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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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On 2012-06-25 20:58:17 +0000, Myth__Buster said:
> 
> On Monday, June 25, 2012 1:53:10 PM UTC-7, Myth__Buster wrote:
>> 
>> What would that achieve?  malloc returns a void* and a void* is not> > 
>> compatible with a function pointer.  Would you add a note for every> > 
>> function that returns something incompatible with something else?
>> 
>> -- 
>> Ian Collins
> 
> Achievement : Clarity and completeness of malloc's behavior in malloc's 
> documentation. More appropriately, a word of caution similar to this - 
> In C, malloc when used to directly allocate memory for function 
> pointers(without any third-variable's allocation such as encompassing 
> struct variable, etc) on machines wherein the data and function 
> pointers are of different sizes, the return data pointer wouldn't be a 
> proper pointer with which the entire memory allocated for function 
> pointer could be accessed. Hence, care should be exercised before the 
> allocated memory could be used. (On similar lines, the applicable 
> alternative approaches can be mentioned therein to accomplish the same 
> and thereby help the reader/programmer).

Given the declaration
    int (*p)(void) = malloc(sizeof(int (*)(void)));
and reading it "for intent" rather than as written:
 - 'p' identifies a function pointer value.
 - malloc is being (mis-)used to allocate memory for a function, and is 
incorrectly being expected to return a pointer to that function.

The second thing is flatly nonsensical. Nothing malloc could reasonably 
do with the block of memory it allocates could make that block into a 
function, and C does not provide tools for composing functions in 
random blocks of memory, so there's no way subsequent code could turn 
the block into a legitimate function either.

I don't know how you've gotten to "malloc is allocating a function 
pointer" here. It's not: it's allocating a block of memory, then 
*returning* a (value) pointer to that block. Trying to interpret the 
pointed-to block as a function will at best crash your program, so the 
returned pointer cannot be interpreted as a pointer to a function. 
(Ignoring the case where malloc returns a null pointer.)

The only sensible values for function pointers (which 'p' already 
identifies, without any need for dynamic allocation of a pointer-sized 
block) are the values obtainable from the addresses of functions in 
your program:

	int random_number(void) {
		return 4; /* generated by fair die roll */
	}

	int main(void) {
		int (*p)(void) = random_number;
		/* and so on */
		return 0;
	}

It's completely unnecessary to note in the docs that malloc can't 
return a function pointer for the same reason that's unnecessary to 
mention that memcpy can't receive a function pointer as an argument: 
it's taken as given that if you use function pointers, you understand 
how they're different from value pointers, and how the pointed-to 
entities differ in both allocation and behaviour. Non-null function 
pointers are *never* compatible with value pointers, and vice versa, as 
far as I know.

If you really wanted to use malloc to "allocate a function pointer" 
(for whatever unlikely reason), you'd need to add another step of 
indirection, and declare 'p' as a pointer to a function pointer:

	int (**p)(void) = malloc(sizeof(*p));
	if (p != NULL) {
		*p = random_number;
		/* and so on */
	}

Here, malloc allocates a block at least the size of a function pointer, 
and returns a pointer *to* that block. I can't imagine why you'd write 
code this way, though.

-o

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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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