chinn@butler.UUCP (David Chinn) (01/14/86)
*** REPLACE THIS LINE WITH YOUR MESSAGE ***
I have been looking at hoc, which appeared on the net
some time ago. I am relatively new to the language (C)
and so was puzzled by the following ....
typedef int (*Inst)(); /* machine instruction */
/* Inst is typedefed to a "pointer to a function
returning an integer */
.
Inst *pc; /* program counter during execution */
/* pc is declared to be a pointer to an Inst;
. or, a pointer to a pointer to a function */
ifcode()
{
Inst *savepc= pc; /* then part */
/* savepc is the same as pc */
pc = *((Inst **)(savepc+2)); /* next stmt */
^^^^^^^^^^^^^^^^^^^^^^
So what is this for?
}
I understand that the cast makes it (savepc+2) a pointer to a pointer to
an Inst (or a pointer to a pointer to a pointer to a function returning
an integer). But then the de-referencing star on the outside takes one
level of indirection away, making savepc+2 what it was anyway ( a
pointer to a pointer to a function returning an integer.)
I compiled hoc and tested the if statement; it worked. I edited code.c
and removed the seemingly extraneous cast and de-ref: it still worked.
Any enlightenment would be appreciated....
... uw-beaver david m chinn
!{tikal,teltone} box 2249
!dataio!butler!chinn kirkland, wash 98083gwyn@BRL.ARPA (VLD/VMB) (01/17/86)
pc = *((Inst **)(savepc+2)); is not the same as pc = (Inst *)(savepc+2); In the former case, you are saying that savepc+2 points at an (Inst *), which you retrieve and stuff into pc. In the latter case, you are setting pc to be equal to the pointer savepc+2. Not at all the same thing. Figuring out how this section of hoc works is great C practice.
thomas@utah-gr.UUCP (Spencer W. Thomas) (01/18/86)
In article <162@butler.UUCP> chinn@butler.UUCP (David Chinn) writes: >Inst *pc; /* program counter during execution */ > /* pc is declared to be a pointer to an Inst; > . or, a pointer to a pointer to a function */ > pc = *((Inst **)(savepc+2)); /* next stmt */ > ^^^^^^^^^^^^^^^^^^^^^^ > So what is this for? What appears to be happening here is that the location savepc+2 is a POINTER to a location containing the desired pc value. Certainly (in this case) removing the cast will not affect the code (you are still dereferencing savepc+2, and the value there is still a pointer), but the types on the left and right of the assignment DO NOT MATCH! If you run lint over the code, it will complain. The right hand side of the assignment will be an Inst, while the left side is an Inst *. The cast keeps the types right. -- =Spencer ({ihnp4,decvax}!utah-cs!thomas, thomas@utah-cs.ARPA) "Ask not what your kernel can do for you, but rather what you can do for you kernel!"
throopw@dg_rtp.UUCP (01/20/86)
> I am relatively new to the language (C) and so was puzzled by the > following .... > typedef int (*Inst)(); > Inst *pc; > ifcode() { > Inst *savepc= pc; > pc = *((Inst **)(savepc+2)); > } > I understand that the cast makes it (savepc+2) a pointer to a pointer to > an Inst (or a pointer to a pointer to a pointer to a function returning > an integer). But then the de-referencing star on the outside takes one > level of indirection away, making savepc+2 what it was anyway ( a > pointer to a pointer to a function returning an integer.) Yes and no. Mostly no. It makes it the same *type* it was before, but the *meaning* is completely different. This is a trick that was worried over in net.lang.c some time before. Actually, Inst *wants* to be declared to be typedef Inst (*Inst)(); and used as pc = *(savepc+2); but C doesn't allow this type of recursive definition. The alternative is to use "int" as a primitive type to "bottom out" the recursion, and then use the cast to keep lint happy about assigning a pointer to a function returning int to what wants to be a pointer to a pointer to a function returning int. Clear? If not (and there is demand by mail) I can repost some of the explaination that went by here a while back. > I compiled hoc and tested the if statement; it worked. I edited code.c > and removed the seemingly extraneous cast and de-ref: it still worked. I am not surprised. The statement pc = *((Inst **)(savepc+2)); implies the same bit-wise manipulation (on most machines) as pc = *(savepc+2); But there is a crucial difference. In the latter case, "pc" is being assigned a pointer of the wrong type, and lint will complain. In the former case, the types match, and no complaint ensues (unless you have a barbaric lint that complains about all pointer casts, as some seem to do). Now, why is the difference crucial? Because on some machines, it may conceiveably be important to have these types match correctly. -- Wayne Throop at Data General, RTP, NC <the-known-world>!mcnc!rti-sel!dg_rtp!throopw