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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: isspace() executes without ctype header?
Date: Wed, 14 Jun 2017 09:35:00 -0700
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bartc <bc@freeuk.com> writes:
> On 14/06/2017 02:13, DFS wrote:
>> Seems strange, but the code compiles (Tiny C on Windows) and runs just 
>> fine with only stdio.h and stdlib.h includes.
>> 
>> Online resources say ctype.h is the required header (which I didn't know 
>> obviously).
>> 
>> I compiled the program with Pelles and got warnings:
>> 
>> Undeclared function 'isspace' (did you mean 'vsscanf'?); assuming 
>> 'extern' returning 'int'.
>> 
>> 
>> Why is it working without ctype.h?
>
>
> Try this compiler** (choose either -32 or -64 version depending on your 
> Tiny C):
>
> https://github.com/bartg/langs/blob/master/bccproj/mcc32.c
> https://github.com/bartg/langs/blob/master/bccproj/mcc64.c
>
> Use tcc to compile it, eg:
>
>    tcc mcc32.c -omcc.exe
>
> Then run as:
>
>    mcc program.c
>
> You won't be able to run the result (as it needs extra bits and pieces, 
> code is only for x64 etc), but can give a second opinion on code that 
> apparently passes error-free with other compilers.

Why would a second opinion be needed?  The standard is clear.

Under C90, calls to undeclared functions are permitted.  Under C99 and
C11, they're a constraint violation.  <stdio.h> and <stdlib.h> do not
declare isspace(); <ctype.h> does.

This:

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    puts(isspace(' ') ? "OK" : "Huh?");
    return 0;
}

is a valid program with well defined behavior under C90, and violates
a constraint in C99 and C11.  An implementation that compiles it
with no diagnostic is not a conforming C99 or C11 implementation
(a bug if it claims to conform to C99 or C11).

Of course the solution for programmers is easy.  If you call isspace(),
add "#include <ctype.h>", even if your code is only intended to be
compiled as C90.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"