Thread: Double Pointer Question

Hi, new to these boards and they seem like a great place to spend some time. I have a question about double pointers to structs that I'm hoping someone can help me out with. If I have a double pointer to an array of structs, how can I get at a member of the nth struct?

Let me explain that in code. The func is a struct:

Code:

func ** functions = xmalloc( foo * sizeof( func * ) );
unsigned int numDone = 0;

while( i = 0; i < bar; ++i )
{
    *( func + numDone++ ) = makeFunc(); //returns a func *
}

//print the member "numInsts" from the nth function:
printf( "%u\n", *(*(func + n )).numInsts );

But, that doesn't compile. So, how do I dereference the double pointer and get myself a struct to work with. I feel like I'm just missing a subtlety in the syntax.

Thanks for any help,
Matt

Originally Posted by krock923

Code:

func ** functions = xmalloc( foo * sizeof( func * ) );
unsigned int numDone = 0;

while( i = 0; i < bar; ++i )
{
    *( func + numDone++ ) = makeFunc(); //returns a func *
}

But, that doesn't compile. So, how do I dereference the double pointer and get myself a struct to work with. I feel like I'm just missing a subtlety in the syntax.

Thanks for any help,
Matt

I don't know what error you're getting, since I don't feel like making a testable .c file out of this, but I'm going to guess: *(func + numDone++) complains that func is a type (not the variable functions) and can't be added to. Do I win?

Not compile errors, but: shouldn't bar = foo? (Unless you're allocating more memory than you need.) Why not use bar inside the loop as

Code:

*(functions + bar) = makeFunc(); //?

[Of course, if this for loop occurs in multiple places so that you're not doing them all at once, then, well, ok, I guess.]

Yes, bar should = foo. That was just a typo on my part and they are indeed the same values in my code.

Unfortunately, you don't win at the error guessing game Here's the error I get:

error: request for member 'numInsts' in something not a structure or union

Try (**(func+n)).numInsts instead. Dot associates before the outer * otherwise, IIRC. (Actually I didn't recall that at all, I had to try things )

Yup, that did it. Thank you for the help.

xmalloc()?? Is that one of your functions? I've never heard of it.

It's something I read in a GNU doc somewhere - I just had to implement it. It calls malloc, checks for the null pointer and kills the program if it finds one.

I figure if the runtime system can't get me anymore memory, a kernel panic isn't too far anyway and there's no use trying to forge ahead.

Sorry, I was trying to not show parts that are unnecessary. To point out your arguments, which are absolutely rational from only seeing that fragment, I'll admit:

The foo/bar thing was pointed out a few posts up.
numDone has other stuff done to it between its initialization and that loop. Those things just aren't necessary in the context of the question I asked.
The func thing was a typo. Where it says func in that assignment, it's a variable name in my code.

Thanks for the pointer (groan!) on how to do arrays. I'm not sure why I insist on doing that whenever I write C. I suppose that it makes things harder than necessary. The reason that I didn't use -> was because I honestly had no idea you could do that in C. I thought that was only a C++ thing. It really makes much more sense that way, though.

At first glance, your avatar looks like some variation of selection sort, but that's probably too simple if you really are someone that is into algorithm analysis.

Ouch

It's finals time and I haven't slept much this past week. . .

Rest assured that it all is correct inside the actual source file. gcc is good about complaining when the code is that stupid.