Error Messaging and Recovery

This document will lead you through a simple lex & yacc example which shows one way to implement error recovery and error messaging. You should be able to apply the basic principles below to your own project with relative ease.

The Yacc File

This yacc file demonstrates a number of features you may wish to use in your project. Among these features:

Error Recovery Using the "error" rule
The "main()" function has been overridden
Error Messages Displaying:
- Current Line Number
- Text of the Bad Token

The Error Rule

The effect of the error rule is to make errors admissable in the grammar. After recognizing an error, the "error" rule allows the compiler to continue processing input. The "error" rule is triggered much like a default (otherwise) action in a case statement in that it is triggered by anything other than the expected token within a rule.

In the event of error within the sound rule, the printf next to "error" is executed. Remember that the grammar will require three successive "valid" tokens before it will reset itself and issue further errors.

The Lex File

Notice in this Lex file how the variable "linenum" is incremented. This variable, defined near the bottom of the yacc file, is used in the error messages.

Try It!

Go into the Lex and Yacc files and use "Save As" under the file menu to save these files (error.l and error.y) into your home directory along with the makefile then type "make." The grammar takes a potentially endless string of "ding dong dell" rhymes.

Error Messaging and Recovery - Rev. 30 April '94 / Ben Steel