SIMSCRIPT II.5 Programming Language

Discrete Simulation Concepts
statements (unseen in the normal listing) which are then interpreted by the compiler. A special
compilation option may be specified that allows these statements, and any compiler-generated
routines, to be included in the program listing. It is also possible to obtain a listing of the object
code generated by the compiler. Although these listings are not normally of use, they can serve to
determine the precise actions specified and thus prove helpful in pinpointing the source of some
complex errors.
Eventually all routines in a program will have been compiled without error. These routines can be
submitted, with any required data, for execution. The SIMSCRIPT II.5 user's manual for a specific
implementation should be consulted for details on the management of the object code modules produced
by the compiler and their linking and execution. Errors in program logic may then become
apparent, either by the abnormal termination of the program or by the production of results that are
deemed incorrect.
The possible reasons for abnormal termination are many and varied. They may arise either from
error conditions detected by the operating system of the machine on which the system is implemented
or those detected by the SIMSCRIPT II.5 system. In the first case, the action to be taken must
be determined by the operating system response to the error condition, which may be independent
of the SIMSCRIPT II.5 system.
When the SIMSCRIPT II.5 system detects an error condition, it endeavors to supply meaningful
information that will help you to identify and correct the source of error. An error message is produced
that describes the reason for termination. A traceback is also produced, which names the currently
executing subprogram, usually identifying the line of program source text corresponding to
the error location, and lists any arguments and local variable values. This information is then listed
for each subprogram in the hierarchy of such subprograms, leading back to the program initiation
in the main routine.
The precise format of this traceback, and the options that may be selected to vary its format, are
implementation dependent. Consult the user's manual. In general, variables are listed by name and
mode, and their values interpreted in a meaningful way. A report on the status of any input/output
devices, the current contents of the future events set, and memory usage statistics are usually included.
On completing the error traceback report, the SIMSCRIPT II.5 system attempts to call a routine
with the predefined name snap.r. If you have included a routine of this name, it will be executed
at this point. This routine may include any valid SIMSCRIPT II.5 statements and provides a mechanism
by which a specially written routine may be added to augment the normal traceback output.
Snap.r may also be written to produce additional information on the status of a program, by listing
the contents of selected global variables. Examine the program status at the time of error, in conjunction
with an up-to-date program listing. This will often identify an error in program logic.
Many of the SIMSCRIPT II.5 implementations provide for comprehensive checking and potential
error detection: array subscript bound checking, invalid entity referencing, set membership and
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