CSCI 4500 Operating Systems. In This Module

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1 CSCI 4500 Operating Systems Module 2 OS Concepts: Processes and File Systems 2008 Stanley A. Wileman, Jr. Operating Systems Slide 1 In This Module Operating System Concepts Introduction to Processes Files and Data Sets Name Binding, Catalogs and Directories System Calls for Processes and Tasks Sample Programs Using Processes 2008 Stanley A. Wileman, Jr. Operating Systems Slide 2 Documentation on System Calls On loki, use man name, or man 2 name where name is the system call name. You can also find many UNIX system calls described in the document at the end of the class web page. Windows system calls are described at -- look at the API index, specifically for System Services and Data Access Stanley A. Wileman, Jr. Operating Systems Slide 3 1

2 Operating System Concepts Recall that we will study operating systems from two points of view. These are the views of Application writers, using the various system calls to achieve interaction with the system and its resources. OS implementers, who must write the code for the operating system that implements the system calls. Let s consider two major system call categories, those for processes and for file systems Stanley A. Wileman, Jr. Operating Systems Slide 4 Introduction to Processes (Tasks) A process, sometimes called a task, is essentially a packaged unit of work for a computer system. In many cases when you enter a command or double click on an icon, a command interpreter process (a shell ) will create a new process to run the program. Processes will always have an address space in which they can execute, and a set of processor registers to hold data, next instruction address, and so forth Stanley A. Wileman, Jr. Operating Systems Slide 5 Processes In a multiprogramming system, multiple processes (at least one per user) are being virtually executed. But it s obvious only one process can be actually executed at a time on a single processor. Thus, the process currently being executed is occasionally freeze dried and another process is executed for a while. This is called a context switch, since the processor s context changes. When a process is not being executed, salient details (e.g. CPU register contents and file pointers) are stored away in a process table Stanley A. Wileman, Jr. Operating Systems Slide 6 2

3 Process Example: The CPU executes some instructions from process A LOAD A ADD B. STORE C. DIV X. ADD Z STORE Y Process A LOAD K DIV E MPY T STORE N CMP L JNE LOOP Process B 2008 Stanley A. Wileman, Jr. Operating Systems Slide 7 then saves process A s context and executes some instructions from process B LOAD A ADD B STORE C DIV X ADD Z STORE Y Process A LOAD K. DIV E. MPY T. STORE N CMP L JNE LOOP Process B 2008 Stanley A. Wileman, Jr. Operating Systems Slide 8 then saves process B s context, restores A s context and executes some more instructions. LOAD A ADD B STORE C DIV X ADD Z STORE Y Process A... LOAD K DIV E MPY T STORE N CMP L JNE LOOP Process B 2008 Stanley A. Wileman, Jr. Operating Systems Slide 9 3

4 System Calls for Processes Key process-related system calls include those in the following categories: create or terminate a process; cause a process to wait, if necessary, until it can use a particular resource; manage the memory used by a process; and control the handling of asynchronous signals sent to a process. We will later examine some of these system calls in detail, and use them in a program Stanley A. Wileman, Jr. Operating Systems Slide 10 Process Tables A process table has one entry for every process in the system. Each process table entry contains information about process ownership, relation to other processes, process state, and statistical information. Process tables are frequently organized as static arrays Stanley A. Wileman, Jr. Operating Systems Slide 11 Process Identification and Ownership Each process in a system must be uniquely identified. In UNIX systems, this identification is provided by an integer called the process ID. Each process is also owned by a particular user, identified by a user ID (uid). One uid, associated with the super user, imparts special privileges (e.g. shutdown the system, and read arbitrary files) to processes. (OS/390 UNIX allows multiple super users. ) 2008 Stanley A. Wileman, Jr. Operating Systems Slide 12 4

5 Files and Data Sets Another major operating system function is to provide convenient access to information organized in files, or data sets. (The term data set is frequently used for files on mainframes.) Files are named collections of information, usually stored on disk or magnetic tape. Files can also be stored on other devices, including RAM disks and removable memory cards for use with portable devices (e.g. cameras and handheld computers) Stanley A. Wileman, Jr. Operating Systems Slide 13 File Systems A file system is a storage structure (like a data structure, but normally tailored for a secondary storage device like a disk). There are many different types of file systems, even for the same operating system. Speed, reliability, media type, and other factors influence the design of a file system. For example, CD Rom file systems differ considerably from those for hard drives Stanley A. Wileman, Jr. Operating Systems Slide 14 System Calls for File Manipulation The usual system calls for manipulating files include the following: open prepare a file for input/output read copy data from file to memory write copy data from memory to file close conclude I/O operations on a file Other system calls may be provided for things like file locking, random positioning, concurrent I/O, tape positioning, communication functions, and so forth. We will later take a detailed look at a number of these system calls and use them in a program Stanley A. Wileman, Jr. Operating Systems Slide 15 5

6 Organizing Files Even systems designed for use by a single user may have thousands of files. How are these files organized? Most systems use a hierarchical naming scheme for files. A particular file is accessed by giving first the name at the top level of the hierarchy, then the second level name, and so forth, finishing with the name of the desired file Stanley A. Wileman, Jr. Operating Systems Slide 16 Hierarchical Naming Examples A mainframe computer might use names like STANW.SAMPLE.JOBS and STANW.SAMPLE.CLIST for data sets containing different types of sample code. If these were stored in a UNIX file system, they might be /u/stanw/sample/jobs and /u/stanw/sample/clist. Note that the basic concepts are the same. It s much like describing a path through a tree, from the root to a particular node Stanley A. Wileman, Jr. Operating Systems Slide 17 Binding The term binding is used to indicate the mechanism by which a name is associated with an object. For file systems, binding refers to the mechanism used to associate a name with a file. There are many different binding techniques used in different file systems Stanley A. Wileman, Jr. Operating Systems Slide 18 6

7 Binding Examples The IBM MVS system uses catalogs, each of which stores pairs of the form (name, location), where name might be STANW.SAMPLE.JOBS and location would identify the physical location of the file. UNIX systems use multiple directories are used to store this information, each directory similar to an interior node of a tree. Thus each directory entry (called a link) contains one component of a hierarchical name and a reference to the named object, which could be another directory or a regular file Stanley A. Wileman, Jr. Operating Systems Slide 19 Catalogs STANW.SAMPLE.JOBS STANW.SAMPLE.CLIST Stanley A. Wileman, Jr. Operating Systems Slide 20 Directories u stanw stanw sample sample jobs clist 2008 Stanley A. Wileman, Jr. Operating Systems Slide 21 7

8 Catalogs and Directories We frequently say a catalog or directory contains a group of files, but this is only a conceptual idea. As we have seen, a directory or catalog only contains a list of file identifiers and other information, but not the files themselves. Directories and catalogs are usually themselves stored in files, but with limited access for writing since their organization must be carefully maintained. This is crucial for the proper operation of the file system Stanley A. Wileman, Jr. Operating Systems Slide 22 Common Filesystems Some common file systems include: FAT (used with MS-DOS) NTFS (used with Windows NT) CDFS (used on CD-ROMs) UFS (used with UNIX systems) HFS (used with OS/390 UNIX) MVS (used with mainframe systems) Each of these has a unique way of naming and storing files, and mapping from a file name to its storage. The functions they provide, however, are basically the same Stanley A. Wileman, Jr. Operating Systems Slide 23 Hierarchical File System Conventions The top directory (node) in a hierarchical file system tree is called, naturally, the root node or root directory. This concept does not apply to the MVS file system. MVS may, however, have multiple catalogs, the first of which is called the master catalog. The first component of a data set s name in MVS is called the high level qualifier (HLQ) Stanley A. Wileman, Jr. Operating Systems Slide 24 8

9 Relative and Absolute Path Names In many systems (UNIX, Windows, etc.), a unique file can be identified by giving a path to it, starting from the root directory. This is called an absolute, or complete path path to the file. A path may be given from a working directory, which is just a directory identified as such by the user. A default working directory, called the user s home directory, is in effect at login time. Absolute paths always begin with / ; a relative path is one that does not start with / Stanley A. Wileman, Jr. Operating Systems Slide 25 Example Paths Absolute or complete paths /u/stanw/samples/jobs /usr/man/man2/fork.2 Equivalent relative paths (where cwd means current working directory) jobs (with cwd of/u/stanw/samples) man2/fork.2 (with cwd of/usr/man) 2008 Stanley A. Wileman, Jr. Operating Systems Slide 26 System Calls for Directories Windows NT: CreateDirectory, RemoveDirectory GetCurrentDirectory, SetCurrentDirectory UNIX: mkdir, rmdir link, unlink, mount, unmount chdir, chroot 2008 Stanley A. Wileman, Jr. Operating Systems Slide 27 9

10 Non-Hierarchical File Systems Some file systems do not support hierarchical file naming (in the traditional manner). For example, the CP/M operating system (the predecessor of MS-DOS) only allows each file to have 11 characters in its file name, in the common 8.3 format (e.g. computer.txt). There is only one directory, which contains an entry for every file in the CP/M file system Stanley A. Wileman, Jr. Operating Systems Slide 28 File Permissions Most systems provide some protection for files to restrict who can read, write, create, or delete particular files. UNIX systems use a 9-bit code containing three groups (for the user/owner, the user s group, and all others) of 3 bits each. Each group of bits is interpreted as r w x. If r is set, then reading is allowed; w = write; x = execute Stanley A. Wileman, Jr. Operating Systems Slide 29 RACF (pronounced Rack F ) OS/390 provides optional security systems (like RACF) that provide security external to the file system. Each system call that seeks to access an object (e.g. a file) causes the operating system to query RACF to see if the requested access is permitted. This global approach to providing security is different (and much more comprehensive) than the file-system integral approach used by UNIX Stanley A. Wileman, Jr. Operating Systems Slide 30 10

11 Access Control Lists Windows NT and RACF also provide ACLs, or Access Control Lists. An ACL is a list of pairs, each pair identifying a user or group of users, and the type of access permitted to the file. Each file system object may have an associated ACL. RACF extends ACLs to virtually every object in the system Stanley A. Wileman, Jr. Operating Systems Slide 31 Special Files UNIX allows access to physical devices (for suitably privileged processes) using the same I/O system calls used for regular files. Character special files are used to model devices that do transfer one character at a time. Examples include serial ports, mice, keyboards, printers, etc. Block special files model devices that transfer a block of data at a time (disks and tapes) Stanley A. Wileman, Jr. Operating Systems Slide 32 File Descriptors / Handles Opening a file requires specification of the file name, or the path to the file. The type of access desired (e.g. read, write) is also indicated. After the file is open, other system calls referencing the file use a reference to a data structure containing information about the open file; this reference is returned by the open call. In UNIX, this reference is called a file descriptor. In Windows, it is called a handle. And in MVS it is a pointer to a data control block Stanley A. Wileman, Jr. Operating Systems Slide 33 11

12 Files Automatically Open in UNIX Processes Three (or more) files are usually available (opened automatically) when a process begins execution: Standard input (usually the keyboard) Standard output (usually the CRT) Standard error (usually also the CRT) Standard print (in MS-DOS and Windows) 2008 Stanley A. Wileman, Jr. Operating Systems Slide 34 High Level Language I/O In a high-level language (like C or Pascal), application programs rarely directly use the reference returned by the open system call. Instead, they use a reference to a higher-level data structure managed by standard libraries provided by the language. For example, to read a character c from the keyboard in C, we could use either read (0, &c, 1); (using the system call) c = getchar(stdin); (using a standard library function) 2008 Stanley A. Wileman, Jr. Operating Systems Slide 35 Pipes A pipe is essentially queue of bytes treated as an I/O device. When a pipe is created, the user is usually given two handles (or file descriptors). The first handle is used for reading, and the second handle is used for writing. Data written into the pipe is conceptually written at the tail of the queue; reading from the pipe removes data from the head of the queue. Pipes are frequently used for communication between processes Stanley A. Wileman, Jr. Operating Systems Slide 36 12

13 Implementation of System Calls Q. Since the operating system code usually executes in kernel mode, and applications run in user mode, how can an application make a system call which requires the application to enter the kernel mode? A. Special instructions are used that cause traps - just as in a divide by 0, but these traps indicate to the OS that a system call was made. While in kernel mode, the application can only execute the specified system call s code, thus ensuring security Stanley A. Wileman, Jr. Operating Systems Slide 37 Making System Calls from C Rather than have users struggle with writing special trap instructions, libraries are included with C implementations that define functions that execute the appropriate trap instructions. Example (UNIX): char buffer[80]; int count; count = read (0,buffer,80); The read function handles parameter passing and arranges for the appropriate trap instruction to be executed Stanley A. Wileman, Jr. Operating Systems Slide 38 System Call Results Most system calls are designed to return a unique code indicating failure. In UNIX, this is usually 1. When a system call fails, an additional mechanism is available to identify the particular reason for failure: errno (a global variable in UNIX) GetLastError (a function call in WindowsNT) a processor register s contents in MVS 2008 Stanley A. Wileman, Jr. Operating Systems Slide 39 13

14 Command Interpreters (Shells) Character-oriented interfaces to operating systems (as opposed to GUIs) utilize one or more programs that interpret a command language, frequently resulting in requests to create processes and execute programs. These interpreters are normally called shells (e.g. sh, csh, mush, tcsh and ksh in UNIX). Windows NT uses a program called cmd.exe, and MS-DOS uses command.com Stanley A. Wileman, Jr. Operating Systems Slide 40 Typical Features of Shells A user-configurable prompt is issued to indicate the shell is ready for the next command. prog arg1 arg2 would be typed to request creation of a process for execution of the executable program in file prog (a relative path) with arguments arg1 and arg2 (more on arguments later) Stanley A. Wileman, Jr. Operating Systems Slide 41 More Shell Features prog < input executes prog with its standard input redirected to the file input. prog > output redirects standard output to the named file. prog < input > output redirects both standard input and standard output. prog1 prog2 runs prog1 and prog2; the standard output of prog1 piped to prog2 s input. prog & runs prog concurrently with shell Stanley A. Wileman, Jr. Operating Systems Slide 42 14

15 Other Approaches Note that the mechanism by which an executable program is submitted for execution, and how external objects (e.g. files and terminals) are associated with the process, is usually independent of the system calls made by the process. For example, in MVS, program execution is requested by statements in a data set (file) containing JCL (job command language). The program itself, while unchanged, could have its execution requested by a command line in UNIX Stanley A. Wileman, Jr. Operating Systems Slide 43 MVS Job Control Language JCL statements usually have three fields: a label, a statement type, and parameters. The JOB statement is used to define a job consisting of one or more job steps: //DOIT JOB ACCT#, USERNAME The EXEC statement requests the execution of a program: //STEP1 EXEC PGM=COMPILER The DD statement associates a ddname with a data set (the ddname is referenced inside the program): //SYSIN DD DSN=MY.SOURCE.PROGRAM 2008 Stanley A. Wileman, Jr. Operating Systems Slide 44 A Typical MVS Job //DOIT JOB , USERNAME // EXEC COB2UCLG //COB2.SYSIN DD * The COBOL program goes here /* //GO.OUT DD SYSOUT=* //GO.IN DD DSN=MY.INPUT.DATA,DISP=SHR 2008 Stanley A. Wileman, Jr. Operating Systems Slide 45 15

16 The UNIX fork System Call The UNIX system call fork() duplicates the process that executes it: memory contents, open file handles, quotas, permissions, etc. In the parent (creating) process, fork() returns the unique process id (an integer) of the newly created process. In the child process, fork() returns Stanley A. Wileman, Jr. Operating Systems Slide 46 The UNIX execve System Call The execve() system call replaces the core image (memory contents) of the currently executing process with that of another program. Shells begin the execution of a program (specified on the command line) by first executing fork, and then arranging for the child process to exec the appropriate program Stanley A. Wileman, Jr. Operating Systems Slide 47 The Windows NT CreateProcess System Call The CreateProcess system call combines the effects of the UNIX fork and exec system calls. CreateProcess creates a new process which begins executing a named program with a specified set of arguments Stanley A. Wileman, Jr. Operating Systems Slide 48 16

17 The MVS ATTACHX System Call The ATTACHX system call is used in MVS to create a new subtask (process) and have it execute a named program. MVS System calls are usually made from code generated by assembler macros. The ATTACHX macro places the address of a parameter block in a register, then executes the instruction SVC 42 which invokes the operating system by causing a trap. The parameter block includes the name of the program to be executed by the new task Stanley A. Wileman, Jr. Operating Systems Slide 49 Passing Arguments to UNIX Processes When a UNIX process begins execution, it receives command line parameters in a variable length array of pointers to character strings. In C, the main function uses two (or three) arguments: An integer giving the number of arguments A pointer to an array of character strings which contains the arguments The third argument points to the environment (which we will cover later) Stanley A. Wileman, Jr. Operating Systems Slide 50 Sample Program Using Arguments The program args.c illustrates a C program that references the arguments provided on the command line. The program just prints the arguments. If invoked as args one two three, the program will print the following: Number of arguments = 4 Argument 0: <args> Argument 1: <one> Argument 2: <two> Argument 3: <three> 2008 Stanley A. Wileman, Jr. Operating Systems Slide 51 17

18 UNIX program: args.c /************************************/ /* Print the command line arguments */ /************************************/ #include <stdio.h> main (int argc, char *argv[]) { int i; printf("number of arguments = %d\n, argc); for (i=0;i<argc;i++) printf(" Argument %d: <%s>\n",i, argv[i]); exit(0); } 2008 Stanley A. Wileman, Jr. Operating Systems Slide 52 Sample Program Using Fork/Exec The program prun.c illustrates a C program that creates a new process that then executes a named program with arguments. If invoked as prun pargs one two, the program will create a new process (using fork()) and terminate. The new process will then execute the pargs program (using execve()) which will print the expected output Stanley A. Wileman, Jr. Operating Systems Slide 53 UNIX program: prun.c (part 1) /*********************************************/ /* Create a new process to execute a program */ /*********************************************/ #include <stdio.h> extern char **environ; void main( int argc, char *argv[]) { char *args[10]; int i; /* Eliminate argv[0] prior to use by execve */ for (i=1; i<argc;i++) args[i-1] = argv[i]; args[i-1] = NULL; if (argc < 2) { /* error: too few arguments */ printf("usage: prun <prog> <arg>...\n" ); exit(1); } 2008 Stanley A. Wileman, Jr. Operating Systems Slide 54 18

19 Program: prun.c (part 2) if (fork() == 0) { /* fork, test for error */ /* inside the new process, execute the prog */ execve(argv[1], args, environ); perror("execve"); /* if execve had error */ exit(1); } printf( "prun complete.\n", i ); exit(0); } 2008 Stanley A. Wileman, Jr. Operating Systems Slide 55 Next In the next module we ll examine the process model in more detail. We will examine the states in which processes may exist, and the causes for transitions between these states. Threads will be considered as alternatives to processes. And interprocess communication (IPC) will be introduced Stanley A. Wileman, Jr. Operating Systems Slide 56 19