flex: Generated Scanner
9 The Generated Scanner
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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 ...
}
(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 (;).
'flex' generates 'C99' function definitions by default. Flex used to
have the ability to generate obsolete, er, 'traditional', function
definitions. This was to support bootstrapping gcc on old systems.
Unfortunately, traditional definitions prevent us from using any
standard data types smaller than int (such as short, char, or bool) as
function arguments. Furthermore, traditional definitions support added
extra complexity in the skeleton file. For this reason, current
versions of 'flex' generate standard C99 code only, leaving K&R-style
functions to the historians.
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 NULL,
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 it would be better to use 'YY_FLUSH_BUFFER' (⇒Actions). Note
that 'yyrestart()' does _not_ reset the start condition to 'INITIAL'
(⇒Start Conditions).
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. The calling sequence for 'YY_INPUT()' 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'.
Here is 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); \
}
%}
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.
For scanning from in-memory buffers (e.g., scanning strings), see
⇒Scanning Strings. ⇒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.
Info Catalog
flex: Actions
flex: Top
flex: Start Conditions