5.  Alternative implementations

      These problems were actually the inspiration for the first alternative malloc implementations. Since their main aim was debugging, they would often use techniques like allocating a guard zone before and after the chunk, and possibly filling these guard zones with some pattern, so accesses outside the allocated chunk could be detected with some decent probability. Another widely used technique is to use tables to keep track of which chunks are actually in which state and so on.

      This class of debugging has been taken to its practical extreme by the product "Purify" which does the entire memory-colouring exercise and not only keeps track of what is in use and what isn't, but also detects if the first reference is a read (which would return undefined values) and other such violations.

      Later actual complete implementations of malloc arrived, but many of these still based their workings on the basic schema mentioned previously, disregarding that in the meantime virtual memory and paging have become the standard environment.

      The most widely used "alternative" malloc is undoubtedly ``gnumalloc'' which has received wide acclaim and certainly runs faster than most stock mallocs. It does, however, tend to fare badly in cases where paging is the norm rather than the exception.

      The particular malloc that prompted this work basically didn't bother reusing storage until the kernel forced it to do so by refusing further allocations with sbrk(2). That may make sense if you work alone on your own personal mainframe, but as a general policy it is less than optimal.