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.
Note: if you plan to hack on the driver, you should use MALLOC(x, ...), FREE(x, ...), REALLOC(x, ...), and CALLOC(x, ...) instead of the lowercase counterparts. Be sure to look at how these macros are used in the driver source itself in order to figure out the right syntax. 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. Various debug version of these macros also exist. Look at the macros defined near the end of the file config.h in the src directory.