snmp_select_info()

Get information that select() needs for SNMP

Synopsis:

#include <sys/types.h>
#include <sys/select.h>
#include <sys/time.h>
#include <snmp/snmp_api.h>

int snmp_select_info( int * numfds, 
                      struct fd_set * fdset,
                      struct timeval * timeout, 
                      int * block );

Arguments:

numfds

The number of significant file descriptors in fdset.

fdset

A pointer to a set of file descriptors that contains all of the file descriptors that you've opened for SNMP. If activity occurs on any of these file descriptors, you should call snmp_read() with that file-descriptor set.

timeout

A pointer to a timeval structure that defines the longest time that SNMP can wait for a timeout. You should call select() with the minimum time between timeout and any other timeouts necessary. You should check this on each invocation of select(). If a timeout is received, you should call snmp_timeout() to see if the timeout was for SNMP. (The snmp_timeout() function is idempotent.)

You must provide the timeout, even if block is 1 (see below).

block

Governs the behavior of select():

• If block is 0, select() is requested to time out.
• If block is 1, select() is requested to block indefinitely. The timeout value is treated as undefined, although you must provide it. On return, if block is nonzero, the value of timeout is undefined.

Library:

libsnmp

Use the -l snmp option to qcc to link against this library.

Description:

The snmp_select_info() function is used to return information about what SNMP requires from a select() call.

Asynchronous SNMP transactions:

To have SNMP transactions occur asynchronously, you can invoke the functions snmp_select_info(), snmp_timeout(), and snmp_read() in conjunction with the system call select(). For more information, see select().

For asynchronous transactions, invoke snmp_select_info() with the information you would have passed to select() in the absence of SNMP. The snmp_select_info() function modifies the information, which is subsequently passed to select().

The following code segment shows how to use these SNMP functions in conjunction with select():

FD_ZERO(&fdset);
numfds=sd+1;
FD_SET(sd,&fdset);
block=0;
tvp=&timeout;
timerclear(tvp);
tvp->tv_sec = 5;

snmp_select_info(&numfds, &fdset, tvp, &block);

if(block==1)
{
  tvp = NULL;
}
count = select(numfds, &fdset, 0, 0, tvp);

if(count==0)
  snmp_timeout();
if(count>0)
  snmp_read(&fdset);

Returns:

The number of open sockets (i.e. the number of open sessions).

Classification:

SNMP

See also:

select(), snmp_close(), snmp_open(), snmp_pdu, snmp_read(), snmp_select_info(), snmp_send(), snmp_session, snmp_timeout()

Based on RFC 1157, FAQ in Internet newsgroup comp.protocols.snmp

Marshall T. Rose, The Simple Book: An Introduction to Internet Management, Revised 2nd ed. (Prentice-Hall, 1996, ISBN 0-13-451659-1)