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flex & bison by John Levine

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Chapter 8. Error Reporting and Recovery

The previous chapters discussed techniques for finding errors within bison grammars. In this chapter, we turn our attention to the other side of error detection—how the parser and lexical analyzer detect errors. This chapter presents some techniques to incorporate error detection and reporting into a parser. We’ll make a modified version of the SQL parser from Chapter 4 that demonstrates them.

Bison provides the error token and the yyerror() routine, which are typically sufficient for early versions of a tool. However, as any program begins to mature, especially a programming tool, it becomes important to provide better error recovery, which allows for detection of errors in later portions of the file, and to provide better error reporting.

Error Reporting

Error reporting should give as much detail about the error as possible. The default bison error declares only that it found a syntax error and stops parsing. In our examples, we used yylineno to report the line number. This provides the location of the error but does not report any other errors within the file or where in the specified line the error occurs. The bison locations feature, described later in this chapter, is an easy way to pinpoint the location of an error, down to the exact line and character numbers. In our example, we print out the locations, but precise location information would also allow a visual interface to highlight the relevant text.

It is often useful to categorize ...

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