libc/sys/select.2

.\"	$OpenBSD: select.2,v 1.28 2007/05/31 19:19:33 jmc Exp $
.\"	$NetBSD: select.2,v 1.5 1995/06/27 22:32:28 cgd Exp $
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.\"     @(#)select.2	8.2 (Berkeley) 3/25/94
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.Dd $Mdocdate: May 31 2007 $
.Dt SELECT 2
.Os
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/time.h>
.Fd #include <string.h>
.Fd #include <unistd.h>
.Ft int
.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
.Fn FD_SET fd &fdset
.Fn FD_CLR fd &fdset
.Fn FD_ISSET fd &fdset
.Fn FD_ZERO &fdset
.Sh DESCRIPTION
.Fn select
examines 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 -1
in the descriptor sets are examined.
On return,
.Fn select
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
.Fn select
returns 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
.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 effect a poll, the
.Fa timeout
argument should be non-null, pointing to a zero-valued timeval structure.
.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
Any of
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
may be given as null pointers if no descriptors are of interest.
.Sh RETURN VALUES
.Fn select
returns the number of ready descriptors that are contained in
the descriptor sets, or \-1 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 ERRORS
An error return from
.Fn select
indicates:
.Bl -tag -width Er
.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 EBADF
One of the descriptor sets specified an invalid descriptor.
.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 .
.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 1024),
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
.Aq Pa 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
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:
.Bd -literal -offset indent
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
Alternatively, it is possible to use the
.Xr poll 2
interface.
.Xr poll 2
is more efficient when the size of
.Fn select Ns 's
.Ft fd_set
bit-arrays are very large, and for fixed numbers of
file descriptors one need not size and dynamically allocate a
memory object.
.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 brain-damaged
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.
Given that, it is rather surprising to see that many daemons are
written that way (i.e.,
.Xr httpd 8 ) .