error: invalid operands to binary &

[FlashBurn]

I get this error because I want to "and" a pointer to test if it is aligned on a 4kb pg.

Is there another way to do that? At the moment I do:

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if((*p & 0xfff) != 0)
//code

Which does not work

[eboyd]

first of all, what language is this?

[eboyd]

if it's c/c++ 2 "&"'s are used, like so:

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if((a == b) && (b == c))
{
    ...
}

But what you have doesn't seem valid:

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if((*p && 0xfff) != 0))

Might I suggest:

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if((p != 0) && (0xfff != 0))
{
    ...
}

I omitted the "*" becasue I', assuming "p" is a pointer anyway.

[FlashBurn]

I want to take the value of the pointer (the address) "and" it with 0xfff to look if the address is aligned!

There has to be a way how I can do this.

The language I use is "C"!

[Alboin]

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if(((*p) & 0xfff) != 0)

[Brynet-Inc]

Clearly 'eboyd' doesn't know anything about bitwise operations..

[bluecode]

I think what you want is

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if (((uintptr_t)p & 0xFFF) != 0)

where p is the pointer (say a 'void*'), right?

[FlashBurn]

Yeah, but how do you define uintptr_t?

And my pointer is a ptr to a struct and then then pointer which is in the struct:

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struct test_t
{
 void *base;
};

struct test_t *p;

if((p->base & 0xfff) != 0)
 //code

If I make this:

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uint32_t temp;
temp= p->base;

if((temp &0xfff) != 0)
 //code

It does compile, but is it also working?

[pcmattman]

Code: Select all

struct test_t
{
 void *base;
};

struct test_t *p;

if((p->base & 0xfff) != 0)
 //code 

Are you trying to find out if the address of 'p' is page aligned, or if the address pointed to by 'base' is page-aligned?

[bluecode]

Yeah, but how do you define uintptr_t?

Depends on your Compiler and on the architecture you are targeting. The only requirements of an uintptr_t are that
a) if you convert a pointer to a uintptr_t and back to a pointer you do not loose information
b) uintptr_t should be of integer type (so that you can do math with it)
For GCC + x86 or x86-64 'unsigned long' does the thing. On GCC + x86 'unsigned long' is 32bit and on GCC + x86-64 it is 64bit, so in every case it matches the size of a pointer.

@FlashBurn: Your last code snippet looks ok to me (But I would use uintptr_t for more portability). Just to clarify: C does not offer binary operations for pointers, so you have to cast the pointer to a type where you can do binary operations.

[Solar]

Yeah, but how do you define uintptr_t?

Depends on your Compiler and on the architecture you are targeting.

Actually, no. You define uintptr_t like this:

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#include <stdint.h>

Well, at least if you're working in a C99 environment.

[FlashBurn]

Ok, I solved it with a typcast to an uint32_t.

[bluecode]

Actually, no. You define uintptr_t like this:

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#include <stdint.h>

Well, at least if you're working in a C99 environment.

Actually I was thinking he is using this in his OS and does not have a full C99 environment and has to provide his own C99 environment. But thanks for mentioning that what I said should happen within stdint.h.

[eboyd]

Clearly 'eboyd' doesn't know anything about bitwise operations.. Laughing

Thanks. I love being publicly ridiculed.

But you're right. I don't use bitwise operations. I didn't realize that's what his post was all about.

[Solar]

Actually I was thinking he is using this in his OS and does not have a full C99 environment and has to provide his own C99 environment.

As he posted to "General Programming"... besides, it's hard to remember how much of an environment everyone's OS provides. For him, the solution was to typedef uint32_t... had I suggested that, it would have sounded a bit silly if he didn't have an uint32_t, right?