.\" Copyright (c) 1983, 1991 The Regents of the University of California.
.\" %sccs.include.redist.man%
.\" @(#)select.2 6.8 (Berkeley) %G%
.Nd synchronous I/O multiplexing
.Fd #include <sys/types.h>
.Fd #include <sys/time.h>
.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
examines the I/O descriptor sets whose addresses are passed in
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
descriptors are checked in each set;
i.e., the descriptors from 0 through
in the descriptor sets are examined.
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
returns the total number of ready descriptors in all the sets.
The descriptor sets are stored as bit fields in arrays of integers.
The following macros are provided for manipulating such descriptor sets:
initializes a descriptor set
includes a particular descriptor
The behavior of these macros is undefined if
a descriptor value is less than zero or greater than or equal to
which is normally at least equal
to the maximum number of descriptors supported by the system.
is a non-nil pointer, it specifies a maximum interval to wait for the
selection to complete. If
is a nil pointer, the select blocks indefinitely. To affect a poll, the
argument should be non-nil, pointing to a zero-valued timeval structure.
may be given as nil pointers if no descriptors are of interest.
returns the number of ready descriptors that are contained in
or -1 if an error occurred.
If the time limit expires,
including one due to an interrupted call,
the descriptor sets will be unmodified.
One of the descriptor sets specified an invalid descriptor.
A signal was delivered before the time limit expired and
before any of the selected events occurred.
The specified time limit is invalid. One of its components is
Although the provision of
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.
(currently 256) is somewhat larger 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
should probably return the time remaining from the original timeout,
if any, by modifying the time value in place.
This may be implemented in future versions of the system.
Thus, it is unwise to assume that the timeout value will be unmodified
function call appeared in