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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: Word For Today: =?utf-8?Q?=E2=80=9CUglification=E2=80=9D?=
Date: Thu, 14 Mar 2024 10:23:01 -0700
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Xref: csiph.com comp.lang.c:383607

Kaz Kylheku <433-929-6894@kylheku.com> writes:

[some editing of white space done]

> On 2024-03-12, Lawrence D'Oliveiro <ldo@nz.invalid> wrote:
>
>> From /usr/include/<<arch>>/bits/select.h on my Debian system:
>>
>>   #define __FD_ZERO(s)                                                  \
>>     do {                                                                \
>>       unsigned int __i;                                                 \
>>       fd_set *__arr = (s);                                              \
>
> This assignment has value;  it checks that, loosely speaking,
> s is an "assignment compatible" pointer with a fd_set *,
> so that there is a diagnostic if the macro is applied to
> an object of the wrong type.

More to the point, if the macro is applied to a value of the wrong
type.

>>       for (__i = 0; __i < sizeof (fd_set) / sizeof (__fd_mask); ++__i)  \
>>         __FDS_BITS (__arr)[__i] = 0;                                    \
>
> Here, I would have done memset(__arr, 0, sizeof *__arr).

That assumes that it is the entire fd_set that needs to be zeroed,
which may not be right.  Note the call to the __FDS_BITS() macro.

Better:

  #define __FD_ZERO(s) (                                                  \
    (void) memset(                                                        \
      __FDS_BITS( (fd_set*){(s)} ), 0, sizeof __FDS_BITS( (fd_set*){0} )  \
    )                                                                     \
  )

This definition:  avoids introducing any new identifiers;  checks
that the argument s yields an assignment compatible pointer;  and
provides a macro that can be used as a void expression (unlike the
original macro definition, which can be used only as a statement).