System call is a programming interface provided by the operating system to applications. Applications can request the operating system to perform privileged operations through system calls, such as accessing hardware devices, file systems, networks, etc. The purpose of system calls is to protect the operating system and hardware from direct access by applications, while providing a safe and controlled way for applications to use system resources.
The underlying principle is that the operating system maintains a system call table, which contains the system call number and the corresponding function pointer. When an application calls a system call, it passes parameters to the operating system and executes the corresponding system call processing function in kernel mode. After the system call processing function completes the corresponding task, the result is returned to the application program and control is returned to the user state. During the system call process, there will be a certain performance overhead due to the operation of switching from user mode to kernel mode, so the number of system calls should be minimized.