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I'm making a compiler in BNFC and it's got to a stage where it already compiles some stuff and the code works on my device. But before shipping it, I want my compiler to return proper error messages when the user tries to compile an invalid program.

I found how bison can write error on the stderr stream and I'm able to catch those. Now suppose the user's code has no syntax error, it just references an undefined variable, I'm able to catch this in my visitor, but I can't know what the line number was, how can I find the line number?

Eduardo Wada
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2 Answers2

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In bison you can access the starting and ending position of the current expression using the variable @$, which contains a struct with the members first_column, first_line, last_column and last_line. Similarly @1 etc. contain the same information for the sub-expressions $1 etc. respectively.

In order to have access to the same information later, you need to write it into your ast. So add a field to your AST node types to store the location and then set that field when creating the node in your bison file.

sepp2k
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(previous answer is richer) but in some simple parsers if we declare

%option yylineno

in flex, and print it in yyerror,

yyerror(char *s) { 
  fprintf(stderr,"ERROR (line %d):before '%s'\n-%s",yylineno, yytext,s);
}

sometimes it help...

JJoao
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