yylineno

yylineno is a global integer variable used in lexical scanners generated by Flex to track the current line number of the input being tokenized. It is provided when the scanner is built with the option %option yylineno and is intended for use by the scanner, the parser, or host code to generate accurate line-based messages.

The value of yylineno is updated as the scanner processes input. By default, the initial value is 1, and the counter is incremented for every newline that is matched by the scanner rules. Users can reset yylineno if they restart or reuse a scanner, typically by setting it back to 1.

Access to yylineno is usually through external linkage. The scanner defines the variable, and outside code can reference it by declaring extern int yylineno; or by including the scanner’s generated header. This makes it convenient to incorporate line information into error messages, debugging output, or reporting.

Usage considerations include the fact that yylineno reflects line boundaries as recognized by the scanner’s newline matches. It does not automatically track lines for input that bypasses the scanner or undergoes external preprocessing. For more robust location information across parsing boundaries, you might combine yylineno with location-tracking mechanisms such as Bison’s YYLTYPE or a custom locator.

In practice, enabling yylineno is common when detailed line reporting is needed in error handling. A typical pattern is to enable the option, reference yylineno in error messages, and reset it as appropriate when reinitializing scanners.