SELECT(2) FreeBSD System Calls Manual
SELECT(2)
NAME select - synchronous I/O multiplexing
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <sys/types.h> #include <sys/time.h> #include <unistd.h>
int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);
FD_SET(fd, &fdset);
FD_CLR(fd, &fdset);
FD_ISSET(fd, &fdset);
FD_ZERO(&fdset);
DESCRIPTION Select() examines the I/O descriptor sets whose addresses are passed in readfds, writefds, and
exceptfdsto see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional condi- tion pending, respectively. The only
exceptionalcondition detectable
is out-of-band data received on a socket. The first nfds descriptors are checked in each set; i.e., the
descriptorsfrom 0 through nfds-1 in the descriptor sets are examined. On return, select() replaces the given
descriptorsets with subsets consisting of those descriptors that are ready for the requested operation. Select()
returnsthe 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: FD_ZERO(&fdset) initializes a descriptor
setfdset to the null set. FD_SET(fd, &fdset) includes a particular descriptor fd in fdset. FD_CLR(fd, &fdset)
removesfd from fdset. FD_ISSET(fd, &fdset) is non- zero if fd is a member of fdset, zero otherwise. The behavior
ofthese macros is undefined if a descriptor value is less than zero or greater than or equal to FD_SETSIZE, which
isnormally at least equal to the
max- imum number of descriptors supported by the system.
If timeout is a non-nil pointer, it specifies the maximum interval to wait for the selection to complete.
Systemactivity can lengthen the interval by an indeterminate amount.
If timeout is a nil pointer, the select blocks indefinitely.
To effect a poll, the timeout argument should be non-nil, pointing to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as nil pointers if no descriptors are of interest.
RETURN VALUES Select() returns the number of ready descriptors that are contained in the descriptor sets, or -1
ifan error occurred. If the time limit expires, select() returns 0. If select() returns with an error,
includ- ing one due to an interrupted call, the descriptor sets will be unmodi- fied.
ERRORS An error return from select() indicates:
[EBADF] One of the descriptor sets specified an invalid descriptor.
[EINTR] A signal was delivered before the time limit expired and before any of the
selectedevents occurred.
[EINVAL] The specified time limit is invalid. One of its
com- ponents is negative or too large.
[EINVAL] nfds was invalid.
SEE ALSO accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2),
clocks(7)
NOTES The default size of FD_SETSIZE is currently 1024. In order to accommo- date programs which might
potentiallyuse a larger number of open files with select(), it is possible to increase this size by having the
program define FD_SETSIZE before the inclusion of any header which includes <sys/types.h>.
If nfds is greater than the number of open files, select() is not
guaran- teed to examine the unused file descriptors. For historical reasons, select() will always examine the
first256 descriptors.
BUGS Version 2 of the Single UNIX Specification (``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 select() call.
HISTORY The select() function call appeared in 4.2BSD.
FreeBSD 4.4 March 25, 1994 FreeBSD
4.4
>From: jco@cornelius-olsen.dk
>To: pgsql-general@postgresql.org, pgsql-interfaces@postgresql.org
>Subject: [INTERFACES] libpq C library Client Interface - select()
>Date: Sat, 18 Jan 2003 16:08:52 +0100
>MIME-Version: 1.0
>Received: from mc2-f15.law16.hotmail.com ([65.54.237.22]) by
>mc2-s10.law16.hotmail.com with Microsoft SMTPSVC(5.0.2195.5600); Sat, 18
>Jan 2003 07:14:46 -0800
>Received: from relay2.pgsql.com ([64.49.215.143]) by
>mc2-f15.law16.hotmail.com with Microsoft SMTPSVC(5.0.2195.5600); Sat, 18
>Jan 2003 07:14:46 -0800
>Received: from postgresql.org (postgresql.org [64.49.215.8])by
>relay2.pgsql.com (Postfix) with ESMTPid 7C708EDFB33; Sat, 18 Jan 2003
>10:12:07 -0500 (EST)
>Received: from localhost (postgresql.org [64.49.215.8])by postgresql.org
>(Postfix) with ESMTPid CE1E2475A3F; Sat, 18 Jan 2003 10:09:51 -0500 (EST)
>Received: from cornelius-olsen.dk (0x50c4f773.arcnxx7.adsl-dhcp.tele.dk
>[80.196.247.115])by postgresql.org (Postfix) with ESMTPid ECDD3474E5C; Sat,
>18 Jan 2003 10:09:50 -0500 (EST)
>X-Mailer: Lotus Notes Release 5.0.9 November 16, 2001
>Message-ID: <OFE0A20DB5.46EB5199-ONC1256CB2.00532607@dk>
>X-MIMETrack: Serialize by Router on sonic/cgroup(Release 5.0.11 |July 24,
>2002) at 01/18/2003 04:08:54 PM,Serialize complete at 01/18/2003 04:08:54
>PM
>X-Virus-Scanned: by AMaViS new-20020517
>Precedence: bulk
>Sender: pgsql-interfaces-owner@postgresql.org
>Return-Path: pgsql-interfaces-owner+M3576@postgresql.org
>X-OriginalArrivalTime: 18 Jan 2003 15:14:46.0266 (UTC)
>FILETIME=[55F48DA0:01C2BF04]
>
>Hi,
>
>Several places in the docs references something called "select()". One
>example:
>
>A better way to check for NOTIFY messages when you have no useful queries
>to make is to call PQconsumeInput(), then check PQnotifies(). You can use
>select() to wait for backend data to arrive, thereby using no CPU power
>unless
>there is something to do. (See PQsocket() to obtain the file descriptor
>number to use with select().)
>
>My problem is that I can find no information on what "select()" is or how
>it's used. Does anyone have any pointers?
>My particular interest is that I'd like to find out how to do modify the
>test program "testlibpq2.c" to use "select()" as suggested in the comments:
>
> /*
> * wait a little bit between checks; waiting with select()
> * would be more efficient.
> */
>Thanks in advance.
>/J�rn
>
>---------------------------(end of broadcast)---------------------------
>TIP 6: Have you searched our list archives?
>
>http://archives.postgresql.org
_________________________________________________________________
STOP MORE SPAM with the new MSN 8 and get 2 months FREE*
http://join.msn.com/?page=features/junkmail
