Thoroughly Understanding C++ ABI (2024)

C++ ABIs are why Win32 settled on things like COM to do cross-module passing of objects. Because nobody could agree on a standard, the COM standard enforced a very specific calling convention, and a very specific vtable layout for COM objects.

Re: Thoroughly Understanding C++ ABI (2024)

#5

Post by jdw64 » Fri, Aug 07, 2026, 2:35 PM UTC

Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it.

In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI.

In the end, I think the ABI stays stable because of community conventions established by compiler vendors.

Re: Thoroughly Understanding C++ ABI (2024)

#6

Post by jdw64 » Fri, Aug 07, 2026, 2:44 PM UTC

rramadass wrote: post #3

GNU libstdc++ "Dual ABI" issue - https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_a...

I understand that Move Semantics led to better implementations and that this is why things changed. I know that in this case, problems arose when transitioning from C++98 to what's commonly called Modern C++ (C++11). However, I'm not very familiar with what the specific issues were with COW. If you happen to know of any documents that describe this problem in more detail, I'd appreciate it if you could let me know. I'm sorry for always asking questions. I've looked up a few documents, but since you clearly know much more about this than I do, I'm asking to study it myself as well. If it's too much trouble, feel free not to reply.

Re: Thoroughly Understanding C++ ABI (2024)

#7

Post by pjmlp » Fri, Aug 07, 2026, 2:49 PM UTC

Dwedit wrote: post #4

C++ ABIs are why Win32 settled on things like COM to do cross-module passing of objects. Because nobody could agree on a standard, the COM standard enforced a very specific calling convention, and a very specific vtable layout for COM objects.

Not only, there is also XPC and IO/DriverKit on Apple, Android IPC, D-BUS, FIDL on Fucshia,....

Also C ABI also does not exist, people keep mistaking the ABI of their favourite C compiler with the OS ABI, which only overlap if the OS was written in C to start with.

For example on mainframes and micros, naturally not written in C, it is either a bytecode based ABI like TIMI on IBM i, or language environments like on z/OS, ClearPath MCP, OS 2200 and so forth.

And as a reminder, from a famous WG14 and WG21 member, and former Rust contributor,

"To Save C, We Must Save ABI"

https://thephd.dev/to-save-c-we-must-save-abi-fixing-c-funct...

Re: Thoroughly Understanding C++ ABI (2024)

#8

Post by andyg_blog » Fri, Aug 07, 2026, 2:50 PM UTC

jdw64 wrote: post #5

Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it. In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI. In the end, I think the ABI stays stable because of community conven…

It doesn't, and FWIW the article says that. A meta note: ABI stability is a holy war inside the C++ community. Those against it argue that it's holding back real language evolution. Herb Sutter is even working on his own C++ offshoot that shows what could be.

Re: Thoroughly Understanding C++ ABI (2024)

#9

Post by jcranmer » Fri, Aug 07, 2026, 2:53 PM UTC

jdw64 wrote: post #5

Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it. In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI. In the end, I think the ABI stays stable because of community conven…

No, ABI is not part of the C++ standard, although there are parts of C++ that do go "this is an ABI thing" (e.g., [[no_unique_address]]).

ABI is more of OS-level thing. Most systems these days follow the SysV ABI, which is largely defined by the hardware manufacturers via the processor-specific supplements (the x86-64 one is here: https://gitlab.com/x86-psABIs/x86-64-ABI). These largely delegate C++-specific conventions to the Itanium C++ ABI (which they likely directly reference), although the ARM ecosystem uses a somewhat different layout for the exception handling tables. Microsoft uses a different ABI for both the underlying C ABI and for the C++ compatibility layer built on top of the C ABI.

One of the issues that crops up is that vendors end up needing to add extensions to the ABI for various reasons, and these extensions tend to end up being incompatible, since they're added before they've had a time to be standardized. 16-bit floats is a particular historical bugbear, as is the C23 _BitInt stuff.

Re: Thoroughly Understanding C++ ABI (2024)

#10

Post by pjmlp » Fri, Aug 07, 2026, 2:53 PM UTC

jdw64 wrote: post #5

Does the C++ standard guarantee binary ABI? Isn't that something that compiler and library contributors handle separately? So I think the arguments you see online are more accurate—vendors are the ones maintaining it. In other words, the ABI we rely on today isn't really part of the C+ standard—it's more like the Itnaium C++ ABI or the MSVC C++ ABI. In the end, I think the ABI stays stable because of community conven…

No, neither does the C standard.

With exception of Swift, D, and bytecode based languages, the ABI is left to the vendors.

For those that think ISO/IEC 9899:2024 PDF has anything related to ABI, the actual ABI used by C compilers, is the OS ABI, if the OS was written in C to start with, and naturally this overlaps quite nicely with UNIX like OSes, and Windows.

It isn't like that on other platforms that decided to either use other systems languages, or expose their OS APIs in a different way, e.g. mainframes, micros, Android, ChromeOS, WebOS,...