On the athena workstations, the malloc.h header file only exists on
DECstation and RS/6000 workstations.  To get malloc defined on other
platforms, just put the following paragaph in your own header file:

#if defined(_AIX) || defined(ultrix)
#include <stdlib.h>
#else
#ifdef __STDC__
extern void *malloc(unsigned int), realloc (void *, unsigned int);
extern void free(void *);
#else
extern char *malloc(), realloc();
#endif
#endif

Then remove references to malloc.h from your source code.