Advanced UNIX progamming

1
Advanced UNIX progamming

• Fall 2002
• Instructor Ashok Srinivasan
• Lecture 6

Acknowledgements The syllabus and power point
presentations are modified versions of those by
T. Baker and X. Yuan
2
Announcements

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

3
Review

• Portability
• Standards ANSI, POSIX, etc
• 32 bit vs 64 bit
• Byte order Little endian vs big endian
• Introduction to the UNIX API
• Environment variables
• Exit status
• Process ID
• User ID
• UNIX file system
• File system abstraction
• Directories
• File descriptors

4
Week 3 Topics

• UNIX file system
• File system abstraction
• Directories
• File descriptors
• Unix API Programming Examples and Techniques
• Example with direct IO
• open, close, fdopen, lseek, unlink
• Variable argument list
• HW1 hints

5
Week 3 Topics ... continued

• File I/O
• File descriptors
• open, creat, close, dup, dup2
• I/O redirection
• Process management
• fork, exit, wait, waitpid, execv
• Pipes
• Named and unnamed pipes
• Implementing pipe in a shell

6
UNIX file system

• File system abstraction
• Directories
• File descriptors

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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

8
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

9
Directories
10
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 exist 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

11
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

12
File Descriptors ... continued
13
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

14
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
• 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

15
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

16
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

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Unix API Programming Examples and Techniques

• Examples with direct IO
• open, close, fdopen, lseek, unlink
• Variable argument list
• Note
• Use man pages to get information on system calls
• Look into the system header files
• /usr/include/sys/types.h)

18
Direct I/O

• Using open()
• The usual C-language stream-oriented I/O
  operations, like printf(), use buffers and
  process data character-by-character
• They are implemented using the lower-level direct
  I/O operations read() and write()
• In situations where we do not want to view the
  data as characters, where we want greater
  efficiency, or where the extra (stream) layer of
  buffering causes us problems with
  synchronization, it is better to use the direct
  I/O operations

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Using man

• Look at the man page for open()
• If there is more than one page on a given name,
  man will give you the one that is first in the
  chapter order of the Unix manual.
• Shell commands are in Section 1, I/O and OS
  interface calls are in Section 2 and Section 3
  respectively
• Specification of section number varies
• On Red Hat Linux, type man 2 open or man -S 2
  open to see the page on open from Section 2 of
  the Unix manual
• On Solaris, you can type man -s 2 open

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man page for open

• include ltsys/types.hgt
• include ltsys/stat.hgt
• include ltfcntl.hgt
• Solaris 2.6 includes the following synopsis
• int open(const char path, int oflag, / mode_t
  mode / ...)
• Red Hat Linux 6.2
• int open(const char pathname, int flags, mode_t
  mode)
• 1996 POSIX standard synopsis is as follows
• int open(const char path, int oflag, ...)
• The latest official POSIX/Unix synopsis
• No ltsys/types.hgt
• What does the ... mean here? Will a compiler
  allow this in an actual program?

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Variable Argument Lists

• The ... indicates a variable number of arguments
• Similar to that in printf()
• For more on variable argument lists, look at the
  file /usr/include/stdarg.h
• Functions with variable argument lists can be
  dangerous
• It is difficult to check types, and the use of a
  correct number of arguments

22
Example Programs

• example1.c illustrates a common programming error
• Failure to provide the correct number of
  arguments to a vararg function
• example2.c illustrates opening a file