Hey guys,
I am an EE that has found myself doing more and more programming lately, so I have a small question for you guys. I have a good understanding of pointers, but I don't really understand the reasoning behind this:
Why are both the * operators necessary? I've seen this around a bunch, and it seems like it's a pointer to a pointer? Is that correct? I don't really understand it. What is the 'name' of this technique, and what is it used for?Code:int i, index = *((int*)ptr);
Thanks!
(int*) says "pretend, for the moment, that ptr is an int*". * means "go to the address held by this pointer and see what's there".
I just started using this one (presuming that ptr is actually a char*). It works if you want to pull a number out of a block of data if the number was stored as an integer, and you know the offset. Unless the block is just an array of ints, you cannot simply use an int pointer -- eg, a struct containing an int:
Internet packets are constructed this way, but unless you can fit a struct onto them, you can't access what would be "somestruct->num". But you can do this:Code:struct { char string[64]; int num; } *somestruct;
Don't ask me how to explain it completely (I think tabstop already did), however, as I probably tried every arrangement of asterisks and parentheses I could think of before it worked (and this is probably the only way it works). Or maybe I got it from some kind of google.Code:struct somestruct *sp; char *cp=(char*)sp+64; int X=*((int*)cp);
So the fact that you've seen it somewhere else is comforting...
C programming resources:
GNU C Function and Macro Index -- glibc reference manual
The C Book -- nice online learner guide
Current ISO draft standard
CCAN -- new CPAN like open source library repository
3 (different) GNU debugger tutorials: #1 -- #2 -- #3
cpwiki -- our wiki on sourceforge
it's not a pointer to a pointer but one that has been cast appropriately and then dereferenced.
I sort of see what you are saying, but why won't this work?Originally Posted by tabstop
I don't see why the casting needs a dereference operator. Is it casting the value as an int pointer type, as opposed to just an int type?int i, index = (int)*ptr
that's exactly it.
let's say ptr is void* then *ptr is just not defined
let's say ptr is char*
then *ptr is a contents of 1 char, and you cast it to int later
*(int*)ptr is contents of 4 (maybe 8 on some compilers) bytes interpreted as int
don't you think that the contents of 1 byte will be slightly different from the contents of 4 or 8 bytes?
All problems in computer science can be solved by another level of indirection,
except for the problem of too many layers of indirection.
– David J. Wheeler
I see what you're saying. I did not realize that it was casting an int pointer type and not just an int type.
Thanks for the help, everyone!
