Date: Sun, 7 Nov 1999 17:28:53 +0200 (IST)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
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To: Dave Dribin <drib AT enteract DOT com>
cc: djgpp AT delorie DOT com
Subject: Re: Allegro 3.12: page fault in generate_optimized_palette()
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On 6 Nov 1999, Dave Dribin wrote:

> As you can see (max - j) ends up being negative and causing havoc on
> the memcpy().  Is this a known problem?  Am I using the function
> wrong?

Yes and yes.  memcpy has the following prototype (see <string.h> and
the documentation of memcpy):

  void * memcpy(void *_dest, const void *_src, size_t _n);

The last argument is type size_t, which is an unisigned integral
type.  So your negative value is interpreted as a very large positive
value, and causes your program to crash because you are writing
outside the limits of the array common[].

In other words, your code has a bug.

In addition, please note that memcpy is not supposed to be used for
copying overlapping regions, and your code does precisely that.  Use
memmove if the source and destination overlap.