Date: Sun, 2 Apr 95 17:28 MDT
From: mat AT ardi DOT com (Mat Hostetter)
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: Dynamic allocation question
References: <01HOV5OOOLOM8ZEIJR AT cc DOT uab DOT es> <199504022219 DOT SAA08085 AT delorie DOT com>

>>>>> "DJ" == DJ Delorie <dj AT delorie DOT com> writes:

    >> If ASSUMPTION 2 is true, is there an ANSI-C manner to get from
    >> the O.S.  the amount of memory a pointer points to? I have
    >> tried "sizeof(pointer)", but it only returns the size of a
    >> single element.

    DJ> There is no portable way of doing this.  ANSI and POSIX
    DJ> specifically stay away from memory allocation techniques.  In
    DJ> theory, you may be able to reverse-engineer malloc() (or, for
    DJ> djgpp, just read the sources) and see how free() figures it
    DJ> out, but in general such functionality is not available.
    DJ> Given the existence of realloc(), it often does not need to be
    DJ> available.

Some people write their own my_malloc/my_realloc/my_free that keep
track of the block size in a portable way and use these routines
consistently.  What you do is allocate four bytes more than you need
with `malloc', store the size at the base of the allocated memory, and
then return the allocated address plus four bytes.  Your
my_realloc/my_free have to know about this four byte offset, and you
should be careful to handle my_realloc(NULL, size) and my_free(NULL)
correctly.

You can then write a simple function which will tell you the size of a
block allocated by these functions, basically just returning
((long *)p)[-1].

Some programmers also install magic sentinel values before and after
the requested block of memory, which can get checked at various times
(e.g. all calls to my_free and my_realloc) for memory smashage.

-Mat