I am new to Linux system programming and I came across API and ABI while reading Linux System Programming.
Definition of API:
An API defines the interfaces by which one piece of software communicates with another at the source level.
Definition of ABI:
Whereas an API defines a source interface, an ABI defines the low-level binary interface between two or more pieces of software on a particular architecture. It defines how an application interacts with itself, how an application interacts with the kernel, and how an application interacts with libraries.
How can a program communicate at a source level? What is a source level? Is it related to source code in any way? Or the source of the library gets included in the main program?
The only difference I know is API is mostly used by programmers and ABI is mostly used by a compiler.
The API is what humans use. We write source code. When we write a program and want to use some library function we write code like:
long howManyDecibels = 123L;
int ok = livenMyHills(howManyDecibels);
and we needed to know that there is a method livenMyHills(), which takes a long integer parameter. So as a Programming Interface it's all expressed in source code. The compiler turns this into executable instructions which conform to the implementation of this language on this particular operating system. And in this case result in some low level operations on an Audio unit. So particular bits and bytes are squirted at some hardware. So at runtime there's lots of Binary level action going on which we don't usually see.
At the binary level there must be a precise definition of what bytes are passed at the Binary level, for example the order of bytes in a 4 byte integer, or the layout of a complex data structure - are there padding bytes to align some values. This definition is the ABI.