Re: Fixing a sample from K&R book using cake static analyser

Ben Bacarisse <ben@bsb.me.uk> writes:

Anton Shepelev <anton.txt@gmail.moc> writes:
>

Ben Bacarisse:
>

I don't get why the goto crowd want to complicate it so
much.

>
Will someone post a goto-less that fixes what I perceive as
bugs in the original:
>
  a.  the failure to free() a newly allocated nlist in case
      of a later error,
>
  b.  the failure to free() a newly allocated np->name in
      case of a later error,
>
  c.  the failure to keep np->defn unchaged if the
      allocation of the new defn value failed.
>
And my perception be wrong, let us first establish the
actual bug(s).

>
With the usual trepidation that this is untested (though I did compile
it), I'd write something like:
>
struct nlist *install(const char *name, const char *defn)
{
     struct nlist *node = lookup(name);
     if (node) {
          char *new_defn = strdup(defn);
          if (new_defn) {
               free(node->defn);
               node->defn = new_defn;
               return node;
          }
          else return NULL;
     }

That's fine, certainly, but I'm a fan of early fail-and-bail, which
permits the didn't-fail side of the if to remain unindented:

      if (node) {
           char *new_defn = strdup(defn);
           if (!new_defn)
               return NULL;

           free(node->defn);
           node->defn = new_defn;
           return node;
      }

     else {
          struct nlist *new_node = malloc(sizeof *new_node);
          char *new_name = strdup(name), *new_defn = strdup(defn);
          if (new_node && new_name && new_defn) {
               unsigned hashval = hash(name);
               new_node->name = new_name;
               new_node->defn = new_defn;
               new_node->next = hashtab[hashval];
               return hashtab[hashval] = new_node;
          }
          else {
               free(new_defn);
               free(new_name);
               free(new_node);
               return NULL;
          }
     }

I think I'd deviate a little more on this side, as I don't like doing
the strdups when you know the malloc has failed. Again, early
fail-and-bail applies:

     else {
          struct nlist *new_node = malloc(sizeof *new_node);
          if (!new_node)
               return NULL;
         
          char *new_name = strdup(name), *new_defn = strdup(defn);
          if (!new_name || !new_defn) {
               free(new_defn);
               free(new_name);
               free(new_node);
               return NULL;
          }
         
          unsigned hashval = hash(name);
          new_node->name = new_name;
          new_node->defn = new_defn;
          new_node->next = hashtab[hashval];
          return hashtab[hashval] = new_node;
     }

You could split the strdups too, of course, but it's itsy-bitsy.  In all
honesty, I'd probably actually code with gotos, and jump to whichever
bit of cleanup was relevant. Obviously, I've not even compiled this,
we're only talking about shape of code, not generation of executables.

}
>
We have four cases:
>
  node with the name found
     new definition allocated
     new definition not allocated
  node with the name not found
     new node, name and definition all allocated
     not all of new node, name and definition allocated.

I once came across a quirky coding standard that would have split
this into 3 functions. The outermost if/else blocks would
have been two separate helper functions.

struct nlist *install(const char *name, const char *defn)
{
     struct nlist *node = lookup(name);
     if (node)
          return update_node(node, defn);
     else
          return new_node(name, defn);
}

The principle being that every function should do one job, and be
visible in one small screenful. The choice of code structure in each of
the two helper functions is now simplified, as you don't need to
consider anything in the other helper function.

Phil
--
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