Note that Digital Unix *does* have mmap. Here is the top of the man page:

NAME
  mmap - Maps file system object into virtual memory

SYNOPSIS

  #include <sys/mman.h>

  void *mmap (
          void *addr,
          size_t len,
          int prot,
          int flags,
          int filedes,
          off_t off );

  [Digital]  The following definition of the mmap() function does not conform
  to current standards and is supported only for backward compatibility (see
  standards(5)):

  caddr_t mmap (
          caddr_t addr,
          size_t len,
          int prot,
          int flags,
          int filedes,
          off_t off );

STANDARDS

  Interfaces documented on this reference page conform to industry standards
  as follows:

  mmap():  XPG4-UNIX

  Refer to the standards(5) reference page for more information about indus-
  try standards and associated tags.

  Standards: standards(5)
----------- end of man page text ---------

Here is the error I got:

cc: Error: file.c, line 1283: In this statement, "p" and "(-1)" cannot be compared for equality or inequality. (noequality)
                        if(p != MAP_FAILED) {
---------------------------^