(flex.info)Misc Macros

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13 Miscellaneous Macros
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The macro 'YY_USER_ACTION' can be defined to provide an action which is
always executed prior to the matched rule's action.  For example, it
could be #define'd to call a routine to convert yytext to lower-case.
When 'YY_USER_ACTION' is invoked, the variable 'yy_act' gives the number
of the matched rule (rules are numbered starting with 1).  Suppose you
want to profile how often each of your rules is matched.  The following
would do the trick:

         #define YY_USER_ACTION ++ctr[yy_act]

   where 'ctr' is an array to hold the counts for the different rules.
Note that the macro 'YY_NUM_RULES' gives the total number of rules
(including the default rule), even if you use '-s)', so a correct
declaration for 'ctr' is:

         int ctr[YY_NUM_RULES];

   The macro 'YY_USER_INIT' may be defined to provide an action which is
always executed before the first scan (and before the scanner's internal
initializations are done).  For example, it could be used to call a
routine to read in a data table or open a logging file.

   The macro 'yy_set_interactive(is_interactive)' can be used to control
whether the current buffer is considered "interactive".  An interactive
buffer is processed more slowly, but must be used when the scanner's
input source is indeed interactive to avoid problems due to waiting to
fill buffers (see the discussion of the '-I' flag in Note: Scanner
Options).  A non-zero value in the macro invocation marks the buffer
as interactive, a zero value as non-interactive.  Note that use of this
macro overrides '%option always-interactive' or '%option
never-interactive' (Note: Scanner Options).  'yy_set_interactive()'
must be invoked prior to beginning to scan the buffer that is (or is
not) to be considered interactive.

   The macro 'yy_set_bol(at_bol)' can be used to control whether the
current buffer's scanning context for the next token match is done as
though at the beginning of a line.  A non-zero macro argument makes
rules anchored with '^' active, while a zero argument makes '^' rules
inactive.

   The macro 'YY_AT_BOL()' returns true if the next token scanned from
the current buffer will have '^' rules active, false otherwise.

   In the generated scanner, the actions are all gathered in one large
switch statement and separated using 'YY_BREAK', which may be redefined.
By default, it is simply a 'break', to separate each rule's action from
the following rule's.  Redefining 'YY_BREAK' allows, for example, C++
users to #define YY_BREAK to do nothing (while being very careful that
every rule ends with a 'break' or a 'return'!)  to avoid suffering from
unreachable statement warnings where because a rule's action ends with
'return', the 'YY_BREAK' is inaccessible.