select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
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);
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 pselect uses a struct
timespec (with seconds and nanoseconds).
(ii) The select function may update the timeout parameter to indicate
how much time was left. The pselect function does not change
(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.
n is the highest-numbered descriptor in any of the three 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 timeout is NULL (no timeout), select can block
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 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. (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. 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.
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.
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.
Some code calls select with all three sets empty, n zero, and a nonnull
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 timeout is ported to other operating systems,
and when 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.
struct timeval tv;
/* Watch stdin (fd 0) to see when it has input. */
/* 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! */
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
printf("No data within five seconds.\n");
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 variants). However, note that the System V variant
typically sets the timeout variable before exit, but the BSD variant
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.
accept(2), connect(2), poll(2), read(2), recv(2), send(2), sigproc-
Linux 1.2 1996-02-11 SELECT(2)
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