lib/libsys/select.2

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.Dd June 25, 2020
.Dt SELECT 2
.Os
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/select.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
The
.Fn select
system call
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 only exceptional condition detectable is out-of-band
data received on a socket.
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.
The
.Fn select
system call
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 not a null pointer, it specifies the maximum interval to wait for the
selection to complete.
System activity can lengthen the interval by
an indeterminate amount.
.Pp
If
.Fa timeout
is a null pointer, the select blocks indefinitely.
.Pp
To effect a poll, the
.Fa timeout
argument should not be a null pointer,
but it should point to a zero-valued timeval structure.
.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
The
.Fn select
system call
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 system call,
the descriptor sets will be unmodified.
.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 of the arguments
.Fa readfds , writefds , exceptfds ,
or
.Fa timeout
points to an invalid address.
.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.
.It Bq Er EINVAL
The
.Fa nfds
argument
was invalid.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr getdtablesize 2 ,
.Xr gettimeofday 2 ,
.Xr kqueue 2 ,
.Xr poll 2 ,
.Xr pselect 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 2 ,
.Xr clocks 7
.Sh NOTES
The default size of
.Dv FD_SETSIZE
is currently 1024.
In order to accommodate programs which might potentially
use a larger number of open files with
.Fn select ,
it is possible
to increase this size by having the program define
.Dv FD_SETSIZE
before the inclusion of any header which includes
.In sys/types.h .
.Pp
If
.Fa nfds
is greater than the number of open files,
.Fn select
is not guaranteed to examine the unused file descriptors.
For historical
reasons,
.Fn select
will always examine the first 256 descriptors.
.Sh STANDARDS
The
.Fn select
system call and
.Fn FD_CLR ,
.Fn FD_ISSET ,
.Fn FD_SET ,
and
.Fn FD_ZERO
macros conform with
.St -p1003.1-2001 .
.Sh HISTORY
The
.Fn select
system call appeared in
.Bx 4.2 .
.Sh BUGS
.St -susv2
allows systems to modify the original timeout in place.
Thus, it is unwise to assume that the timeout value will be unmodified
by the
.Fn select
system call.
.Fx
does not modify the return value, which can cause problems for applications
ported from other systems.