NAME

       snmp_alarm_register, snmp_alarm_register_hr, snmp_alarm_unregister - alarm functions

SYNOPSIS

       #include <net-snmp/utilities.h>

       unsigned int
       snmp_alarm_register(unsigned int seconds,
                           unsigned int flags,
                           SNMPAlarmCallback *f_callback,
                           void *clientarg);

       unsigned int
       snmp_alarm_register_hr(struct timeval t,
                              unsigned int flags,
                              SNMPAlarmCallback *f_callback,
                              void *clientarg);

       void
       snmp_alarm_unregister(unsigned int reg);

DESCRIPTION

       These  functions  implement  support  for  a generic timer handling mechanism for multiple
       parts of an application to register function callbacks to happen at a particular  time  in
       the future.

USAGE

       The usage is fairly simple and straight-forward:  Simply create a function you want called
       back at some point in the future.  The function definition should be similar to:

           void my_callback(unsigned int reg, void *clientarg);

       Then, call snmp_alarm_register() to register your callback to be called seconds from  now.
       The  flags field should either be SA_REPEAT or NULL.  If flags is set with SA_REPEAT, then
       the registered callback function will be called every seconds.  If flags is NULL then  the
       function will only be called once and then removed from the alarm system registration.

       The  clientarg  parameter in the registration function is used only by the client function
       and is stored and passed back directly to them on every call to the system.

       The snmp_alarm_register() function returns a unique unsigned int (which is also passed  as
       the  first  argument of each callback), which can then be used to remove the callback from
       the queue at a later point in the future using the  snmp_alarm_unregister()  function.  If
       the  snmp_alarm_register()  call  fails  it  returns zero.  In particular, note that it is
       entirely permissible for an alarm function to unregister itself.

       The   snmp_alarm_register_hr()   function   is   identical    in    operation    to    the
       snmp_alarm_register()  function,  but  takes  a  struct  timeval as a first parameter, and
       schedules the callback after the period represented by t (the letters hr stand  for  "high
       resolution").   The  operation  of  this  function  is  dependent  on the provision of the
       setitimer(2) system call by the operating system.  If this system call is  not  available,
       the  alarm  will  be  scheduled  as  if snmp_alarm_register() had been called with a first
       argument equal to the value of the tv_sec member of t.  See, however, the notes below.

INITIALIZATION

       The init_snmp() function initialises the snmp_alarm subsystem by calling init_snmp_alarm()
       and  then  init_alarm_post_config()  to  set up the first timer to initialise the callback
       function.  These two functions should not be used directly by applications.

NOTES

       The default behaviour of the snmp_alarm subsystem is to request SIGALRM signals  from  the
       operating   system   via  the  alarm(2)  or  setitimer(2)  system  calls.   This  has  the
       disadvantage, however, that  no  other  part  of  the  application  can  use  the  SIGLARM
       functionality   (or,  if  some  other  part  of  the  application  does  use  the  SIGALRM
       functionality, the snmp_alarm subsystem will not work correctly).

       If your application runs a select(2)-based event loop, however, there is no  need  to  use
       SIGALRM  for  the  snmp_alarm  subsystem,  leaving  it  available  for  other parts of the
       application.  This is done by making the following call:

       netsnmp_ds_set_boolean(NETSNMP_DS_LIBRARY_ID,
                              NETSNMP_DS_LIB_ALARM_DONT_USE_SIG, 1);

       before calling init_snmp().  Then,  snmp_select_info()  takes  alarms  into  account  when
       calculating  the  timeout  value  to  be  used  for select(2).  All you need to do is call
       run_alarms() when select(2) times out (return value of zero).  This is the approach  taken
       in the agent; see snmpd.c.  Furthermore, when using this method, high resolution alarms do
       not depend on the presence of the setitimer(2) system call, although overall precision  is
       of course still determined by the underlying operating system.  Recommended.

SEE ALSO

       netsnmp_session_api(3), default_store(3), alarm(2), setitimer(2), select(2)