System Call

1. User operating-system Interface

• Two main approaches
• command-line interpreter (a.k.a. command interpreter or shell)
• graphical user interface (GUI)
• The shell
• allows users to directly enter commands that are to be performed by the operating system
• is usually a system program (not part of the kernel)
• GUI allows a mouse-based window-and-menu system
• Some systems allow (e.g., X-Windows in Unix)

2. System Calls

• System calls provide the nterface between the a running program and the operating system
• generally available in routines written in C and C++
• certain low-level tasks may have to be written using assembly language
• Typically, application programmers design programs using an application programming interface (API)
• The run-time support system (run-time libraries) provides a system-call interface, that intercepts function calls in the API and invokes the necessary system call within the operating system.

3. Example System-Call Processing

4. System Call: read (fd, buffer, nbvtes)

5. Major System Calls in Unix: Process Management

• pid = fork()
• create a child process identical to the parent
• pid = waitpid( pid, &statloc, options)
• wait for a child to terminate
• s = execve(name, argv, environp)
• repalce a process' core image
• exec()
• i.e., load the selected program into memory
• exit(status)
• terminate process execution and return status 
• s = kill(pid, signal)
• send a signal to a process

6. System program

• System programs provide a convenient environment for program development
• They can provide various services
• status information
• file modification
• programming language support
• program loading and execution
• communications