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.\"     @(#)select.2	8.2 (Berkeley) 3/25/94
.\"
.Dd March 25, 1994
.Dt SELECT 2
.Os BSD 4.2
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/time.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 No -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-nil pointer, it specifies a maximum interval to wait for the
selection to complete.  If 
.Fa timeout
is a nil pointer, the select blocks indefinitely.  To effect a poll, the
.Fa timeout
argument should be non-nil, 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 nil 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 getdtablesize 3 ,
.Xr gettimeofday 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 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 size
.Dv FD_SETSIZE
(currently 256) 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 .
.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.
However, it is unlikely this semantic will ever be implemented, as the
change would cause source code compatibility problems.
In general 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.
.Sh HISTORY
The
.Fn select
function call appeared in
.Bx 4.2 .