System Calls

A system call is a routine that allows a user application to request actions that require special privileges. Adding system calls is one of several ways to extend the functions provided by the kernel.

The distinction between a system call and an ordinary function call is only important in the kernel programming environment. User-mode application programs are not usually aware of this distinction.

Operating system functions are made available to the application program in the form of programming libraries. A set of library functions found in a library such as libc.a can have functions that perform some user-mode processing and then internally start a system call. In other cases, the system call can be directly exported by the library without any user-space code. For more information on programming libraries, see Using Libraries.

Operating system functions available to application programs can be split or moved between user-mode functions and kernel-mode functions as required for different releases or machine platforms. Such movement does not affect the application program. Kernel Environment provides more information on how to use system calls in the kernel environment.

Differences Between a System Call and a User Function

A system call differs from a user function in several key ways:

• A system call has more privilege than a normal subroutine. A system call runs with kernel-mode privilege in the kernel protection domain.
• System call code and data are located in global kernel memory.
• System call routines can create and use kernel processes to perform asynchronous processing.
• System calls cannot use shared libraries or any symbols not found in the kernel protection domain.