16 Scanner Options

The various flex options are categorized by function in the following menu. If you want to lookup a particular option by name, See Index of Scanner Options.

Even though there are many scanner options, a typical scanner might only specify the following options:

%option   8bit reentrant bison-bridge
%option   warn nodefault
%option   yylineno
%option   outfile="scanner.c" header-file="scanner.h"

The first line specifies the general type of scanner we want. The second line specifies that we are being careful. The third line asks flex to track line numbers. The last line tells flex what to name the files. (The options can be specified in any order. We just divided them.)

flex also provides a mechanism for controlling options within the scanner specification itself, rather than from the flex command-line. This is done by including %option directives in the first section of the scanner specification. You can specify multiple options with a single %option directive, and multiple directives in the first section of your flex input file.

Most options are given simply as names, optionally preceded by the word ‘no’ (with no intervening whitespace) to negate their meaning. The names are the same as their long-option equivalents (but without the leading ‘--’ ).

flex scans your rule actions to determine whether you use the REJECT or yymore() features. The REJECT and yymore options are available to override its decision as to whether you use the options, either by setting them (e.g., %option reject) to indicate the feature is indeed used, or unsetting them to indicate it actually is not used (e.g., %option noyymore).

A number of options are available for lint purists who want to suppress the appearance of unneeded routines in the generated scanner. Each of the following, if unset (e.g., %option nounput), results in the corresponding routine not appearing in the generated scanner:

    input, unput
    yy_push_state, yy_pop_state, yy_top_state
    yy_scan_buffer, yy_scan_bytes, yy_scan_string

    yyget_extra, yyset_extra, yyget_leng, yyget_text,
    yyget_lineno, yyset_lineno, yyget_in, yyset_in,
    yyget_out, yyset_out, yyget_lval, yyset_lval,
    yyget_lloc, yyset_lloc, yyget_debug, yyset_debug

(though yy_push_state() and friends won’t appear anyway unless you use %option stack).