What is an application binary interface (ABI)?

Question

I never clearly understood what an ABI is. Please don\'t point me to a Wikipedia article. If I could understand it, I wouldn\'t be here posting such a lengthy post.

Answer 1

The best way to differentiate between ABI and API is to know why and what is it used for:

For x86-64 there is generally one ABI (and for x86 32-bit there is another set):

http://www.x86-64.org/documentation/abi.pdf

https://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/LowLevelABI/140-x86-64_Function_Calling_Conventions/x86_64.html

http://people.freebsd.org/~obrien/amd64-elf-abi.pdf

Linux + FreeBSD + MacOSX follow it with some slight variations. And Windows x64 have its own ABI:

http://eli.thegreenplace.net/2011/09/06/stack-frame-layout-on-x86-64/

Knowing the ABI and assuming other compiler follows it as well, then the binaries theoretically know how to call each other (libraries API in particular) and pass parameters over the stack or by registers etc. Or what registers will be changed upon calling the functions etc. Essentially these knowledge will help software to integrate with one another. Knowing the order of the registers / stack layout I can easily piece together different software written in assemblies together without much problem.

But API are different:

It is a high level functions names, with argument defined, such that if different software pieces build using these API, MAY be able to call into one another. But an additional requirement of SAME ABI must be adhered to.

For example, Windows used to be POSIX API compliant:

https://en.wikipedia.org/wiki/Windows_Services_for_UNIX

https://en.wikipedia.org/wiki/POSIX

And Linux is POSIX compliant as well. But the binaries cannot be just moved over and run immediately. But because they used the same NAMES in the POSIX compliant API, you can take the same software in C, recompile it in the different OS, and immediately get it running.

API are meant to ease integration of software - pre-compilation stage. So after compilation the software can look totally different - if the ABI are different.

ABI are meant to define exact integration of software at the binary / assembly level.