snmp_select_info()

position to a block within an SNMP message structure

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 );

Description:

This function is used to return information about what SNMP requires from a select() call. The arguments are:

numfds

The number of significant file descriptors in fdset.

fdset

A pointer to a set of file descriptors. All file descriptors opened for SNMP are ORed into fdset. If activity occurs on any of these file descriptors, snmp_read() should be called with that file-descriptor set.

timeout

A pointer to a structure defining the longest time that SNMP can wait for a timeout. The select() should be done with the minimum time between timeout and any other timeouts necessary. This should be checked on each invocation of select(). If a timeout is received, snmp_timeout() should be called to see if the timeout was for SNMP. (The snmp_timeout() function is idempotent.)

The timeout must be provided, 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 it must be provided. On return, if block is nonzero, the value of timeout is undefined.

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() in the C Library Reference.

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().

Parameters to select():	Corresponding parameters to snmp_select_info():
nfds	numfds
readfds	fdset
timeout	timeout-must point to an allocated (but not necessarily initialized) `timeval` structure.

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

Safety:
Interrupt handler	No
Signal handler	No
Thread	No