Note: some machines don't allow the system calls to be replaced with
a function of the same name.  In particular, the compile will fail during
the link phase with a 'duplicate symbol' error.  Most of the machines
I've seen that exhibit this behavior use shared libraries (NeXT, RS/6000).
Also note that smalloc.c uses sbrk() and brk() and these routines cannot
be used in a program that uses malloc() (read the sbrk() man page for more
information).  Even if the program doesn't explicitly call malloc, it will
fail if it calls system calls that internally use malloc.  Thus the only
truly safe way to use sbrk and brk is to replace the definition of system
malloc.  And some machines do not allow system malloc to be replaced.
<p>
Note: if you plan to hack on the parser, you should use MALLOC(x),
FREE(x), REALLOC(x), and CALLOC(x) instead of the lowercase counterparts.
The uppercase versions are macros that get replaced with the proper
function names depending upon which malloc package is chosen.  This
change was made in order to provide for those machines that complain
when functions are added with the same names as system calls.  The change
also allows some stats to be collected even when using system malloc.