Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!necntc!ima!haddock!karl
From: karl@haddock.ISC.COM (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: When do you #include <stdlib.h>?
Message-ID: <3122@haddock.ISC.COM>
Date: 22 Mar 88 23:38:34 GMT
References: <4991@sigi.Colorado.EDU>
Reply-To: karl@haddock.ima.isc.com (Karl Heuer)
Distribution: na
Organization: Interactive Systems, Boston
Lines: 21

In article <4991@sigi.Colorado.EDU> murillo@boulder.Colorado.EDU (Rodrigo Murillo) writes:
>I realized today that you can call functions that are in stdlib.h
>without actually including them in your file.  But on the otherhand
>you can include them.  Is there a hard fast rule?  Let me give an
>example:
>     printf("foo: %dl",atol(argv[1]));

The rule is that you are allowed to omit the #include, provided that you
supply a compatible declaration yourself.  Thus, you could have said
"extern long atol(char *);" or "extern long atol();" and your program would
have been correct.  Since nothing explicit was encountered, the implied
declaration was "extern int atol();" which was incorrect.

>Why doesn't the compiler barf when it encounters atol() when the stdlib.h is
>NOT included?

Absence of a prototype has to be permitted for compatibility with pre-ANSI C.
The better compilers will have an option to generate a warning, though.

Btw, that should be "%ld", not "%dl".

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint