ajw@donk.UUCP (ajw) (10/15/88)
Here's a fragment of code I find myself staring at glumly from
time to time. It sets up the nucleus of a 2-way list, into
which extra malloc'd nodes will be inserted.
struct whatever {
struct whatever *next; /* next node */
struct whatever *prev; /* previous node */
int eresting_stuff; /* blah, blah, ... */
};
extern struct whatever tail;
static struct whatever head = { &tail, (struct whatever *)0 };
static struct whatever tail = { (struct whatever *)0, &head };
Without the 'extern', the compiler can't initialize 'head'. But
with it, I get a 'non-standard extension' warning, although
everything works just fine.
Question is, is there a way to achieve this initialization within
the letter of the law, leaving 'head' and 'tail' both static?
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Opinions individual author's. Read before breaking seal. No warranty implied.joe@modcomp.UUCP (10/16/88)
> Here's a fragment of code I find myself staring at glumly from time to time. > It sets up the nucleus of a 2-way list, into which extra malloc'd nodes will > be inserted. > > struct whatever { > struct whatever *next; /* next node */ > struct whatever *prev; /* previous node */ > int eresting_stuff; /* blah, blah, ... */ > }; > extern struct whatever tail; > static struct whatever head = { &tail, (struct whatever *)0 }; > static struct whatever tail = { (struct whatever *)0, &head }; > > Without the 'extern', the compiler can't initialize 'head'. But with it, I > get a 'non-standard extension' warning, although everything works just fine. This kind of initialization can only be safely done at run time. Even if you found a compiler that would accept such an unethical sequence of code, (two variables named "tail" -- wow!), you should not use it -- it would be unportable, even to a different C compiler on the same machine/OS. However, I see how being able to forward reference structures, just as easily as we can forward reference structure pointers today, could be appealing. Joe Korty uunet!modcomp!joe
gwyn@smoke.ARPA (Doug Gwyn ) (10/17/88)
In article <196@donk.UUCP> ajw@donk.UUCP (ajw) writes: >Question is, is there a way to achieve this initialization within >the letter of the law, leaving 'head' and 'tail' both static? Do the following in sequence: declare "tail" to be a struct whatever (an incomplete type) declare what a "struct whatever" looks like initialize a declaration of "tail" I don't have a copy of the standard at hand, but I think that's supposed to work.
peter@ficc.uu.net (Peter da Silva) (10/17/88)
> Without the 'extern', the compiler can't initialize 'head'. But with it, I > get a 'non-standard extension' warning, although everything works just fine. Would this work? static struct whatever emptylist[2] = { { &emptylist[1], NULL, NULL }, { NULL, &emptylist[0], NULL } }; #define head &emptylist[0]; #define tail &emptylist[1]; -- Peter da Silva `-_-' Ferranti International Controls Corporation. "Have you hugged U your wolf today?" peter@ficc.uu.net
scs@athena.mit.edu (Steve Summit) (10/23/88)
In article <196@donk.UUCP> ajw@donk.UUCP (ajw) writes: >Here's a fragment of code I find myself staring at glumly from >time to time. It sets up the nucleus of a 2-way list, into >which extra malloc'd nodes will be inserted. In general, I try not to have statically- and dynamically- allocated elements in the same list, because of the possibility of trying to free one of the statically-allocated ones. Preventing this can require ugly special cases. I usually use something like static struct whatever *head = NULL; static struct whatever *tail = NULL; insert_at_head(stuff) int stuff; { struct whatever *new; new = (struct whatever *)alloc(sizeof(struct whatever)); new->eresting_stuff = stuff; new->next = head; head = new; new->prev = NULL; if(tail == NULL) tail = new; } Note that head and tail are now pointers, not structures. This still requires one special case, to set tail initially, which is one reason I don't like doubly-linked lists. Complexity seems to be O(n**2) with the number of links, so that doubly-linked lists are four times as hard to get right as singly-linked ones. (You can insert at either the head or tail of a singly-linked list without special casing the initial case, if you're clever.) Steve Summit scs@adam.pika.mit.edu