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

Groups > comp.lang.c > #163731

Re: on ``warning: array subscript has type ‘char’''

Show all headers | View raw

Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> 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.

Lol!  That's the C language we love!  Order is back in the house now.
Thank you so much for the great info.  It did not occur to me that
/isdigit/ could be macro.  I will probably never forget this anymore.
Thank you!

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

Thread

on ``warning: array subscript has type ‘char’'' Meredith Montgomery <mmontgomery@levado.to> - 2021-12-06 17:40 -0300
  Re: on ``warning: array subscript has type ‘char’'' Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2021-12-06 13:12 -0800
    Re: on ``warning: array subscript has type ‘char’'' Meredith Montgomery <mmontgomery@levado.to> - 2021-12-07 15:11 -0300

csiph-web