Advanced UNIX progamming

1. Advanced UNIX progamming Fall 2002 Instructor: Ashok Srinivasan Lecture 5 Acknowledgements: The syllabus and power point presentations are modified versions of those by T. Baker and X. Yuan

2. Announcements • Reading assignment • APUE Chapter 1 • APUE Chapter 3 • Pages 47-56, 56-62 • APUE Chapter 4 • Pages 77-81, 92-95 • APUE Chapter 5

3. Review • Review some features of C • Header files • Command line arguments • Utilities • Review some UNIX system calls • system, etc • Portability • Standards: ANSI, POSIX, etc • 32 bit vs 64 bit • Byte order: Little endian vs big endian

4. Week 2 Topics • Review some features of C • Header files • Command line arguments • Utilities • Review some UNIX system calls • system, etc • Portability • Standards: ANSI, POSIX, etc • 32 bit vs 64 bit • Byte order: Little endian vs big endian

5. Week 2 Topics ... continued • Introduction to the UNIX API • Environment variables • Exit status • Process ID • User ID • UNIX file system • File system abstraction • Directories • File descriptors

6. Review some features of C • Header files • Macros • Command line arguments • See example1.c • Utilities

7. Some Unix System Calls • You may use these in your first assignment • system • mkstemp • See example2.c

8. system #include <stdlib.h> int system(const char *string); • Works as if string is typed into the shell at a terminal • Returns the exit status (see man page for waitpid) • Usually -1 is returned if there is an error

9. mkstemp #include <stdlib.h> int mkstemp(char *template) • template should end in XXXXXX • It replaces XXXXXX with unique file name, and returns an open file descriptor for a file available for reading and writing

10. Portability • Standards • Source code portability: ANSI/ISO C • UNIX standards: POSIX, open group • Internet engineering task force (IETF) • 32 bit vs 64 bit • Byte order • Little endian vs big endian

11. Source Code Portability • Standard programming language • Example: ANSI/ISO C • ISO C90 is in use; C99 is latest - should it be used? • Standard libraries • Standard API to operating system • Example: POSIX.1 • Auto-configuration mechanisms • Programmer discipline

12. Unix Standards • POSIX (IEEE STDS 1003.x and ISO/IEC 9945) • POSIX.1: System API for C language • POSIX.2: Shell and utilities • POSIX.5: System API for Ada language • POSIX.9: System API for Fortran language • See also http://www.pasc.org and http://www.standards.ieee.org

13. Unix Standards ... continued • The Open Group • A consortium of vendors and user organizations • Consolidation of X/Open and the Open Software Foundation • Controls the UNIX trademark • The Austin Group combined the IEEE, TOG, and ISO standards • See also http://www.opengroup.org and http://www.opengroup.org/onlinepubs/007904975

14. IETF • Internet Engineering Task Force (IETF) • Network designers, operators, vendors, researchers • Deals with the Internet • Issues RFCs • See also http://www.ietf.org

15. 64-bit vs. 32-bit architecture • Pointers cannot be stored as int • size_t cannot be stored as int • long may not be long enough for size_t and off_t Note: ILP32 and LP64 are not the only two models

16. Byte order • Little-Endian • Low-order byte is stored at lowest address • Big-Endian • High-order byte is stored at lowest address

17. Introduction to the UNIX API • Environment variables • Exit status • Process ID • User ID

18. Environment Variables • Associated with each process is a set of environment variables, each having a value • These values are passed to the process when it is executed • Along with argc and argv, this is another way of passing parameters to a program • Example: when in the Bourne shell you execute export DISPLAY=mysystem:0.0 The environment variable DISPLAY has value mysystem:0.0 • A process can access its environment variables using either (i) external variable environ, or (ii)function getenv() • See example3.c

19. Exit status • Each process (except the system's initial process) has a parent process • When a process terminates, its parent can find out what caused the termination, via a status value • The termination status contains a short integer exit status code • The function exit() allows a process to terminate itself and specify the exit status code that it wants to return to the parent • When a command is run from a shell, the exit status code is returned to the shell, which allows it to be used in job control statements • What is the difference between _exit() and exit()?

20. Process ID • Each process has a unique identifier, of signed arithmetic type pid_t • The process ID of the current process can be obtained by calling the function getpid()

21. User ID • Associated with each process are two user ID's • A fixed real user ID, and • A changeable effective user ID • In order to modify privileges • The real user ID can be obtained by calling getuid() • See example4.c

22. UNIX file system • File system abstraction • Directories • File descriptors

23. File system abstraction • File: a sequence of bytes of data • Filesystem: a space in which files can be stored • Link: a named logical connection from a directory to a file • Directory: a special kind of file, that can contain links to other files • Filename: the name of a link • Pathname: a chain of one or more filenames, separated by /'s

24. File system abstraction ... continued • inode: a segment of data in a filesystem that describes a file, including how to find the rest of the file in the system • File descriptor: a non-negative integer, with a per-process mapping to an open file description • Open file description: an OS internal data-structure, shareable between processes

25. Directories

26. Directories ... continued • Names belong to links, not to files • There may be multiple hard links to one file • Renaming only renames one link to that file • Unix allows both hard and soft links • A file will exit even after the last hard link to it has been removed, as long as there are references to it from open file descriptions • Soft links do not prevent deletion of the file • A directory may have multiple (hard) links to it • But this capability is usually restricted, to prevent creation of directory cycles

27. File Descriptors • Each open file is associated with an open file description • Each process has a (logical) array of references to open file descriptions • Logical indices into this array are file descriptors • These integer values are used to identify the files for I/O operations • The file descriptor 0 is reserved for standard input, the file descriptor 1 for standard output, and the file descriptor 2 for the standard error

28. File Descriptors ... continued

29. File Descriptors ... continued • The POSIX standard defines the following • File descriptor: A per-process, unique, nonnegative integer used to identify an open file for the purposes of file access • Open file description: A record of how a process or group of processes are currently accessing a file • Each file descriptor refers to exactly one open file description, but an open file description may be referred to by more than one file descriptor • A file offset, file status, and file access modes are attributes of an open file description • File access modes: Specification of whether the file can be read and written

30. File Descriptors ... continued • File offset: The byte position in the file where the next I/O operation through that open file description begins • Each open file description associated with a regular file, block special file, or directory has a file offset • A character special file that does not refer to a terminal device may have a file offset • There is no file offset specified for a pipe or FIFO (described later) • File status: Includes the following information • append mode or not • blocking/nonblocking • Etc

31. File Descriptors ... continued • FIFO special file: A type of file with the property that data written to such a file is read on a first-in-first-out basis • Pipe: An object accessed by one of the pair of file descriptors created by the pipe() function • Once created, the file descriptors can be used to manipulate the pipe, and it behaves identically to a FIFO special file when accessed this way • It has no name in the file hierarchy

32. File Descriptors ... continued • Important points • A file descriptor does not describe a file • It is just a number that is ephemerally associated with a particular open file description • An open file description describes a past "open" operation on a file; its does not describe the file • The description of the file is in the inode • There may be several different open file descriptors (or none) referring at it any given time