The RenderMan Interface - Paul Bourke

C.2.2
Error handling
There are two types of errors that may be encountered while processing a RIB input stream:
syntactic errors and semantic errors. Syntactic errors occur when the stream of tokens fails to
form a syntactically legal statement or request. For example, a syntactic error occurs when
a required parameter is missing, or when a string is left unterminated. Semantic errors
occur when a syntactically legal statement contains incorrect data; e.g., when a parameter
that must be non-negative is specified to be -1.
RIB defines a number of syntactic errors and a limited number of semantic errors. In theory
RIB should be responsible only for syntactic errors. However, due to the weak typing of
programming languages such as C, semantic errors that can not be easily recognized within
the RenderMan Interface software are checked at the RIB level. For example, RIB checks
arrays that are to be converted to matrices to be sure they have 16 values.
Table C.2 shows the set of errors recognized by RIB. Detailed descriptions of the errors are
given below.
All errors encountered by a RIB interpreter require some associated action to be performed.
In the case of syntax errors, if input processing is to be continued, the input scanner must
resynchronize itself with the input stream. This synchronization is done by reading and
discarding tokens from the input stream until a valid RIB request token is encountered.
That is, any tokens between the point of the syntax error and the next request token are
discarded. The protocol has been designed so that no more than one request (along with
any associated parameters) must be discarded when recovering from an error.
Errors are handled in one of three ways:
• They are ignored and the rendering process will proceed to its completion no matter
what input stream is provided.
• They cause diagnostic messages to be generated on the renderer’s standard error
stream, but they are otherwise ignored (the default).
• The first error causes a diagnostic message to be generated and the renderer terminates
immediately without creating an image.
If the RIB interpreter is acting as a network server, in direct communication with a client
application, the interpreter may send parsing error signals back to the client. These signals
take the form of the following RIB requests, though they are not valid in the client-to-server
stream. None of these error requests have arguments. Note that some errors may not be
recognized immediately by a RIB interpreter upon parsing a request. This may be due to
buffering or queuing built into the interface between the intepreter and the renderer. In a
client-server environment this may have implications for the client application.
arraytoobig
The interpreter was unable to allocate sufficient memory to store an
array specified in the input stream. This error is dependent on the
interpreter’s implementation. Good implementations of a RIB interpreter
support arrays as large as memory will permit.
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