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Re: libpq C library Client Interface - select()

From: "Key88 SF" <key88sf(at)hotmail(dot)com>
To: jco(at)cornelius-olsen(dot)dk, pgsql-interfaces(at)postgresql(dot)org
Subject: Re: libpq C library Client Interface - select()
Date: 2003-01-18 18:34:37
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Lists: pgsql-interfaces
SELECT(2)                 FreeBSD System Calls Manual                

     select - synchronous I/O multiplexing

     Standard C Library (libc, -lc)

     #include <sys/types.h>
     #include <sys/time.h>
     #include <unistd.h>

     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);


     Select() examines the I/O descriptor sets whose addresses are passed in
     readfds, writefds, and exceptfds to see if some of their descriptors 
     ready for reading, are ready for writing, or have an exceptional condi-
     tion pending, respectively.  The only exceptional condition detectable 
     out-of-band data received on a socket.  The first nfds descriptors are
     checked in each set; i.e., the descriptors from 0 through nfds-1 in the
     descriptor sets are examined.  On return, select() replaces the given
     descriptor sets with subsets consisting of those descriptors that are
     ready for the requested operation.  Select() returns the total number 
     ready descriptors in all the sets.

     The descriptor sets are stored as bit fields in arrays of integers.  
     following macros are provided for manipulating such descriptor sets:
     FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.
     FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
     FD_CLR(fd, &fdset) removes fd from fdset.  FD_ISSET(fd, &fdset) is non-
     zero if fd is a member of fdset, zero otherwise.  The behavior of these
     macros is undefined if a descriptor value is less than zero or greater
     than or equal to FD_SETSIZE, which is normally at least equal to the 
     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.  System activity 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.

     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, select() returns 0.  If select() returns with an error, 
     ing one due to an interrupted call, the descriptor sets will be unmodi-

     An error return from select() indicates:

     [EBADF]            One of the descriptor sets specified an invalid

     [EINTR]            A signal was delivered before the time limit expired
                        and before any of the selected events occurred.

     [EINVAL]           The specified time limit is invalid.  One of its 
                        ponents is negative or too large.

     [EINVAL]           nfds was invalid.

     accept(2), connect(2), getdtablesize(2), gettimeofday(2), read(2),
     recv(2), send(2), write(2), clocks(7)

     The default size of FD_SETSIZE is currently 1024.  In order to accommo-
     date programs which might potentially use a larger number of open files
     with select(), it is possible to increase this size by having the 
     define FD_SETSIZE before the inclusion of any header which includes

     If nfds is greater than the number of open files, select() is not 
     teed to examine the unused file descriptors.   For historical reasons,
     select() will always examine the first 256 descriptors.

     Version 2 of the Single UNIX Specification (``SUSv2'') allows systems 
     modify the original timeout in place.  Thus, it is unwise to assume 
     the timeout value will be unmodified by the select() call.

     The select() function call appeared in 4.2BSD.

FreeBSD 4.4                     March 25, 1994                     FreeBSD 

>From: jco(at)cornelius-olsen(dot)dk
>To: pgsql-general(at)postgresql(dot)org, pgsql-interfaces(at)postgresql(dot)org
>Subject: [INTERFACES] libpq C library Client Interface - select()
>Date: Sat, 18 Jan 2003 16:08:52 +0100
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>Several places in the docs references something called "select()". One
>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 
>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.
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