4 ms·

> Still Objective-C objects have underlying C functions that are being called whenever you try to call a method or anything on an Objective-C object. The same

> Still Objective-C objects have underlying C functions that are being called whenever you try to call a method or anything on an Objective-C object. The same goes for C++ or Swift. Getting the function pointer for a method can be involved, but all functions adhere to the C ABI.

poizan42 11y ago
At least MSVC on x86 uses "thiscall" by default which puts the this pointer in ecx. You cannot call those functions from C without specific compiler support.
- vardump 11y ago
  Sure you can. Just write an asm stub to call, that sets up ecx and other things right.
  - poizan42 11y ago
    The claim was: The same goes for C++ or Swift. Getting the function pointer for a method can be involved, but all functions adhere to the C ABI. This is clearly false since at least one C++ compiler generates functions that does not adhere to any of the common calling conventions for C on that platform (or "C ABIs" if you will). Besides that, I said you could not call them from C. An asm stub is not C. Neither is inline assembly even though it's a common extension to C.
- jheriko 11y ago
  thats not true if you inspect the details of the specific ABI and cheat a little or use some inline assembler... but cl is easily the worst compiler for a C++ ABI. they break it on tiny incremental updates sometimes... at least historically (in fairly recent history at least).
  - poizan42 11y ago
    > thats not true if you inspect the details of the specific ABI and cheat a little or use some inline assembler... Some x86 C compilers supports calling conventions that lets you put things in registers, e.g. Microsofts "fastcall". But I wouldn't consider that part of the standard "C ABI" for Windows given that it's practically never used for externally visible functions. As I said "without specfic compiler support". Using inline assembler is just cheating, it's not C. > but cl is easily the worst compiler for a C++ ABI. they break it on tiny incremental updates sometimes... at least historically (in fairly recent history at least). That is more about name mangling schemes and layout of standard library classes though. As far as I know the calling conventions has been quite fixed for a long time.
chris_wot 11y ago
What C ABI? Can you point me to the spec?
- deathanatos 11y ago
  I don't believe C standardizes the ABI across all OS/hardware combinations. But some OS/hardware combinations are standardized; for example, there's the AMD64 ABI[1], which I think just about every operating system that runs on amd64 uses … except Windows. [1]: http://www.x86-64.org/documentation/abi.pdf http://www.x86-64.org/documentation/abi.pdf
  - chris_wot 11y ago
    Yeah, but that's a bit of a long bow... That's not a C ABI :-)

poizan42 11y ago

At least MSVC on x86 uses "thiscall" by default which puts the this pointer in ecx. You cannot call those functions from C without specific compiler support.

vardump 11y ago
Sure you can. Just write an asm stub to call, that sets up ecx and other things right.
- poizan42 11y ago
  The claim was: The same goes for C++ or Swift. Getting the function pointer for a method can be involved, but all functions adhere to the C ABI. This is clearly false since at least one C++ compiler generates functions that does not adhere to any of the common calling conventions for C on that platform (or "C ABIs" if you will). Besides that, I said you could not call them from C. An asm stub is not C. Neither is inline assembly even though it's a common extension to C.
jheriko 11y ago
thats not true if you inspect the details of the specific ABI and cheat a little or use some inline assembler... but cl is easily the worst compiler for a C++ ABI. they break it on tiny incremental updates sometimes... at least historically (in fairly recent history at least).
- poizan42 11y ago
  > thats not true if you inspect the details of the specific ABI and cheat a little or use some inline assembler... Some x86 C compilers supports calling conventions that lets you put things in registers, e.g. Microsofts "fastcall". But I wouldn't consider that part of the standard "C ABI" for Windows given that it's practically never used for externally visible functions. As I said "without specfic compiler support". Using inline assembler is just cheating, it's not C. > but cl is easily the worst compiler for a C++ ABI. they break it on tiny incremental updates sometimes... at least historically (in fairly recent history at least). That is more about name mangling schemes and layout of standard library classes though. As far as I know the calling conventions has been quite fixed for a long time.

chris_wot 11y ago

What C ABI? Can you point me to the spec?

deathanatos 11y ago
I don't believe C standardizes the ABI across all OS/hardware combinations. But some OS/hardware combinations are standardized; for example, there's the AMD64 ABI[1], which I think just about every operating system that runs on amd64 uses … except Windows. [1]: http://www.x86-64.org/documentation/abi.pdf http://www.x86-64.org/documentation/abi.pdf
- chris_wot 11y ago
  Yeah, but that's a bit of a long bow... That's not a C ABI :-)