what is ABI?

http://en.wikipedia.org/wiki/Application_binary_interface
Application binary interface (ABI)

application binary interface (ABI) describes the low-level interface between an application program and the operating system, or the interface between an application and its libraries, or that between component parts of an application. An ABI is similar to an application programming interface (API); however, the latter defines a source code interface.[1] Simply put, ABIs enable binary compatibility, whereas APIs enable source code compatibility.

For example, the POSIX standard defines an API that allows a wide range of common computing functions to be written such that they may operate on many different systems (Mac OS X, various BSDs and Microsoft Windows all implement this interface); however, making use of this requires re-compilation for each platform.  A compatible ABI, on the other hand, allows compiled object code to function without any changes, on any system implementing that ABI. This is advantageous to both software providers (where they may distribute existing software on new systems without producing/distributing upgrades) and users (where they may install older software on their new systems without purchasing upgrades), although this generally requires various software libraries implementing the necessary APIs too.

ABIs cover details such as the calling convention, which controls how functions' arguments are passed and return values retrieved; the system call numbers and how an application should make system calls to the operating system; and in the case of a complete operating system ABI, the binary format of object files, program libraries and so on. A complete ABI, such as the Intel Binary Compatibility Standard (iBCS),[2] allows a program from one operating system supporting that ABI to run without modifications on any other such system. Other ABIs standardize details such as the C++ name decoration,[3] exception propagation,[4] and calling convention between compilers on the same platform, but do not require cross-platform compatibility.