Re: libpq C library Client Interface - select()

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
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>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
>
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