Hello friends, Welcome to Gate Smashers The topic is system call. . .
In the last video we discussed how we work in user mode & kernel mode If the user is using any application or API or writing any program then we generally writes it in user mode But if I wants to access any functionality of operating system then I have to go in kernel mode So because our access is on the user mode because we are accessing the user so to go in kernel mode we use system call System call is a programmatic way through which we shifts from user mode to kernel mode or can access the functionalities of kernel and operating system Like if I'd take a simple example if we are writing a C program and I want to get any value printed on monitor let's say 2 + 2 = 4 then 2 + 2 is up to user mode, processor will do 2 + 2 no problem But we have to get it printed on the monitor or if I want to get something printed or I want to access printer, so wherever I have to access devices, I've to take help and do it through kernel Kernel means through operating system `Same way if I want to access some file Let's say I want o perform a transaction want to change data in some file File is in hard disk. . so again, to access any file or hard disk, I've to go in kernal mode Because these all devices and resources are being governed by operating system so in operating system kernal is the main logical unit So here I've to take help of system call and system call is generally.
. . Many times many operating systems are like if we talk about Linux, then when we write C program in the text editor then we can use system call there directly Like we can use read, write, close, open, fork etc and many other system calls directly And generally in Linux operating system or if we'll talk about any other system also There're hundreds of system call.
. . same way if we'd talk about window's then there're around 700 system calls But there are many operating systems where we don't write system calls directly We rather use APIs or we uses the local libraries like printf Or scanner.
. . printf means that we want to get some value printed, then we're writing Printf.
But that printf or scanner is not system call Printf is a function, but that function is accessing the system call, so print means writing on the monitor so whether you use system call directly or you use some API or any function but at the end work will be done through system call because system call will invoke kernal to perform this work so as I said system call generally varies according to operating system Many of them have around 300 system calls Window 7 have around 700 system calls so we categories these system calls First we have file related system call First we have file related system call means where we want to access any file Let me tell you with a simple example What happens generally is. . .
when we write any C program then we are writing that program using some API or application But when we execute that program then we says in operating system that the program has become process now means when it's in running mode then it became process And running mode means that process will in the RAM Means that process has arrived in the RAM now and our operating system also i. e. kernal is in RAM also I'm just taking a general architecture.
. . many times just a part of code lies in the RAM rather than whole code or kernal so we are talking through a simple example where it says that our kernal is in main memory and process also Now if we want to do any execution in the process Let's say I want to access some file, then this process don't have privilege that privilege will be given by kernal So process will invoke system call and will access that file So all this work here has been done through system call So if we'd talk about file, then what we do in file?
Let's say I've to create some file. . .
Create a file let's say I want to open a file, want to read a file or write some file so all the file related work that we are doing here we are doing all that through file related system calls Next is device related system call, so device related system call means if I want to access any device or hardware here we are calling device as hardware If we want to access hardware whether it is hard disk, printer or monitor. . .
so to access all these devices. . .
User can never do direct access, he has to take privilege Privilege will be given by operating system so here we'll again take privilege through system call Whether we are reading any device or writing on a device or repositioning value of that device or we are accessing its attributes Same way we use IOctrl. . .
we use this to control input output devices or to control a file also. . .
although, file control is a header file but we generally use it here Next we have. . .
Information related Information means regarding processes or devices, if I want to take information of any attribute there we use information related system call like getpid is a simple example We use getpid if I want to know about some process's ID, getppid means if I want to know about parent ID Wherever we need any process related information which we call metadata in general term Means metadata related to a data is. . .
data is like there's a audio file. . .
and the content in audio file is its data But data related to it. . like its size, file name, file's permission, file's extension.
. . All this is called attribute So if I want to access a data like this for this we use information related system call Same way if I want access system's time, system's date, system's data.
. . then in that case we use information related Then process control.
. . this is also a major system call that we use where if you are getting a process loaded As I just told you, generally the programs that we write or save, we are doing that in secondary memory in hard disk because we want to save it permanently But when we have to load them in main memory or want execute them for that we use process control system call if you want to abort a process.
. . Fork system call.
. . this is a very important system call fork system call means like we talk about multi processing environment in operating system Means we want to do multiple processes and tasks at a time for that we use multi threading environment means we run multiple threads related to the process or we can use fork system call.
. . In fork system call, a process creates its own child process And that child process is doing its work parent process is also doing its work this way we can create a multiprocessing environment here so we create fork system call also through process control system call wait & signal.
. . we use it under process synchronization if you want some process to wait or want to increase semaphore values, or want to allocate memory, so we can access all these things through this system call Then communication.
. . In communication we generally use inter process communication Inter process communication means.
. . two processes are communicating with each other they can either use this Pipe() method or shared memory so here we have pipe(0 system call, Shmget() that we use to get value of shared memory or if I want to create or terminate connections, we have system calls related to it here So these are major categorizations of system call we have system calls other than these also Like system calls related to protection and security where we use chmod, privileges.
. . if I want to change on any file then we can use chmod, umask and many other privileges and system calls that we use so you con't have to directly memorize these system calls but at least you should be clear with their idea that why we use system call and what is there importance in operating system So this is all about the system calls Thank You!
