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From: herndon@umn-cs.UUCP (Robert Herndon)
Newsgroups: comp.lang.c
Subject: Portable Memory Allocation
Message-ID: <1645@umn-cs.UUCP>
Date: Mon, 8-Jun-87 23:45:23 EDT
Article-I.D.: umn-cs.1645
Posted: Mon Jun  8 23:45:23 1987
Date-Received: Thu, 11-Jun-87 05:54:17 EDT
Distribution: comp.lang.c
Organization: University of Minnesota, Minneapolis
Lines: 33
Keywords: variant records
Summary: Genuine C Question


  I have an application where I want to represent nodes with
a finite, fixed number of possible fields, e.g., applications
equivalent to pascal's variant records.  Some of these nodes
will be relatively small, others large.  I would like to allocate
only enough memory for the union to hold the info necessary.
Is there any portable, COMPACT way to do this in C?

E.g., I have nodes which look like:
  struct node {
     struct this_stuff x;
     struct that_stuff y;
     struct othr_stuff z;
     union  var_part {
        struct childrenA a[27];
        struct childrenB b[3];
        struct childrenC c[42];
     } c;
  };

I would like to be able to make a call something like
    nodeptr = malloc(sizeof(node)-sizeof(node.c)+sizeof(node.c.b))
to reserve just enough space to hold fields x, y, z, and c.b[3].
It seems that the above expression might allocate more space than
necessary (due to packing considerations), but should work.  It
is also a kluge.  Is there a more elegant method?  Besides using
pascal?

-- 
Robert Herndon				Dept. of Computer Science,
...!ihnp4!umn-cs!herndon		Univ. of Minnesota,
herndon@umn-cs.ARPA			136 Lind Hall, 207 Church St. SE
herndon.umn-cs@csnet-relay.ARPA		Minneapolis, MN  55455