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Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc

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Myth__Buster <raghavanil4m@gmail.com> writes:
[...]
> Well, am aware of the possible implications of converting a void* into
> a function pointer. But, the initialization of function pointer p is
> present only for the completeness of the code. For the question being
> discussed, let's consider the code as under :
>
> #include <stdlib.h>
> int main(void)
> {
>     (void)malloc(sizeof(int (*)(void)));
>         // Ignore the usability of this statement, possible
>         // memory-leaks, and any other trivial factors involved
>         // which are irrelevant to the question being discussed.
>      return 0;
> }
>
> Now, would like to reiterate myself - malloc() returns a void*(data
> pointer as always known) even when we have allocated memory for a
> function pointer on a machine wherein data and function pointers are
> of different sizes. This behavior of malloc appears to me as improper
> and hence I am questioning its generality of returning a void*(data
> pointer) always. Shouldn't there be a note in malloc's documentation
> regarding such usage?

The expression

    sizeof(int (*)(void))

yields a value of type size_t.  The particular value is likely to be 4
or 8 on many typical systems.

malloc() only sees this value as its parameter; it doesn't know or care
that it happens to be the size of a function pointer.  So you might as
well have written:
    (void)malloc(4);
or
    (void)malloc(8);

malloc will attempt to allocate that many bytes of memory, and will
return a void* value that points to it (or a null pointer if the
allocation fails).

If you like, you can then use that allocated memory as an object of
pointer-to-function type.  There's nothing problematic about that.  A
function pointer is just another data type, like an object pointer, an
integer, a floating-point object, or a struct object.

But if you want to use malloc() to allocate an object of some type, you
need to convert the result to a pointer to that type.  Your original
code didn't do that correctly; it attempted to convert the result of
malloc() to a pointer-to-function type rather than to a
pointer-to-pointer-to-function type.

A typedef might make this a bit clearer:

    typedef void (*func_ptr)(void);
    ...
    func_ptr *p = malloc(sizeof *p);

p is a pointer to a pointer to function.

Another thing that you might *think* you can do is to set a function
pointer to point to space allocated by malloc():

    func_ptr wrong = malloc(some_size);

For this to work, some_size would probably have to indicate the size of
the function.  (That's assuming function pointers are implemented as
pointers to the function's code, which is not specified; a function
pointer might instead point to some kind of descriptor, for
example). You'd also need some way to initialize the newly allocated
memory so it acts like a function. This can be done on some systems, but
it's *extremely* system-specific.

What I *think* you're trying to do is allocate a single
pointer-to-function object using malloc() (that's the closest thing to
the code you showed us that makes any sense).  But it's not clear *why*
you're doing this. If you're only allocating space for a single pointer
object, why not just declare the object directly?

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
    Will write code for food.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 09:08 -0700
  Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-06-25 09:16 -0700
    Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 12:08 -0700
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc James Kuyper <jameskuyper@verizon.net> - 2012-06-25 15:27 -0400
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-25 15:30 -0400
        Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 12:47 -0700
          Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-25 16:10 -0400
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-06-25 20:42 +0100
        Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 13:14 -0700
          Re: Different sizes of data and function pointers on a machine --   void* return type of malloc, calloc, and realloc Ian Collins <ian-news@hotmail.com> - 2012-06-26 08:30 +1200
            Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 13:53 -0700
              Re: Different sizes of data and function pointers on a machine --   void* return type of malloc, calloc, and realloc Ian Collins <ian-news@hotmail.com> - 2012-06-26 09:00 +1200
              Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc James Kuyper <jameskuyper@verizon.net> - 2012-06-25 17:13 -0400
              Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 13:58 -0700
                Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Owen Jacobson <angrybaldguy@gmail.com> - 2012-06-26 00:03 -0400
          Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc James Kuyper <jameskuyper@verizon.net> - 2012-06-25 16:47 -0400
          Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc gordonb.vk4xa@burditt.org (Gordon Burditt) - 2012-06-25 16:06 -0500
          Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Keith Thompson <kst-u@mib.org> - 2012-06-25 14:17 -0700
          Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-06-25 17:24 -0400
  Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc James Kuyper <jameskuyper@verizon.net> - 2012-06-25 12:52 -0400
    Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Myth__Buster <raghavanil4m@gmail.com> - 2012-06-25 12:28 -0700
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc James Kuyper <jameskuyper@verizon.net> - 2012-06-25 15:59 -0400
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Keith Thompson <kst-u@mib.org> - 2012-06-25 14:08 -0700
      Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Nobody <nobody@nowhere.com> - 2012-06-26 21:29 +0100
  Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Keith Thompson <kst-u@mib.org> - 2012-06-25 13:13 -0700
  Re: Different sizes of data and function pointers on a machine -- void* return type of malloc, calloc, and realloc Paul N <gw7rib@aol.com> - 2012-06-25 13:17 -0700

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