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You must use the EVM read and write functions to write a program
that performs any of the following operations: Passes events to another process through a pipe
or socket connection Analyzes events stored in a file Receives events from a process other than the EVM
daemon
You cannot write events directly
using the standard UNIX write functions, because the event handle contains only a
pointer to the body of the event and because the location of the
body can 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 “Reading and Writing
Events” shows how to
write events to a file, to read them from a file into your program,
and to validate them. Example 4-5 “Reading and Writing
Events” 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 b but 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);} |
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