Chapter 5. System Calls

The kernel provides a set of interfaces by which processes running in user-space can interact with the system. These interfaces give applications access to hardware and other operating system resources. The interfaces act as the messengers between applications and the kernel, with the applications issuing various requests, and the kernel fulfilling them (or telling the application to go away). The fact that these interfaces exist, and that applications are not free to do directly whatever they please, is key to providing a stable system and avoiding a big mess.

System calls provide a layer between the hardware and user-space processes. This layer serves three primary purposes. First, it provides an abstracted hardware interface for user-space. When reading or writing from a file, for example, applications need not concern themselves with the type of disk, media, or even the filesystem on which the file resides. Second, system calls ensure system security and stability. With the kernel acting as a middleman between system resources and user-space, the kernel can arbitrate access based on permissions and other criteria. For example, this prevents applications from incorrectly using hardware, stealing other processes' resources, or doing harm to the system. Finally, a single common layer between user-space and the rest of the system allows for the virtualized system provided to processes, discussed in Chapter 3, "Process Management." If applications were free to access system resources without the kernel's knowledge, it would be nearly impossible to implement multitasking and virtual memory, and certainly impossible to do so with stability and security. In Linux, system calls are the only means user-space has of interfacing with the kernel; they are the only legal entry point into the kernel other than exceptions and traps. Indeed, other interfaces, such as device files or /proc, are ultimately accessed via system calls. Interestingly, Linux implements far fewer system calls than most systems^[1].

^[1] About 250 system calls are on x86. (Each architecture is allowed to define unique system calls.) Although not all operating systems publish their exact system calls, some operating systems are estimated to have over one thousand.

This chapter addresses the role and implementation of system calls in Linux.