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From: Kaz Kylheku <330-706-9395@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: warning: assignment makes pointer from integer without a cast[enabled by default] yylval=atoi(yytext);
Date: Wed, 24 Feb 2016 15:56:17 +0000 (UTC)
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On 2016-02-22, Danny Pereira <dannyjpereira@gmail.com> wrote:
> *purelex.l:12:8: warning: assignment makes pointer from integer without a
> cast [enabled by default] yylval=atoi(yytext); *

This tells you that yylval does not have type int as you expect; it is
in fact a pointer to something.

Though presented as a "warning" in GCC, assigning an integer to a
pointer is a constraint violation according to ISO C, requiring a
diagnostic from a conforming implementation, and the behavior
is not well defined if the code is translated and executed anyway.

It is a bad idea to ignore this diagnostic.

Your yylval is a pointer because of the "Bison bridge" option.
This is documented in the GNU Flex manual:

  A.2 C Scanners with Bison Parsers

  [... few paragraphs down ...]

  Note that the macros `yylval' and `yylloc' evaluate to pointers.