Guardian Programmer's Guide

Fault-Tolerant Programming in C
Guardian Programmer’s Guide — 421922-014
27 - 43
Primary and Backup Processing
Primary and Backup Processing
The original application program can now be restructured into the following three
functions:
• Function primary_processing, which performs the primary process activities: it
does the work of the application and updates state information in the backup
process.
• Function backup_processing, which performs the backup process activities: it
receives state information from the primary process and monitors the primary
process CPU. If a message indicating CPU failure or process termination is
received, the backup process takes over execution.
• Function main, which is where execution of the active backup program begins.
Function main determines whether it is running as the primary process or the
backup process: if it is the primary process, it initializes the backup process and
calls primary_processing; if it is the backup process, it calls
backup_processing.
A separate function, is_backup_process, is written to determine whether the
program is running as the primary process or the backup process. main calls this
function at the beginning of its execution.
Function primary_processing
Function primary_processing does the work of the application (which, in this
example, is to execute an infinite loop that increments a counter). Within the innermost
loop, function update_backup is called to update the state information in the backup
process.
/*This function does the work of the application, which is
to increment a counter. To avoid overflow, the
application wraps around after 10,000 iterations*/
void primary_processing (void)
{
for (;;)
{
counter++;
printf ("Counter is %d\n", counter);
if (counter > 10000) counter = 0;
update_backup (counter);
}
}