Guardian Programmer's Guide

Fault-Tolerant Programming in C
Guardian Programmer’s Guide — 421922-014
27 - 59
Primary and Backup Processing
/*This function is called by the primary process to update the
state of the backup process. It creates and sends a message to
the backup process.*/
void update_backup (void)
{
short cond_code;
short error;
message_format message;
/*Create update message*/
message.msgnumber = UPDATE_MESSAGE;
error = __ns_fget_file_state (infile,
&message.msg_variants.update_info.infile_state);
error = __ns_fget_file_state (outfile,
&message.msg_variants.update_info.outfile_state);
error = __ns_fget_file_state (stdout,
&message.msg_variants.update_info.stdout_state);
/*Check for existence of backup; attempt to recreate if
necessary*/
if (!backup_exists())
initialize_backup();
/*Send update message to backup*/
cond_code = WRITEX (backup_filenum,
(char*) &message,
(short) sizeof message);
/*Check condition code and include check for existence of
backup*/
}
Primary and Backup Processing
At this stage of the example, functions have been written to perform the following
active backup tasks:
• Create and start the backup process (function initialize_backup).
• Update state information (functions update_backup and backup_exists).
The next step is to restructure the application into separate functions for primary
pr
ocessing and backup processing, and a main function where execution will begin.
The following new functions are created:
• Function primary_processing, which does the work of the application and
updates state information in the backup process.
• Function backup_processing, which receives state information from the primary
process, checks for failure of the primary process or CPU, and takes over
execution if the primary process or CPU fails.
• Function main, which tests whether it is running as the primary process or the
backup process. If it is the primary process, it creates and starts the backup