HP-UX Event ManagerProgrammer's Guide

change each time the event is modified. Conversely, when you read an event, it is not enough
to read the body; a handle must be created to enable you to reference the event through the API
functions.
Example 4-5 shows how to write events to a file, to read them from a file into your program, and
to validate them.
Example 4-5 introduces the following functions:
• EvmEventWrite — Writes an event to an open file descriptor. For more information about
this function, see EvmEventWrite(3).
• EvmEventRead — Creates a new event structure and populates it with an event read from
a file descriptor. EvmEventDestroy must be used to free the new event. For more
information about this function, see EvmEventRead(3).
• EvmEventValidate — Performs a data integrity check on the event. This check validates
an event that is received over a connection or retrieved from storage. For more information
about this function, see EvmEventValidate(3).
Example 4-5 Reading and Writing Events
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <evm/evm.h>
int main()
{ EvmEvent_t event_in,event_out;
EvmStatus_t status;
EvmItemValue_t itemval,itemval_name;
int fd; /* Create an event containing a name. */
itemval_name.NAME="sys.unix.evm";
EvmEventCreateVa(&event_out,EvmITEM_NAME, itemval_name, EvmITEM_NONE);
/* Create an output log file and use EvmEventWrite() to write the event to it.
You can write the event to any file descriptor, including a pipe to another process because
the event is a binary data package, do not write it directly to a terminal or printer.*/
fd = open("eventlog",O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
if (fd < 0)
{fprintf(stderr,"Failed to open output log file\n");
exit(1); }
status = EvmEventWrite(fd,event_out);
if (status != EvmERROR_NONE)
{fprintf(stderr,"Failed to write event to log file\n");
exit(1); }
/* Read the event back in using EvmEventRead(). Note that you produce a different event this time,
and you have to supply a pointer to the event handle, not the handle itself. */
lseek(fd,0,SEEK_SET);
status = EvmEventRead(fd,&event_in);
if (status != EvmERROR_NONE)
{fprintf(stderr,"Failed to read event from log file\n");
exit(1); }
/* Because the incoming event has been outside the control of this process, it is important to verify its
integrity.
Use the EvmEventValidate() function to do this each time you either read an event from a file or receive it
from
any process other than the EVM daemon.*/
status = EvmEventValidate(event_in);
if (status != EvmERROR_NONE)
{fprintf(stderr,"Event read from logfile is invalid");
exit(1); }
/* You can show that the event just read is the same event that was just written out by retrieving and
displaying its name. */
status = EvmItemGet(event_in,EvmITEM_NAME,&itemval);
if(status == EvmERROR_NONE)
{ fprintf(stdout,"Event name: %s\n",itemval.NAME);
EvmItemRelease(EvmITEM_NAME,itemval); } /* Free the space used by the event.*/
EvmEventDestroy(event_in);
EvmEventDestroy(event_out);}
Subscribing Event Notifications
A program that subscribes for receiving event notifications must perform the following operations:
• Create a listening connection to the EVM daemon
• Communicate to the daemon which events it is interested in by passing a filter string
• Monitor event activity on the connection, and be prepared to handle each event as it arrives
Example 4-6 shows how to wait for events to arrive, and display each incoming event on stdout.
Example 4-6 introduces the following functions:
Subscribing Event Notifications 43