Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #20028

Re: A short-but-bizarre program that shouldn't work but does.

Show all headers | View raw

On 2012-04-28, Robbie Hatley <see.my.signature@for.my.contact.info> wrote:
> Actuality: gcc compiles this without error.  It does give

GCC also compiles this without error:

   int x = ({ puts("foo"); 42 });

if you don't give it any options to control dialect.

There is no restriction on what may compile. Firstly, compilers often
do not accept the one of the ISO C dialects but their own, unless they are
asked to accept one of the ISO C dialects (and then it's still a question of
which one).

Then, ISO C only requires programs which break syntax rules and semantic
constraints to be diagnosed. But to be diagnosed does not mean to be
rejected.

For instance GCC compiles this also, even in some ISO C mode, like -ansi
or -std=c99.

   double x = 3.0;
   int *p = &x;

There is a diagnostic, but it is treated as a warning and the code
is translated anyway.

Of course, the behavior is then undefined. As far as ISO C is concerned,
if the translation unit requires a diagnostic, it is incorrect. If the
implementation chooses to run it, that is in the domain of behavior that
is not deined by ISO C.

Note there is no diagnostic required if a program uses a nonstandard form of
the main startup function. That is just undefined behavior without
the requirement for a diagnostic.

> C99 section 5.1.2.2.1(2) clearly states:
> "The function called at program startup is named main.
> The implementation declares no prototype for this function.

See, since there is no prototype, no rule is being broken that requires
a diagnostic (no conflict between declaration and definition). The behavior is
simply undefined.

That just means that ISO C gives no opinion on what the behavior should be,
nor any recommended handling of the situation. Not that it is wrong.

Your environment does have an opinion: it supports the additional parameter for
passing the environment pointer.

> Is this some kind of non-standard extension or something?

Yes, this extra argument to main is a traditional extension found in Unix
environments which evidently has not been standardized.

It is not documented in POSIX 1003/Single Unix Spec.

What /is/ documented is the "extern char *environ[]" array; so it is better
to rely on that (if you're targetting POSIX and you need to enumerate
the environment variables rather than just perform lookup).

Reference: http://pubs.opengroup.org/onlinepubs/9699919799/functions/environ.html

Further down in the page it talks about main. Nothing is mentioned about
any third parameter, and I cannot find anything else in the document that
gives requirements about main.

-- 
If you ever need any coding done, I'm your goto man!

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

A short-but-bizarre program that shouldn't work but does. Robbie Hatley <see.my.signature@for.my.contact.info> - 2012-04-28 14:59 -0700
  Re: A short-but-bizarre program that shouldn't work but does. Kaz Kylheku <kaz@kylheku.com> - 2012-04-28 22:58 +0000
  Re: A short-but-bizarre program that shouldn't work but does. pete <pfiland@mindspring.com> - 2012-04-28 19:40 -0400
    Re: A short-but-bizarre program that shouldn't work but does. Robbie Hatley <see.my.signature@for.my.contact.info> - 2012-04-28 22:15 -0700
      Re: A short-but-bizarre program that shouldn't work but does. Philip Lantz <prl@canterey.us> - 2012-04-29 01:13 -0700
      Re: A short-but-bizarre program that shouldn't work but does. pete <pfiland@mindspring.com> - 2012-04-29 10:02 -0400
  Re: A short-but-bizarre program that shouldn't work but does. James Kuyper <jameskuyper@verizon.net> - 2012-04-28 22:15 -0400
  Re: A short-but-bizarre program that shouldn't work but does. Keith Thompson <kst-u@mib.org> - 2012-04-28 20:18 -0700
  Re: A short-but-bizarre program that shouldn't work but does. "Heinrich Wolf" <invalid@invalid.invalid> - 2012-04-29 11:20 +0200
    Re: A short-but-bizarre program that shouldn't work but does. "Heinrich Wolf" <invalid@invalid.invalid> - 2012-04-29 11:31 +0200
    Re: A short-but-bizarre program that shouldn't work but does. "Heinrich Wolf" <invalid@invalid.invalid> - 2012-04-29 11:38 +0200
    Re: A short-but-bizarre program that shouldn't work but does. "Heinrich Wolf" <invalid@invalid.invalid> - 2012-04-29 12:26 +0200

csiph-web