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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: on ``warning: array subscript has type =?utf-8?B?4oCYY2hhcg==?= =?utf-8?B?4oCZJyc=?=
Date: Mon, 06 Dec 2021 13:12:40 -0800
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Meredith Montgomery <mmontgomery@levado.to> writes:
> I can't explain this.
>
> %make warning
> gcc -Wall -x c -g -std=c99 -pedantic-errors    warning.c   -o warning
> In file included from warning.c:2:
> warning.c: In function ‘getop’:
> warning.c:38:27: warning: array subscript has type ‘char’ [-Wchar-subscripts]
>    38 |     while (isdigit(s[++i] = c = getch()))
>       |                    ~~~~~~~^~~~~~~~~~~~~
>
> I understand the warning is trying to tell me that a char is not a good
> idea as an array subscript as a char could be negative.  However, the
> array subscript is /i/, not /c/.  So I'm puzzled.  Thank you!
[...]

The isdigit() function is probably implemented as a macro that uses
array indexing.  (All library functions can also be implemented as
macros.)  The part of the compiler that generates the warning message
doesn't know that the array indexing operator is the result of a macro
expansion, so it warns you as if you had written the indexing operator
directly.

If you replace `isdigit` by `(isdigit)` or precede the call by
    #undef isdigit
it will bypass the macro and call the actual function, and you probably
won't see the warning.  I suggest this as a diagnostic method, not as a
solution.

Having said that, the warning is quite useful even if it's less than
clear.  The argument to isdigit() is of type int, and is required to be
either within the range of unsigned char *or* equal to EOF (which is
typically -1).  Which means, annoyingly, that a char value passed to
isdigit() and friends should be cast to unsigned char.  Passing a
negative char value (other than EOF) to isdigit() has undefined behavior.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Philips
void Void(void) { Void(); } /* The recursive call of the void */