How does scanf() work inside the OS?

Question

I've been wondering how scanf()/printf() actually works in the hardware and OS levels. Where does the data flow and what exactly is the OS doing around these times? What calls does the OS make? And so on...

Answer 1

scanf() and printf() are functions in libc (the C standard library), and they call the read() and write() operating system syscalls respectively, talking to the file descriptors stdin and stdout respectively (fscanf and fprintf allow you to specify the file stream you want to read/write from).

Calls to read() and write() (and all syscalls) result in a 'context switch' out of your user-level application into kernel mode, which means it can perform privileged operations, such as talking directly to hardware. Depending on how you started the application, the 'stdin' and 'stdout' file descriptors are probably bound to a console device (such as tty0), or some sort of virtual console device (like that exposed by an xterm). read() and write() safely copy the data to/from a kernel buffer called a 'uio'.

The format-string conversion part of scanf and printf does not occur in kernel mode, but just in ordinary user mode (inside 'libc'), the general rule of thumb with syscalls is you switch to kernel mode as infrequently as possible, both to avoid the performance overhead of context switching, and for security (you need to be very careful about anything that happens in kernel mode! less code in kernel mode means less bugs/security holes in the operating system).

btw.. all of this was written from a unix perspective, I don't know how MS Windows works.

Answer 2

scanf , printf etc internally all these types of functions can't be directly written in c/c++ language. internally they all are written in assembly language by the use of keword "asm", any thing written with keyword "asm" are directly introduced to object file irrespective of compilation (not changed even after compilation), and in assembly language we have got predefined codes which can implement all these functions ...... so in short SCANF PRINTF etc ALL ARE WRITTEN IN ASSEMBLY LANGUAGE INTERNALLY. YOU CAN DESIGN YOUR OWN INPUT FUNCTION USING KEYWORD "ASM".

Answer 3

I think the OS just provides two streams, one for input and the other for output, the streams abstract away how the output data gets presented or where the input data comes from.

so what scanf & printf are doing are just adding bytes (or consuming bytes) from either streams.

Answer 4

On my OS I am working with scanf and printf are based on functions getch() ant putch().