21. Diagnostics

‘warning, rule cannot be matched’

indicates that the given rule cannot be matched because it follows other rules that will always match the same text as it. For example, in the following "foo" cannot be matched because it comes after an identifier "catch-all" rule:

[a-z]+    got_identifier();
foo       got_foo();

Using REJECT in a scanner suppresses this warning.

‘warning, -s option given but default rule can be matched’

means that it is possible (perhaps only in a particular start condition) that the default rule (match any single character) is the only one that will match a particular input. Since ‘-s’ was given, presumably this is not intended.

‘reject_used_but_not_detected undefined’

‘yymore_used_but_not_detected undefined’

These errors can occur at compile time. They indicate that the scanner uses REJECT or ‘yymore()’ but that flex failed to notice the fact, meaning that flex scanned the first two sections looking for occurrences of these actions and failed to find any, but somehow you snuck some in (via a #include file, for example). Use ‘%option reject’ or ‘%option yymore’ to indicate to flex that you really do use these features.

‘flex scanner jammed’

a scanner compiled with ‘-s’ has encountered an input string which wasn’t matched by any of its rules. This error can also occur due to internal problems.

‘token too large, exceeds YYLMAX’

your scanner uses ‘%array’ and one of its rules matched a string longer than the ‘YYL-’ MAX constant (8K bytes by default). You can increase the value by #define’ing YYLMAX in the definitions section of your flex input.

‘scanner requires -8 flag to use the character 'x'’

Your scanner specification includes recognizing the 8-bit character x and you did not specify the -8 flag, and your scanner defaulted to 7-bit because you used the ‘-Cf’ or ‘-CF’ table compression options. See section ‘-7’flag.

‘flex scanner push-back overflow’

you used ‘unput()’ to push back so much text that the scanner’s buffer could not hold both the pushed-back text and the current token in yytext. Ideally the scanner should dynamically resize the buffer in this case, but at present it does not.

‘input buffer overflow, can't enlarge buffer because scanner uses REJECT’

the scanner was working on matching an extremely large token and needed to expand the input buffer. This doesn’t work with scanners that use REJECT.

‘fatal flex scanner internal error--end of buffer missed’

This can occur in an scanner which is reentered after a long-jump has jumped out (or over) the scanner’s activation frame. Before reentering the scanner, use:

yyrestart( yyin );

or, as noted above, switch to using the C++ scanner class.

‘too many start conditions in <> construct!’

you listed more start conditions in a <> construct than exist (so you must have listed at least one of them twice).

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