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Re: Why sizeof(main) = 1?

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<kzelechowski@e3tech.local> writes:
> (sizeof main == 1) because (main) is treated as a logical value.  Compare: 
> { if (main) yadda; }

No, that's not the reason.

Consider that an `int` can also be treated as a logical value;
`if (1)` is valid, but `sizeof 1` is the same as `sizeof (int)`,
which is rarely 1.

The expression `main`, or any function name, is implicitly converted,
in most contexts, to a pointer whose type depends on how you declared
and/or defined the function.

Any pointer value can be used as a condition; it's implicitly
checked for inequality to 0, i.e., to the null pointer.

    if (main) yadda;

is well defined but not useful, since `main`, after conversion,
can never be a null pointer.

But when it's the argument of `&` or `sizeof`, this implicit
conversion doesn't happen.  `&main` gives you the address of the
`main` function, just like `main` by itself does.  `sizeof main`
*would* be the size of a function, but there is no such concept in C,
so it's a constraint violation, requiring a diagnostic.

> Użytkownik "Myth__Buster"  napisał w wiadomości grup 
> dyskusyjnych:clcm-20130104-0002@plethora.net...
>
> On a Linux system with gcc, I am just wondering why sizeof(main) can
> be 1 or sizeof when applied on any function name can yield 1 ever? Or
> is it only gcc's perspective to say sizeof of an implicit function
> pointer to be 1 since it gives sizeof(void) to be 1 based on the
> backward compatibility with the pre C99 notion that void* had its
> predecessor char* and usually sizeof(char) being 1?
>
> Also, I tried the invariably buggy code to see if at all sizeof(main)
> = 1 can be realized.

I think you mean pre C89 or pre-C90.  C89/C90 introduced the `void`
keyword and the `void*` pointer type; C99 didn't make any real
changes in that area.  But this has nothing to do with backward
compatibility; the same standard that introduced the `void` keyword
and type also prohibited applying `sizeof` to it.

(And `sizeof (char) == 1` by definition.)

`sizeof main == 1` because of a gcc extension.  Standard C does
not permit pointer arithmetic on void* or on function pointers.
Gcc permits both, and specifies that, for example, adding 1 to a
void* or function pointer advances it by one byte (like adding 1 to
a char*).  The authors of gcc chose to implement this by pretending
that the size of the type `void`, and the size of any function type,
is 1 (neither has a defined size in standard C).

I can see that pointer arithmetic on `void*` might be convenient in
some cases.  The usefulness of arithmetic on function pointers is
less obvious, but I suppose it could have its uses.  It's a pity
that implementing such arithmetic had the size effect of making
both `sizeof (void)` and `sizeof main` equal to 1; both results
are nonsensical.

-- 
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"
-- 
comp.lang.c.moderated - moderation address: clcm@plethora.net -- you must
have an appropriate newsgroups line in your header for your mail to be seen,
or the newsgroup name in square brackets in the subject line.  Sorry.

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Thread

Why sizeof(main) = 1? Myth__Buster <raghavanil4m@gmail.com> - 2013-01-04 18:14 -0600
  Re: Why sizeof(main) = 1? Barry Schwarz <schwarzb@dqel.com> - 2013-02-26 10:51 -0600
    Re: Why sizeof(main) = 1? Keith Thompson <kst-u@mib.org> - 2013-03-11 18:25 -0500
  Re: Why sizeof(main) = 1? Jasen Betts <jasen@xnet.co.nz> - 2013-02-26 10:51 -0600
  Re: Why sizeof(main) = 1? gordonb.k8xjg@burditt.org (Gordon Burditt) - 2013-02-26 10:51 -0600
  Re: Why sizeof(main) = 1? <kzelechowski@e3tech.local> - 2013-09-02 04:07 -0500
    Re: Why sizeof(main) = 1? James Kuyper <jameskuyper@verizon.net> - 2013-09-06 23:25 -0500
      Re: Why sizeof(main) = 1? Keith Thompson <kst-u@mib.org> - 2013-09-11 17:26 -0500
        Re: Why sizeof(main) = 1? James Kuyper <jameskuyper@verizon.net> - 2013-09-12 11:29 -0500
    Re: Why sizeof(main) = 1? Keith Thompson <kst-u@mib.org> - 2013-09-06 23:25 -0500
      Re: Why sizeof(main) = 1? Robert Wessel <robertwessel2@yahoo.com> - 2013-09-11 17:27 -0500
        Re: Why sizeof(main) = 1? James Kuyper <jameskuyper@verizon.net> - 2013-09-12 11:29 -0500
        Re: Why sizeof(main) = 1? Keith Thompson <kst-u@mib.org> - 2013-09-12 11:30 -0500
    Re: Why sizeof(main) = 1? Ken Brody <kenbrody@spamcop.net> - 2013-09-06 23:25 -0500

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