CODEWITHSUNDEEP
debuggingyacc
WallyZ
WallyZ

Reputation: 332

debugging yacc YYDEBUG where is y.debug

I trying to debug a the yacc generated component for awk (awk.g.c) but when I define YYDEBUG it includes y.debug which I don't seem to have.

Where does y.debug come from?

Without it there are several references that are undefined.

I'm compiling the old 32v or V7 version of awk so I'm not sure if this is something that still exists.

Upvotes: 1

Views: 887

Answers (1)

rici
rici

Reputation: 241881

Some versions of yacc (in particular, the AT&T version, still available as part of Plan 9) generated an additional file with the suffix .debug containing debugging information, notably the table which translated symbol numbers back into names. Modern yacc-alikes just insert this information into the generated C file, on the grounds that the memory consumption is basically trivial these days.

The name table might not be generated if you don't request it, but the way you ask for it depends on the yacc version:

  1. Most bison versions only generate the table if the trace option is enabled. (Posix mandates -t for this, but bison provides a host of alternatives and not all historical yaccs complied.)

  2. As indicated above, some really old yaccs put the name table into y.debug. The AT&T implementation, as I mentioned above, always did this, but guarded the #include line with a preprocessor conditional on YY_DEBUG

  3. However, the yacc implementation you pointed to in a comment, which uses the conditionally-included y.debug mechanism, only generates the y.debug file if you invoke it with the -D flag. So that's what you need to do.

Background notes

I unearthed the information in point 3 from the V10 source linked in a comment. The download link is at the top of this page; that wasn't immediately obvious from the link in the comment. (That's the complete source tarball, which is about 70MB. The individual files linked to by the link in the comment have been HTMLised, which makes them a pain to work with.) I could have saved myself some time by reading the release notes (called yaccnews rather than CHANGES). The last note in that file describes the implementation, and I include the paragraph here since it has all the details on how debugging works in this particular yacc version.

8/11/81 Debugging changed. If the parser starts with %{#define YYDEBUG %} and yacc is invoked as yacc -D (for Debugging), then the parser uses an external variable named yydebug to control debugging output. If yydebug == 1, the parser prints out the text of the reduction when it performs one. If yydebug == 2, the parser also prints out the name of the token returned by each call to yylex, and if yydebug == 3, the parser also prints out the active items each time it changes state (this is uninteresting).

For what it's worth, it should be possible to generate a working, compilable parser using a modern yacc (such as bison or byacc). In the long run, that will probably be easier. (If you use bison and you require legacy yacc compatibility, you can use the -y flag. That flag is not supported by byacc, which claims to be legacy compatible regardless.)

Upvotes: 1

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