What is the difference between a system call and a normal user program, and what is the difference b

The implementation of each operating system is different, and the implementation of the operating system on different CPUs is also different, but in general, the operating system distinguishes at least two running states - core mode and user mode, the system call runs in the kernel state, and the user program runs in the user state. All ** of the user mode cannot directly access the address space of the kernel state, and can only access the system functions through system calls (which are actually the limited interfaces left by the system for the user). In this way, the safe and reliable operation of the system is ensured.

If it is combined with propositions, it is also divided into two categories, one is the system library function, which is generally provided by the writer of the operating system and can only be used by the system call, which is inaccessible in the user mode. It is generally not as rich as the user library functions, and is only designed to implement the essential functions of the operating system. User library functions are generally designed and provided by the vendors that support the language (or add them if they wish), and can be quite rich, but they are also essentially user programs, and when they want to run system functions, they are still implemented through system calls.

Therefore, the user library function is just a set of well-functioning user programs provided by the manufacturer.

The difference between system calls and general user programs and library functions is that system callers are executed in the kernel state, and calling them requires a mechanism similar to hardware interrupt handling to provide system services. Ordinary user programs are generally executed in user mode. When I was learning C++ at Chuanzhi Podcast, I had this problem in my first exam.

Library functions are part of the language itself, while system functions are interfaces provided by the kernel to applications and are part of the system.

A library call is a part of a language or application, while a system call is part of an operating system.

System calls are the way a user application accesses and uses the various services provided by the kernel. At the intersection of the kernel and the user application, the kernel provides a set of system call interfaces through which the application can access the system hardware and various operating system resources.

1.System calls are made to facilitate applications to use the operating system's interfaces, while library functions are introduced to facilitate people to write applications.

2.A system call can be understood as an interface function provided by the kernel to us in the user mode, and can be thought of as a library function of some kind of kernel.

It's a system call, and fread is a C standard library function.