January 11, 2006

Coverage: [USP] Chapter 2

Process

executable module of program resides in a program image in main memory
(logical) layout of the program image
has a process id (do a ps UNIX) and a process state (ready, running, blocked, and so on)
variables are either static (exist for the life of the process) or automatic (allocated/deallocated on block entry/exit)

Thread

an ADT within a process
has its own stack, program counter value, register set, and state
share process resources
heavyweight process vs. lightweight process
static variables make the use of multiple threads unsafe

char* strtok (char* restrict s1, const char* restrict delimiter);
- first and subsequent calls are different
- tokenizes string in place
strtok is not thread-safe, e.g., wordcount, wordaverage
- use POSIX thread-safe function strtok_r
- _r stands for reentrant
moral of the story: avoid using static storage

Storage Class Summary

Class	Scope	Life	Storage	Init. arrays/structs	Default value
automatic	block	block active	stack	yes	undefined
register (1)	block	block active	machine register	no	undefined (2)
external (3)	declaration to end-of-file	permanent	data area	yes	0
static external (4)	declaration to end-of-file	permanent	data area	yes	0
static internal	block	permanent	data area	yes	0

Notes:

Speed advantage
For function arguments, value passed
Can be accessed from other files
Can not be accessed from other files

(ref. [C])

`makeargv`

three versions
- int makeargv (const char *s, const char *delimiters, char*** argvp);
e.g., ./a.out -c tom and -m jerry
cleaning up after yourself

Bubbblesort Storage Class Exercise 2.20

count is a static variable with internal linkage (note use of static modifier)
onepass does not have a storage class; its linkage is internal
all other functions have external linkage; functions do not have a storage class
all other variables are automatic with no linkage

Handling Errors

importance cannot be stressed enough; key to reliability
errno, perror (in stdio.h), and strerror (in string.h)
- be careful, strerror may change the value of errno
  - therefore save and restore it
interruptions by signals
- restart.h, use r_ prefix, e.g., r_close
use conditional compilation, #ifdef and #endif, #define DEBUG or gcc -DDEBUG
follow 12 guidelines in [USP], pp. 30-31

Other Things Discussed

system function (combo of fork, wait, and execl)
exec ksh (at command prompt)
extern char** environ variable, getenv function
zombie process, orphans, init process
normal (return or exit) or abnormal (ctrl-c, signal) process termination
lint

Codes Discussed In-class

Examples 2.8 (close), 2.9 (r_close), 2.19 (freemakeargv), 2.23 (mail), 2.24 (getpaths)

Self-study

re-read section 2.9
download, compile, and run codes
convince yourself you understand how the list works
exercise in implementing the third version of makeargv

References

[C] C Language for Experienced Programmers, Version 2.0.0, AT&T, 1988.

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