Translator – Debugging - Sub-String Assignments (II)

The next issue was more difficult to resolve and was a strange problem because statements that appeared to cause the program to crash, worked by themselves. Using the process of elimination, the statement below was finally found to be causing the crash:

RIGHT$(A$+B$,2) = C$

When the comma token is received, the +$ token is added to the output list and is pushed on the done stack. The RIGHT$ token is on top the hold stack with its reference flag set because it is a sub-string assignment statement. Being a sub-string assignment, the first operand must have the reference flag set, which for the +$ token was not, hence an error.

This error is detected in the find code routine where it is checking for a reference of the first operand when the token passed in has its reference flag set. If the reference flag of the operand is not set, an error is returned. The passed in token is first deleted and the error token is set to the token of the first operand.

The problem in this case was that the passed in token, the sub-string function, that gets deleted, is still on top of the hold stack. When an error occurs on a line, one of the things that the cleanup routine does is delete all the tokens on the hold stack. The sub-string function token gets deleted twice, which causes a crash later on – allocated memory must not be freed more than one.

The solution upon these reference errors is to delete the token in the find code routine only if the token is not an internal (sub-string) function. The find code is only called from two places where the token will have its reference flag set: internal sub-string functions in sub-string assignment statements (which are on the hold stack and should not be deleted) and assignment tokens (which should be deleted).

The next problem is that currently, the error reported against the + in the statement above is “item can not be assigned” but should be “expected comma” instead. This problem affects several other statement all related to the fact that a string variable is expected and the error message should reflect this...