both when Linux code which reads
code is ported to Linux that reuses a struct timeval for
multiple selects in a loop without reinitializing it.
Consider timeout to be undefined after select returns.
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)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");
exit(0);
}
CONFORMING TO
4.4BSD (the select function first appeared in 4.2BSD).
Generally portable to/from non-BSD systems supporting
clones of the BSD socket layer (including System V vari
ants). However, note that the System V variant typically
sets the timeout variable before exit, but the BSD variant
does not.
The pselect function is defined in IEEE Std 1003.1g-2000
(POSIX.1g). It is found in glibc2.1 and later. Glibc2.0
has a function with this name, that however does not take
a sigmask parameter.
SEE ALSO
accept(2), connect(2), poll(2), read(2), recv(2), send(2),
sigprocmask(2), write(2)
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