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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Abstract Syntax Tree for C programs
Date: Tue, 04 Jun 2019 14:46:59 -0700
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James Kuyper <jameskuyper@alumni.caltech.edu> writes:
> On Tuesday, June 4, 2019 at 6:24:44 AM UTC-4, Bart wrote:
[...]
>> How difficult can it be? For example:
>> 
>>   int main (void) {
>>      fn(1,2,3,4);
>>      fn("hello world");
>>   }
>> 
>> There are no declarations of 'fn' visible; should this code pass or 
>> fail? Apparently gcc passes this unless I do:
>
> It should pass, if you're compiling for a language like C90 or Gnu90
> which does not require functions to be declared in advance. It should
> fail if compiling for a language like C99 or any version of C++ where
> functions are required to be declared in advance. If you refuse to tell
> gcc which of those languages you want it to compile, how can you
> reasonably expect it to guess? The developers of gcc being also the
> developers of GnuC, that is, quite reasonably, the language that it
> compiles by default.

Just to be clear, none of the differences between ISO C and GNU C
are relevant to this example.  In both C90 and GNU C90, the program
does not require a compile-time diagnostic but its run-time behavior
is undefined.  In C99, C11, GNU C99, and GNU C11, a diagnostic is
required, and recent versions of gcc do in fact emit a diagnostic,
even in default mode.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */