Date: Wed, 4 Jun 2008 17:37:09 -0700 (PDT)
From: Dante Allegria <danteallegria AT yahoo DOT ca>
Reply-To: danteallegria AT yahoo DOT ca
Subject: flex 2.5.35-1: isatty() problem (and solution)
To: cygwin AT cygwin DOT com
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Hi,

Recently, I updated my cygwin flex from 2.5.4 to 2.5.35-1 and my build has been breaking because of changes in flex.skl.

Specifically, I have a problem with extern declaration of isatty in flex.skl. In 2.5.35-1, it is done like this in flex.skl:

%if-c-only
m4_ifdef( [[M4_YY_ALWAYS_INTERACTIVE]],,
[[
    m4_ifdef( [[M4_YY_NEVER_INTERACTIVE]],,
    [[
#ifdef __cplusplus
extern int isatty M4_YY_PARAMS( int );
#endif /* __cplusplus */
    ]])
]])
%endif

The problem is that I've been long using flex to generate a C skeleton and then compiling it with a C++ compiler. Thus the extern declaration in the generated lexer is #if'ed out since __cplusplus is defined. One might say, well, just don't do that. But AFAICT, this is a sanctioned usage of flex. The flex manual states that it should be possible:

"flex provides two different ways to generate scanners for use with C++. The first way is to simply compile a scanner generated by flex using a C++ compiler instead of a C compiler." [1]

My proposal is to replace:

#ifdef __cplusplus
extern int isatty M4_YY_PARAMS( int );
#endif /* __cplusplus */

with a more C/C++ portable construct:

#ifdef __cplusplus
extern "C"
{
#endif /* __cplusplus */
extern int isatty M4_YY_PARAMS( int );
#ifdef __cplusplus
}
#endif /* __cplusplus */

This solves the problem by allowing both C and C++ compilers to work with generated lex.yy.c files.

Thanks,
dante

1. http://www.gnu.org/software/flex/manual/html_chapter/flex_19.html#SEC19


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