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From: "marb...@yahoo.co.uk" <marblypup@yahoo.co.uk>
Newsgroups: comp.compilers
Subject: Re: Using a C struct to represent a node in a parse tree ... how many fields in the struct?
Date: Sat, 9 Apr 2022 05:03:55 -0700 (PDT)
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Hmm! The last time I wrote a (simple) compiler, it never occurred to me to use any of those methods! Here's my code (with the 'payload' removed from the struct and new_node):
I guess Johann would say I'm being extremely anal :-D

struct tree {
  enum token_type type;
  struct tree *child, *sibling;
};

static struct tree *new_node(int type) {
  struct tree *res=malloc_or_bomb(sizeof *res);
  res->type=type;
  res->child=res->sibling=NULL;
  return res;
}

static struct tree *add_child(struct tree *parent, struct tree *child) {
  /* Add child to end of parent's list of children. */
  struct tree **p;
  for (p=&parent->child; NULL!=*p; p=&(*p)->sibling)
    ;
  *p=child;
  return child;
}

static struct tree *add_first_child(struct tree *parent, struct tree *child) {
  /* Add child to start of parent's list of children. */
  child->sibling=parent->child;
  parent->child=child;
  return child;
}

(I think I pinched the child/sibling technique for storing trees with arbitrary numbers of children from an Infocom game!)
[The child/sibling approach is the way Lisp lists have worked since the last 1950s.
For reasons related to the architecture of the IBM 709, they're usually spelled CAR and CDR. -John]