Understanding the Linux Kernel

8.1. POSIX APIs and System Calls

Let us 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, that is, routines whose only purpose is to issue a system call. Usually, each system call corresponds to a wrapper routine; the wrapper routine defines the API that application programs should refer to.

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 like 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( ) POSIX APIs are implemented in the libc library: the code in that library keeps track of the allocation and deallocation requests and uses the brk( ) system call in order to enlarge or shrink the process heap (see Section 7.6 in Chapter 7).

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