Understanding the Linux Kernel, 3rd Edition

POSIX APIs and System Calls

Let’s start by stressing the difference between an application programmer interface (API) and a system call. The former is a function definition that specifies how to obtain a given service, while the latter is an explicit request to the kernel made via a software interrupt.

Unix systems include several libraries of functions that provide APIs to programmers. Some of the APIs defined by the libc standard C library refer to wrapper routines (routines whose only purpose is to issue a system call). Usually, each system call has a corresponding wrapper routine, which defines the API that application programs should employ.

The converse is not true, by the way—an API does not necessarily correspond to a specific system call. First of all, the API could offer its services directly in User Mode. (For something abstract such as math functions, there may be no reason to make system calls.) Second, a single API function could make several system calls. Moreover, several API functions could make the same system call, but wrap extra functionality around it. For instance, in Linux, the malloc( ) , calloc( ) , and free( ) APIs are implemented in the libc library. The code in this library keeps track of the allocation and deallocation requests and uses the brk( ) system call to enlarge or shrink the process heap (see the section "Managing the Heap" in Chapter 9).

The POSIX standard refers to APIs and not to system calls. A system can be certified as POSIX-compliant ...