Unix BSD Part 2

Programmer Interface

1. Like most computer systems, UNIX consists of two separable parts:
2. Kernel: everything below the system-call interface and above the physical hardware
   
   
   • Provides file system, CPU scheduling, memory management, and other OS functions through system calls
     
     
3. Systems programs: use the kernel-supported system calls to provide useful functions, such as compilation and file manipulation
   

System Calls

1. System calls define the programmer interface to UNIX
2. The set of systems programs commonly available defines the user interface
3. The programmer and user interface define the context that the kernel must support
4. Roughly three categories of system calls in UNIX
   
   
   • File manipulation (same system calls also support device manipulation)
   • Process control
   • Information manipulation
     

File Manipulation

1. A file is a sequence of bytes; the kernel does not impose a structure on files
2. Files are organized in tree-structured directories
3. Directories are files that contain information on how to find other files
4. Path name: identifies a file by specifying a path through the directory structure to the file
   
   
   • Absolute path names start at root of file system
   • Relative path names start at the current directory
   • System calls for basic file manipulation: create, open, read, write, close, unlink, trunc
     

Process Control

1. A process is a program in execution
2. Processes are identified by their process identifier, an integer
3. Process control system calls
   
   
   • fork creates a new process
   • execve is used after a fork to replace on of the two processes’s virtual memory space with a new program
   • exit terminates a process
   • A parent may wait for a child process to terminate; wait provides the process id of a terminated child so that the parent can tell which child terminated
   • wait3 allows the parent to collect performance statistics about the child
     
     
4. A zombie process results when the parent of a defunct child process exits before the terminated child
5. Processes communicate via pipes; queues of bytes between two processes that are accessed by a file descriptor
6. All user processes are descendants of one original process, init
7. init forks a getty process: initializes terminal line parameters and passes the user’s login name to login
   
   
   • login sets the numeric user identifier of the process to that of the user
   • executes a shell which forks subprocesses for user commands
     
     
8. setuid bit sets the effective user identifier of the process to the user identifier of the owner of the file, and leaves the real user identifier as it was
9. setuid scheme allows certain processes to have more than ordinary privileges while still being executable by ordinary users
   

SIGNALS

1. Facility for handling exceptional conditions similar to software interrupts
2. The interrupt signal, SIGINT, is used to stop a command before that command completes (usually produced by ^C)
3. Signal use has expanded beyond dealing with exceptional events
   
   
   • Start and stop subprocesses on demand
   • SIGWINCH informs a process that the window in which output is being displayed has changed size
   • Deliver urgent data from network connections

Process Groups

1. Set of related processes that cooperate to accomplish a common task
2. Only one process group may use a terminal device for I/O at any time
   
   • The foreground job has the attention of the user on the terminal
   • Background jobs – nonattached jobs that perform their function without user interaction
     
     
3. Access to the terminal is controlled by process group signals
4. Each job inherits a controlling terminal from its parent
   
   • If the process group of the controlling terminal matches the group of a process, that process is in the foreground
   • SIGTTIN or SIGTTOU freezes a background process that attempts to perform I/O; if the user foregrounds that process, SIGCONT indicates that the process can now perform I/O
   • SIGSTOP freezes a foreground process

Information Manipulation

1. System calls to set and return an interval timer:getitmer/setitmer
2. Calls to set and return the current time:gettimeofday/settimeofday
3. Processes can ask for
   
   
   • their process identifier: getpid
   • their group identifier: getgid
   • the name of the machine on which they are executing: gethostname
   
   

Library Routines

1. The system-call interface to UNIX is supported and augmented by a large collection of library routines
2. Header files provide the definition of complex data structures used in system calls
3. Additional library support is provided for mathematical functions, network access, data conversion, etc
4. 
   

User Interface

1. Programmers and users mainly deal with already existing systems programs: the needed system calls are embedded within the program and do not need to be obvious to the user
2. The most common systems programs are file or directory oriented
   
   • Directory: mkdir, rmdir, cd, pwd
   • File: ls, cp, mv, rm
3. Other programs relate to editors (e.g., emacs, vi) text formatters (e.g., troff, TEX), and other activities
   

Shells and Commands

1. Shell – the user process which executes programs (also called command interpreter)
2. Called a shell, because it surrounds the kernel
3. The shell indicates its readiness to accept another command by typing a prompt, and the user types a command on a single line
4. A typical command is an executable binary object file
5. The shell travels through the search path to find the command file, which is then loaded and executed
6. The directories /bin and /usr/bin are almost always in the search path
7. Typical search path on a BSD system:
   ( ./home/prof/avi/bin /usr/local/bin /usr/ucb/bin /usr/bin )
   
   
8. The shell usually suspends its own execution until the command completes
   
   

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