api 是应用程序和操作系统之间的接口，凡是符合该api标准的应用程序都可以在支持该api的操作系统上编译通过。
abi 是二进制级别的接口，规定了二进制文件的格式、内容、装载/卸载程序的要求、函数调用时的参数传递
规则、寄存器、堆栈的使用。
如果操作系统都支持该api，但是机器的体系结构不同即他们的abi不同，那么在一个机器上生成的二进制代码是不可以在另外一台机器上面运行成功的，可能因为函数调用的参数传递规则不同或者其他由abi定义的行为产生的差异性。
其实关键的是abi定义了运行时的兼容性问题，这个问题是api无法解决的。api所能解决的是静态状态下的兼容性问题。
以下摘自encyclopedia of PCMAGZINE
Definition of: API
(Application Programming Interface)
A language and message format used by an application program to
communicate with the operating system or some other control program
such as a database management system (DBMS) or communications protocol.
APIs are implemented by writing function calls in the program, which
provide the linkage to the required subroutine for execution. Thus, an
API implies that some program module is available in the computer to
perform the operation or that it must be linked into the existing
program to perform the tasks.
Definition of ABI
(Application Binary Interface) A specification for a specific hardware platform combined with the operating system. It is one step beyond the application program interface (API), which defines the calls from the application to the operating system. The ABI defines the API plus the machine language for a particular CPU family. An API does not ensure runtime compatibility, but an ABI does, because it defines the machine language, or runtime, format.