синхронное мультиплексирование ввода / вывода (synchronous I/O multiplexing)
Имя (Name)
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
I/O multiplexing
Синопсис (Synopsis)
#include <sys/select.h>
int select(int nfds, fd_set *restrict readfds,
fd_set *restrict writefds, fd_set *restrict exceptfds,
struct timeval *restrict 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 *restrict readfds,
fd_set *restrict writefds, fd_set *restrict exceptfds,
const struct timespec *restrict timeout,
const sigset_t *restrict sigmask);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
pselect():
_POSIX_C_SOURCE >= 200112L
Описание (Description)
WARNING: select() can monitor only file descriptors numbers that
are less than FD_SETSIZE (1024)—an unreasonably low limit for
many modern applications—and this limitation will not change.
All modern applications should instead use poll(2) or epoll(7),
which do not suffer this limitation.
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.
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 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,
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
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.
