NetNews Usenet Archive 1993 #3/NN_1993_3.iso/spool/comp/lang/fortran/5133

Text File | 1993-01-23 | 2.3 KB | 59 lines

Newsgroups: comp.lang.fortran
Path: sparky!uunet!cs.utexas.edu!hellgate.utah.edu!lanl!cochiti.lanl.gov!jlg
From: [e-mail] (J. Giles)
Subject: Re: Block Data is evil
Message-ID: <[e-mail]>
Sender: [e-mail]
Organization: Los Alamos National Laboratory
References: <[e-mail]> <[e-mail]>
Date: Fri, 22 Jan 1993 18:31:06 GMT
Lines: 47

In article <[e-mail]>, lfini@sisifo (Luca Fini) writes:
|> W. Donald Rolph ([e-mail]) wrote:
|> :
|> : Equivalence is in some cases the only possiblity for run time memory allocation
|> :
|>
|> Equivalence is *not* a run rime memory allocation, you simply call the
|> same memory area with different names (and possibly use it as different
|> data types).

Yes, and that permits you to do run-time management of a statically
allocated "maximum" space.  You figure out what the maximum memory
you ever need is and you statically allocate that much:

      PARAMETER (MAXMEM=1000000)
      COMMON /MEMORY/ MEM(MAXMEM)

This reserves one million integer sized words.  Now, you can write
a MALLOC routine which returns avalable space from within this array
in the form of an index (you actually have two mallocs, one for
integers and reals, the other for complex and doubles - since the
indices must be scaled differently).  To use:

      PARAMETER (MAXMEM=1000000)
      COMMON /MEMORY/ MEM(MAXMEM)
      ...
      REAL RMEM(1)            !declare length one, you use more
      EQUIVALENCE (MEM(1),RMEM(1))
      ...
      RA = MALLOC(100)            !set RA to index of a 100-long block
      ...
      DO 10 I=0,99
         RMEM(RA+I) = ...
      ...
10    CONTINUE

You could even write a preprocessor which allowed you to write RA[I]
and the processor would automatically turn it into RMEM(RA+I).  (Note
the similarity to the C rule of turning a[i] into *(a+i) - the only
difference is that C calls the common memory area `*' instead of `MEM'.)
To use MEM as a double or complex, you simply declare DMEM and CMEM
arrays and equivalence them to MEM as well.  You should then use DALLOC
to reserve space and your `pointers' should be named beginning with D
or C so the preprocessor can figure out which equivalenced array to use.

--
J. Giles