What alternatives to __attribute__ exist on 64-bit kernels?

Question

Hi:

Is there any alternative to non-ISO gcc specific extension __attribute__ on 64-bit kernels ?

Three types that i've noticed are: function attributes, type attributes and variable attributes.

eg. i'd like to avoid using __attribute__((__packed__)) for structures passed over the network, even though some gcc based code do use it.

Any suggestions or pointers on how to entirely avoid __attribute__ usage in C systems/kernel code ?

thanks Saifi.

Answer 1

Any suggestions or pointers on how to entirely avoid attribute usage in C systems/kernel code?

You can build your network packets piece by piece, copying each data element into the correct place in a char* buffer.

Pros: you don't have any alignment issues and it's generally portable, especially if you use the exact-width integer types from <stdint.h>

Cons: it's tedious and potentially error-prone

Answer 2

I'm assuming based on your comments that its your code you want to change, not the whole Linux kernel (etc).

Not sure about function attributes etc but specifically for attribute packed, I have done the following with no issues so far.

Basically instead of relying on the compiler to pack, you could use manual pad fields coupled with compile-time asserts.

struct foo {
   u32 field1;
   u16 field2;
   u16 pad; // manual padding
   // continue for other fields that the compiler would automatically pad for you with attribute packed
   u32 field3;
};

To check your structure you can use a compile time assert, something like this:

#define CASSERT(cond, name) typedef cassert__##name[cond ? 1 : -1]

CASSERT(offsetof(foo, field1) == 0, field1_wrong);
CASSERT(offsetof(foo, field2) == 4, field2_wrong);

When your assertions are wrong, the build will fail with a helpful error and line number