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

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Robbie Hatley wrote:
> 
> Greetings, group.  I was just looking through my source code
> folders and found the following program, author unknown
> (did I write this myself years ago and forget? dunno).
> According to the C Standard, as I interpret it, this should
> not work... and yet, it does:
> 
> #include <stdio.h>
> int main(int argc, char **argv, char **envp)
> {
>     char **envstr = envp;
>     while(*envstr)
>     {
>        puts(*envstr);
>        ++envstr;
>     }
>     return 0;
> }
> 
> My expectation: either won't compile, or will crash if ran.
> 
> Actuality: gcc compiles this without error.  It does give
> warnings about the unused "argv" and "argc", but it does NOT
> give any error or warning about the illegal third parameter
> "envp".  On execution, the program does not crash, but prints
> a list of all environmental variables and their values.
> 
> How can this even compile?
> 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.
> It shall be defined with a return type of int and with no
> parameters:
> int main(void) { /* ... */ }
> or with two parameters (referred to here as argc and argv,
> though any names may be used, as they are local to the function
> in which they are declared):
> int main(int argc, char *argv[]) { /* ... */ }
> or equivalent; or in some other implementation-defined manner."
> 
> So, how can main() have *three* parameters if the standard
> demands it have either zero or two?  Why does the above
> program even compile?  Is this some kind of non-standard
> extension or something?

It's called a "Common extension".

ISO/IEC 9899:1999 (E)

Annex J
(informative)
Portability issues
1 This annex collects some information about portability
  that appears in this International Standard.
 
J.5 Common extensions

J.5.1 Environment arguments
1 In a hosted environment, 
  the main function receives a third argument, 
  char *envp[],
  that points to a null-terminated array of pointers to char,
  each of which points to a string 
  that provides information about the environment 
  for this execution of the program (5.1.2.2.1).

-- 
pete

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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

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