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The output of flex is the file `lex.yy.c', which contains
the scanning routine `yylex()', a number of tables used by
it for matching tokens, and a number of auxiliary routines
and macros. By default, `yylex()' is declared as follows:
int yylex()
{
... various definitions and the actions in here ...
}
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(If your environment supports function prototypes, then it will be "int yylex( void )".) This definition may be changed by defining the "YY_DECL" macro. For example, you could use:
#define YY_DECL float lexscan( a, b ) float a, b; |
to give the scanning routine the name lexscan, returning a
float, and taking two floats as arguments. Note that if
you give arguments to the scanning routine using a
K&R-style/non-prototyped function declaration, you must
terminate the definition with a semi-colon (`;').
Whenever `yylex()' is called, it scans tokens from the
global input file yyin (which defaults to stdin). It
continues until it either reaches an end-of-file (at which
point it returns the value 0) or one of its actions
executes a return statement.
If the scanner reaches an end-of-file, subsequent calls are undefined
unless either yyin is pointed at a new input file (in which case
scanning continues from that file), or `yyrestart()' is called.
`yyrestart()' takes one argument, a `FILE *' pointer (which
can be nil, if you've set up YY_INPUT to scan from a source
other than yyin), and initializes yyin for scanning from
that file. Essentially there is no difference between just assigning
yyin to a new input file or using `yyrestart()' to do so;
the latter is available for compatibility with previous versions of
flex, and because it can be used to switch input files in the
middle of scanning. It can also be used to throw away the current
input buffer, by calling it with an argument of yyin; but
better is to use YY_FLUSH_BUFFER (see above). Note that
`yyrestart()' does not reset the start condition to
INITIAL (see Start Conditions, below).
If `yylex()' stops scanning due to executing a return
statement in one of the actions, the scanner may then be called
again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner
uses block-reads rather than simple `getc()' calls to read
characters from yyin. The nature of how it gets its input
can be controlled by defining the YY_INPUT macro.
YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place
up to max_size characters in the character array buf and
return in the integer variable result either the number of
characters read or the constant YY_NULL (0 on Unix
systems) to indicate EOF. The default YY_INPUT reads from
the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions section of the input file):
%{
#define YY_INPUT(buf,result,max_size) \
{ \
int c = getchar(); \
result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \
}
%}
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This definition will change the input processing to occur one character at a time.
When the scanner receives an end-of-file indication from
YY_INPUT, it then checks the `yywrap()' function. If
`yywrap()' returns false (zero), then it is assumed that the
function has gone ahead and set up yyin to point to
another input file, and scanning continues. If it returns
true (non-zero), then the scanner terminates, returning 0
to its caller. Note that in either case, the start
condition remains unchanged; it does not revert to INITIAL.
If you do not supply your own version of `yywrap()', then you must either use `%option noyywrap' (in which case the scanner behaves as though `yywrap()' returned 1), or you must link with `-lfl' to obtain the default version of the routine, which always returns 1.
Three routines are available for scanning from in-memory buffers rather than files: `yy_scan_string()', `yy_scan_bytes()', and `yy_scan_buffer()'. See section Multiple Input Buffers.
The scanner writes its `ECHO' output to the yyout global
(default, stdout), which may be redefined by the user
simply by assigning it to some other FILE pointer.
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