Advanced UNIX progamming

1
Advanced UNIX progamming

• Fall 2002
• Instructor Ashok Srinivasan
• Lecture 4

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

• Assignment 1 announced
• Due Sep 13, 2002
• Quiz next week, on week 1 2 topics
• Reading assignment
• APUE chapter 1
• APUE Chapter 2
• Pages 25-30, 44-46

3
Review of week 1 topics

• UNIX programming environment
• Editors
• C compilers and debuggers
• Makefiles

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
• Signals and signal masking
• UNIX file system
• File system abstraction
• Directories
• File descriptors
• Unix API Programming Examples and Techniques
• Example with direct IO
• open, close, etc
• Variable argument list

6
Review some features of C

• Header files
• Macros
• Command line arguments
• Utilities

7
Header files

• Usually define interfaces between separately
  compiled modules
• May contain macro definitions, preprocessor
  directives, declarations of types, and function
  prototypes
• Should not contain variable definitions or
  executable code

8
Some header file errors

• Improper header file use can cause problems
• Try compiling example2.c
• Including a header file multiple times may cause
  redefinition errors
• Why does including stdio.h twice not cause any
  problem?
• Look at /usr/include/stdio.h

9
Conditional Code in Headers

• Preprocessor directives are used to prevent the
  body of a header file from being used multiple
  times.
• ifndef MYHEADER
• define MYHEADER
• / the body of the header file /
• endif

10
Macros with and without Parameters

• define MAX_LENGTH 256
• ... for (i 0 i lt MAX_LENGTH i) ...
• Macros can have parameters
• define max(a,b) (a gt b) ? a b
• What is wrong with the following?
• define sum(a, b) a b
• define product(a, b) ab
• See example3.c, example3b.c, example3c.c, and
  example3d.c

11
Some useful functions

• include ltstdio.hgt
• int sprintf(char s, const char format, ...)
• int sscanf(const char s, const char format,
  ...)
• How would these be used to get all the fields
  from the output of the shell command ps?
• See example4.c

12
Some Unix System Calls

• You may use these in your first assignment
• system
• mkstemp

13
system

• include ltstdlib.hgt
• 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

14
mkstemp

• include ltstdlib.hgt
• 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

15
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

16
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

17
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

18
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

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IETF

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

20
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
  offset_t

Datatype ILP32 LP64
char 8 8
short 16 16
int 32 32
long 32 64
pointer 32 64
(long long) 64 64
Note ILP32 and LP64 are not the only two models
21
Byte order

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