CODEWITHSUNDEEP
linuxapisystem-callsterminology
Rari
Rari

Reputation: 47

Difference between system API and system call API

I have read "system call APIs are for user-space access and system APIs are for system space access". I am new to Linux OS concepts, I don't have any knowledge about the System API. Can anyone explain the difference between these two?

Upvotes: 4

Views: 7807

Answers (2)

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

Read system call and linux kernel wikipages first.

As Rahul Triparhi answered, system calls are the elementary operations, as seen from a user-mode application software. Use strace(1) to find out which syscalls are done by some program.

The system calls are well documented in the section 2 of the man pages (type first man man in a terminal on your Linux system). So read intro(2) and then syscalls(2).

Stricto sensu, syscalls have an interface, notably specified in ABI specifications like x86-64 ABI, defined at the lowest possible machine level - in terms of machine instructions and registers, etc... The functions in section 2 are tiny C wrappers above them. See also Linux Assembly HowTo

Please read also Advanced Linux Programming which explains quite well many of them.

BTW, I am not sure that "System API" has a well defined meaning, even if I guess what it could be. See also the several answers to this question.

Probably "System API" refers to the many functions standardized by POSIX, implemented in the POSIX C library such as GNU libc (but you could use some other libc on Linux, like MUSL libc, if you really wanted to). I am thinking of functions like dlopen (to dynamically load a plugin) or getaddrinfo(3) (to get information about some network things) etc... The Linux implementation (e.g. dlopen(3)) is providing a super-set of it.

More generally, the section 3 of man pages, see intro(3), is providing a lot of library functions (most of them built above system calls, so dlopen actually calls mmap(2) syscall, and getaddrinfo may use syscalls to connect to some server - see nsswitch.conf(5), etc...). But some library functions are probably not doing any syscall, like snprintf(3) or sqrt(3) or longjmp(3) .... (they are just doing internal computations without needing any additional kernel service).

Upvotes: 1

Rahul Tripathi
Rahul Tripathi

Reputation: 172428

A system call is an explicit request to the kernel made via a software interrupt. It is the lowest level thing which talks to the Operating system. System call is when you call the kernel. System-calls are actually intended to be very low-level interfaces, you can say to a very specific functionality which your program cannot accomplish on its own.

Whereas a System API are used to invoke system call

Upvotes: 4

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