SYSTEMCALL.TXT

//WE KNOW WE ARE ACCESSING USER MODE AS WE WANT TO USE ANY APPLICATION 
BUT IN CASE WE WANT TO ACCESS SOME FUNCTIONALITY THEN WE NEED KERNEL MODE

TO GO TO KERNEL MODE WE USE SYSTEM CALL

in some system we can directly use kernel system calls like linux 
in some system we use api and local libraries like printf scanf but printf is not an system call ,these are all functions which access system calls 

in the end system calls are invoked to do our work 

SYSTEM CALL-
1 FILE RELATED
open(),read(),write(),close(),createfile-example whenever we write a c++ code and run it,it becomes a process and enters ram and our kernel is also in ram then if process wants to access a file it needs to invoke kernel system calls ,or create or open or write in a file ,any related to file 

2 device related-read ,write,reposition,ioctl,fcntl-to access any devices ,we cant direct access them ,but we need privilage given by the system 

3 information-getpid ,attrobutes,get system time and datat -whenever we want a meta data related to a data ,like the file name permission size etc and not the content of the file itself

4 process control-load execute abort fork wait signal allocate 

5 communication- pipe(), create delete connections,-these when 2 or more processes communicate with each other to do some work 

6 security system calls-chmod