Open System Services Programmer's Guide

facility; individual threads are not independently dispatched (that is, not “kernel” threads), so only
one can be active on a processor at a time. There are no facilities to checkpoint a hidden pthread
state.
Designing, implementing, and testing a correct OSS process pair is not a simple task. Often it is
preferable to use middleware subsystems that provide the necessary fault-tolerance features.
However, sometimes a tailored process pair is the best or most efficient mechanism. For more
information about process-pair programming, see the Fault-Tolerant Programming in C chapter in
the Guardian Programmer's Guide.
One reason to use an OSS process pair is to ease interactions with other OSS processes in a
multi-process OSS application. For example, an OSS process opens OSS files for stdin,
stdout, and stderr streams. Some functions, such as freopen(), do not interoperate between
OSS and Guardian files; even freopen_oss()does not permit a Guardian process to reopen
a Guardian stdout stream as an OSS stream.
OSS Process Pair Creation
Beginning with the J06.14 and H06.25 RVUs, the PROCESS_SPAWN_ procedure accepts the
value ZSYS_VAL_PCREATOPT_CALLERSNAME (3) in the z_nameoptions (also known as
pe_name_options) field of the process-extension parameter to create an OSS backup
process. The z_processname (pe_process_name) field is ignored and should be NULL (zero,
the default). The caller must be a named OSS process. The first parameter, oss-program-file
(the pathname), is typically absent or null, so that the backup process runs the same program as
the primary.
NOTE: The PROCESS_SPAWN_ procedure takes several parameters that are pointers to structures,
which are defined in both the tdmext.h header file and the ZSYSDDL family of header files
(including ZSYSTAL for TAL/pTAL and ZSYSC for C/C++). The process-extension structure
has members with names pe_... defined in tdmext.h. The member called pe_name_options
is also called Z^NAME^OPTIONS in ZSYSTAL and z_name_options in ZSYSC.
ZSYS_VAL_PCREATOPT_CALLERSNAME is defined in ZSYSC but not in tdmext.h. For details
about these structures and other PROCESSS_SPAWN_ parameters, see the Guardian Procedure
Calls Reference Manual.
Because the process is created with PROCESS_SPAWN_, it has Guardian ancestry, and not OSS
ancestry.
OSS Process Attributes
The characteristics of processes in OSS process pairs are:
• Both the primary and backup processes have the same process name.
• The PROCESS_GETPAIRINFO_ procedure works the same as it does for Guardian.
• The program can reside in either the Guardian or the OSS file system.
• Each process has its own PID, just as it has its own CPU, PIN identity.
• The primary and backup processes communicate the same way as usual, using (Guardian)
opens of the backup process in the primary and $RECEIVE in the backup.
• If the OSS process pair implements a server, the primary also opens $RECEIVE and reads
client requests from it.
• To enable automatic message switching, $RECEIVE must be opened with receive depth greater
than 0.
OSS Process Pairs 159