|
HP OpenVMS Systemsask the wizard |
|
The Question is: Hi, What is the traditional (correct) way of determining whether a process is still alive (exists) at C level? I know I could do a sys$getjpi() to get it, but I have to ask some specific info about the process to call getjpi(). 1. Is there JPI$_<something> that tells me only whether it is alive and nothing else (extra info is all right, except that no extra info m ight make it faster)? 2. If no such flag exists, which flag is the fastest? Thanks a lot, Xianguan The Answer is :
If you insist on using a polling-based scheme -- an approach that the
OpenVMS Wizard would not recommend, then there are several techniques
that are available. Most commonly...
If the targetted process can handle spurious wakes, then the cheapest
method might be to call sys$wake for the process identification (PID)
and check the status.
Another approach would involve the use of sys$getjpi to ask for nothing
from the target process, the call will succesfully complete -- not doing
anything, but successfully -- if the PID exists and the call will fail
to do nothing if the PID is no longer valid. Check the return status
from the sys$getjpi call.
An example program follows:
#include <efndef.h>
#include <ssdef.h>
#include <starlet.h>
#include <stdio.h>
#include <stsdef.h>
main(int argc, char *argv[])
{
int RetStat, Null = 0, Pid;
unsigned short int IOSB;
sscanf( argv[1], "%x", &Pid );
RetStat = sys$getjpiw( EFN$C_ENF, &Pid, 0, &Null, IOSB, 0, 0);
if (!$VMS_STATUS_SUCCESS( RetStat ))
return RetStat;
if (!$VMS_STATUS_SUCCESS( IOSB))
return IOSB;
return SS$_NORMAL;
}
Alternatively, use the process termination mailbox mechanism.
Polling based on scans for a process name is NOT recommended.
A technique that would be greatly prefered by the OpenVMS Wizard would
involve the use of the OpenVMS Distributed Lock Manager. Have the
target process queue an exclusive lock on a resource such as FOO_ALIVE
(where FOO is a prefix unique to this application) or FOO_SCSNODE_ALIVE
(where the SCSNODE string is replaced with the name of the local node,
if more than one copy of this monitored process will be running within
an OpenVMS Cluster), and queue a request (with a granting AST specified)
with an incompatible lock mode (eg: exclusive) in the monitoring process.
When (if) the monitored process exits (or if the node itself exits within
an OpenVMS Cluster), the monitoring process will receive the lock and the
grant AST will be triggered.
For details on this, please see the $enq and $deq documentation in
the OpenVMS documentation set.
Using the lock manager has the advantage of immediate notification,
and also avoids the need to track the target PID.
http://www.openvms.compaq.com:8000
/72final/5841/5841pro_050.html#synch_accs_res_chap
|