memory allocation problems

[Rog Skubowius]

In article <11589 at arctic.nprdc.navy.mil> asbury at nprdc.navy.mil (Mike Asbury) writes:
>HELP!  I have a large program (over 100,000 lines of code) written in Turbo C
>with memory leakage somewhere.  I am looking for any programs that will help me 
>find C memory allocation errors.  Any pointers to shareware or commercial 
>programs will be appreciated.  
>					Thanks in advance,
>					Mike

	My first question to you is whether or not you have any function 
	calls 'farmalloc(), farfree(), or farrealloc()'? Also, what version
	of the compiler are you using? If it is Turbo C++ v.1.0, the problem
	may not be yours. I found a bug in their farrealloc() call when
	I was asking for about 650K of total memory. The screen memory
	is overwritten by Turbo as it thinks this is valid 'usable' memory.
	Anyway, back to your problem. I encourage you to write 3 ( or
	possibly 6 ) routines, 1 per C dynamic function routine. For
	instance, CheckMalloc(), CheckFree(), CheckRealloc(). If you
	use a prefixing ( & possibly postfixing ) signature on the allocated
	buffer, & embed the buffer size as well, you can easily fix this
	problem & find the bug. Let me show this:

	unsigned short CheckMalloc( void ** buffer, unsigned short size )
	{
		The application on top of CheckMalloc() wants 'size' bytes,
		but instead, now call malloc() for 'size' + say 3. Then, write
		the letters 'foo' in the first 3 bytes of the allocated memory
		& assign (*buffer) to the next byte following the 'foo' signature.

		fooXXXXXXXXX	-> where a pointer to the 1st X is returned 
						-> through (*buffer) to the calling function.

	}

	void CheckFree( void * buffer )
	{
		Now, if the application has corrupted the buffer, you can
		determine it here. You see, *(buffer - 3) == 'f', 
		*(buffer - 2) == 'o', *(buffer - 1) == 'o'. You should really
		macro the checking signature, its length and then use 
		strncpy() to do the checking of signature. Anyway, if this
		doesn't check out...bang...you've caught some memory corruption
		as either the original 'CheckMalloc()'d pointer was moved, or
		someone overwrote part of your buffer.
	}

	CheckRealloc()
	{
		Pretty obvious what to do here --> Check out the signature 
		& act accordingly.
	}

	This whole scheme can be improved by using a signature at the
	start & one at the end as well, and then putting the allocated
	buffer size in the buffer itself so that the end of application
	buffer & start of end signature can be found. Dig?