Flex - a scanner generator - 1.13 Miscellaneous macros

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## 1.13 Miscellaneous macros

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 below). 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'` (see Options below). ``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

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.

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