UNIX SYSTEM CALLS

                   

     A system call is nothing but a call to an operating system to do some thing   behalf of user. In UNIX also the system calls  does the same. In UNIX there are two modes.

User Mode: In user mode all the user level programs will be executed.

Kernel Mode: In kernel mode all the system level programs will be executed i.e.,whenever there is a need to access system files. When the system starts by default the system is in kernel mode.

     When the user logs in the system he is in user mode. UNIX provides system calls to interact with operating system files. When a system call was issued the control goes from user mode to kernel mode and execution continues in kernel mode. After completed execution in kernel mode the control transfers back to user mode.

Unix system calls can be categorized into  

1) File related system calls

       File related system calls are used to do operations 

on  a file like creating files reading files etc. 

open(), read(), write(), lseek(), stat(),fstat(), chmod(),

  chown() are some examples of  file related system calls.

2) Directory related system calls

       Directory related system calls are used to do 

 operations like creating a directory,getting present 

 working directory etc.

 mkdir(), chdir(),rmdir() getcwd() are some examples of 

directory related system calls.     

3)Process related system calls

         Process related calls are used to do operations on 

processes like creating a new process, loading new process

 code into executing  process address space etc.

fork(),vfork(),exec(),wait(),getpid(),getppid() are some

examples of process related system calls.

4) Inter process communication

          Inter process related system calls are used to 

establish communication between two processes.

pipe(),msgget()msgsnd(),msgrcv(),semget(),semctl(),

 shmget(),shmat() etc. are some examples of inter 

process related system calls.

                                        Directory handling system calls

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