Advanced UNIX progamming

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

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

Review of week 1 topics • UNIX programming environment • Editors • C compilers and debuggers • Makefiles

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

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

Review some features of C • Header files • Macros • Command line arguments • Utilities

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

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

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

Macros with and without Parameters • #define MAX_LENGTH 256 • ... for (i = 0; i < MAX_LENGTH; i++) ... • Macros can have parameters • #define max(a,b) (a > b) ? a : b • What is wrong with the following? • #define sum(a, b) a + b • #define product(a, b) a*b • See example3.c, example3b.c, example3c.c, and example3d.c

Some useful functions • #include <stdio.h> • 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

Some Unix System Calls • You may use these in your first assignment • system • mkstemp

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

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

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

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

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

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

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

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 Note: ILP32 and LP64 are not the only two models

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

Advanced UNIX progamming