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The following phenomenon has several symptoms, resulting in the following typical questions:
I invoke
yyparse
several times, and on correct input it works properly; but when a parse error is found, all the other calls fail too. How can I reset the error flag ofyyparse
?
or
My parser includes support for an ‘#include’-like feature, in which case I run
yyparse
fromyyparse
. This fails although I did specify ‘%define api.pure full’.
These problems typically come not from Bison itself, but from Lex-generated scanners. Because these scanners use large buffers for speed, they might not notice a change of input file. As a demonstration, consider the following source file, first-line.l:
%{ #include <stdio.h> #include <stdlib.h> %}
%% .*\n ECHO; return 1; %%
int yyparse (char const *file) { yyin = fopen (file, "r"); if (!yyin) { perror ("fopen"); exit (EXIT_FAILURE); }
/* One token only. */ yylex (); if (fclose (yyin) != 0) { perror ("fclose"); exit (EXIT_FAILURE); } return 0; }
int main (void) { yyparse ("input"); yyparse ("input"); return 0; }
If the file input contains
input:1: Hello, input:2: World!
then instead of getting the first line twice, you get:
$ flex -ofirst-line.c first-line.l $ gcc -ofirst-line first-line.c -ll $ ./first-line input:1: Hello, input:2: World!
Therefore, whenever you change yyin
, you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one. This depends upon your implementation of Lex; see its
documentation for more. For Flex, it suffices to call
‘YY_FLUSH_BUFFER’ after each change to yyin
. If your
Flex-generated scanner needs to read from several input streams to
handle features like include files, you might consider using Flex
functions like ‘yy_switch_to_buffer’ that manipulate multiple
input buffers.
If your Flex-generated scanner uses start conditions (see Start conditions in The Flex Manual), you might also want to reset the scanner’s state, i.e., go back to the initial start condition, through a call to ‘BEGIN (0)’.
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