libc/sys/select.2

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.Dd August 1, 2023
.Dt SELECT 2
.Os
.Sh NAME
.Nm select ,
.Nm pselect ,
.Nm FD_SET ,
.Nm FD_CLR ,
.Nm FD_ISSET ,
.Nm FD_ZERO
.Nd synchronous I/O multiplexing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/select.h
.Ft int
.Fn select "int nfds" "fd_set * restrict readfds" "fd_set * restrict writefds" "fd_set * restrict exceptfds" "struct timeval * restrict timeout"
.Ft int
.Fn 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"
.Fn FD_SET "int fd" "fd_set *fdset"
.Fn FD_CLR "int fd" "fd_set *fdset"
.Fn FD_ISSET "int fd" "fd_set *fdset"
.Fn FD_ZERO "fd_set *fdset"
.Sh DESCRIPTION
.Fn select
and
.Fn pselect
examine the I/O descriptor sets whose addresses are passed in
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The first
.Fa nfds
descriptors are checked in each set;
i.e., the descriptors from 0 through
.Fa nfds Ns No \-1
in the descriptor sets are examined.
This means that
.Fa nfds
must be set to the highest file descriptor of the three sets, plus one.
On return,
.Fn select
and
.Fn pselect
replace the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
.Fn select
and
.Fn pselect
return the total number of ready descriptors in all the sets.
.Pp
The descriptor sets are stored as bit fields in arrays of integers.
The following macros are provided for manipulating such descriptor sets:
.Fn FD_ZERO fdset
initializes a descriptor set pointed to by
.Fa fdset
to the null set.
.Fn FD_SET fd fdset
includes a particular descriptor
.Fa fd
in
.Fa fdset .
.Fn FD_CLR fd fdset
removes
.Fa fd
from
.Fa fdset .
.Fn FD_ISSET fd fdset
is non-zero if
.Fa fd
is a member of
.Fa fdset ,
zero otherwise.
The behavior of these macros is undefined if
a descriptor value is less than zero or greater than or equal to
.Dv FD_SETSIZE ,
which is normally at least equal
to the maximum number of descriptors supported by the system.
.Pp
If
.Fa timeout
is a non-null pointer, it specifies a maximum interval to wait for the
selection to complete.
If
.Fa timeout
is a null pointer, the select blocks indefinitely.
To poll without blocking, the
.Fa timeout
argument should be non-null, pointing to a zero-valued timeval or timespec
structure, as appropriate.
.Fa timeout
is not changed by
.Fn select ,
and may be reused on subsequent calls; however, it is good style to
re-initialize it before each invocation of
.Fn select .
.Pp
If
.Fa sigmask
is a non-null pointer, then the
.Fn pselect
function shall replace the signal mask of the caller by the set of
signals pointed to by
.Fa sigmask
before examining the descriptors, and shall restore the signal mask
of the calling thread before returning.
.Pp
Any of
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
may be given as null pointers if no descriptors are of interest.
.Sh NOTES
It is recommended to use the
.Xr poll 2
interface instead, which tends to be more portable and efficient.
.Sh RETURN VALUES
.Fn select
returns the number of ready descriptors that are contained in
the descriptor sets,
or returns \-1 and sets
.Dv errno
if an error occurred.
If the time limit expires,
.Fn select
returns 0.
If
.Fn select
returns with an error,
including one due to an interrupted call,
the descriptor sets will be unmodified.
.Sh EXAMPLES
.Bd -literal
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <err.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/time.h>

int
main(int argc, char **argv)
{
	fd_set read_set;
	struct timeval timeout;
	int ret, fd, i;

	/* file descriptor 1 is stdout */
	fd = 1;

	/* Wait for ten seconds. */
	timeout.tv_sec = 10;
	timeout.tv_usec = 0;

	/* Initialize the read set to null */
	FD_ZERO(&read_set);

	/* Add file descriptor 1 to read_set */
	FD_SET(fd, &read_set);

	/*
	 * Check if data is ready to be read on
	 * file descriptor 1, give up after 10 seconds.
	 */
	ret = select(fd + 1, &read_set, NULL, NULL, &timeout);

	/*
	 * Returned value is the number of file
	 * descriptors ready for I/O, or -1 on error.
	 */
	switch (ret) {
	case \-1:
		err(EXIT_FAILURE, "select() failed");
		break;

	case 0:
		printf("Timeout, no data received.\en");
		break;

	default:
		printf("Data received on %d file descriptor(s)\en", ret);

		/*
		 * select(2) hands back a file descriptor set where
		 * only descriptors ready for I/O are set. These can
		 * be tested using FD_ISSET
		 */
		for (i = 0; i <= fd; i++) {
			if (FD_ISSET(i, &read_set)) {
				printf("Data on file descriptor %d\en", i);
				/* Remove the file descriptor from the set */
				FD_CLR(fd, &read_set);
			}
		}
		break;
	}

	return 0;
}
.Ed
.Sh ERRORS
An error return from
.Fn select
indicates:
.Bl -tag -width Er
.It Bq Er EBADF
One of the descriptor sets specified an invalid descriptor.
.It Bq Er EFAULT
One or more of
.Fa readfds ,
.Fa writefds ,
or
.Fa exceptfds
points outside the process's allocated address space.
.It Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.It Bq Er EINVAL
The specified time limit is invalid.
One of its components is negative or too large.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr gettimeofday 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 2 ,
.Xr getdtablesize 3
.Sh HISTORY
The
.Fn select
function call appeared in
.Bx 4.2 .
The
.Fn pselect
function call first appeared in
.Nx 3.0 .
.Sh BUGS
Although the provision of
.Xr getdtablesize 3
was intended to allow user programs to be written independent
of the kernel limit on the number of open files, the dimension
of a sufficiently large bit field for select remains a problem.
The default bit size of
.Ft fd_set
is based on the symbol
.Dv FD_SETSIZE
(currently 256),
but that is somewhat smaller than the current kernel limit
to the number of open files.
However, in order to accommodate programs which might potentially
use a larger number of open files with select, it is possible
to increase this size within a program by providing
a larger definition of
.Dv FD_SETSIZE
before the inclusion of
.In sys/types.h .
The kernel will cope, and the userland libraries provided with the
system are also ready for large numbers of file descriptors.
.Pp
Note:
.Xr rpc 3
library uses
.Ft fd_set
with the default
.Dv FD_SETSIZE
as part of its ABI.
Therefore, programs that use
.Xr rpc 3
routines cannot change
.Dv FD_SETSIZE .
.Pp
Alternatively, to be really safe, it is possible to allocate
.Ft fd_set
bit-arrays dynamically.
The idea is to permit a program to work properly even if it is
.Xr execve 2 Ns 'd
with 4000 file descriptors pre-allocated.
The following illustrates the technique which is used by
userland libraries:
.Pp
.Bd -literal -offset indent -compact
	fd_set *fdsr;
	int max = fd;

	fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
	    sizeof(fd_mask));
	if (fdsr == NULL) {
		...
		return (-1);
	}
	FD_SET(fd, fdsr);
	n = select(max+1, fdsr, NULL, NULL, &tv);
	...
	free(fdsr);
.Ed
.Pp
.Fn select
should probably have been designed to return the time remaining from the
original timeout, if any, by modifying the time value in place.
Even though some systems stupidly act in this different way, it is
unlikely this semantic will ever be commonly implemented, as the
change causes massive source code compatibility problems.
Furthermore, recent new standards have dictated the current behaviour.
In general, due to the existence of those
non-conforming systems, it is unwise to assume that the timeout
value will be unmodified by the
.Fn select
call, and the caller should reinitialize it on each invocation.
Calculating the delta is easily done by calling
.Xr gettimeofday 2
before and after the call to
.Fn select ,
and using
.Fn timersub
(as described in
.Xr getitimer 2 ) .
.Pp
Internally to the kernel,
.Fn select
works poorly if multiple processes wait on the same file descriptor.