4. 4
MULTICS (1965) – parent of UNIX
Have a look at http://www.multicians.org/
Written in PL/1, and was portable. It had
Hierarchical directories
A CLI interface
Utilities (mail, editors, etc.)
The last Multics system shut down in
2000 (!)
Courtesy of Hugh Anderson
5. 5
UNIX history
Well – they needed a computer to run
Ken Thompson’s “Space Travel” game…
The UNIX Time-Sharing System, 1974
http://citeseer.ist.psu.edu/10962.html
Please read sections: I. Introduction, II. Hardware
and Software Environment, VIII. Perspective, IX.
Statistics.
The Evolution of the Unix Time-sharing System,
1984
http://citeseer.ist.psu.edu/ritchie84evolution.html
Please read sections: Introduction, Origins, The
advent of the PDP-11, The first PDP-11 system,
High-level languages, Conclusion.
Courtesy of Hugh Anderson
6. 6
UNIX and C
C was developed in order to re-implement
UNIX (most work down in 1972)
The Development of the C language, 1993
http://citeseer.ist.psu.edu/ritchie93development.html
Please read sections: Introduction, History: The
Setting, Embryonic C, Neonatal C, Successors,
Critique.
In summary, C was a high-level language (for the
1970’s), and small and efficient enough to implement
an OS, even on the small machines of the day.
Courtesy of Hugh Anderson
7. 7
UNIX 35 years on…
According to Netcraft (August 2005) 78% of
(72M) web servers are UNIX based.
Peer review, open source, standards, design,
GNU, stability, Portability-to-the-max (PDAs,
MacOSX, servers…)
Virtually all complex S/W products started in
UNIX
TCP/IP, web browsers, theorem provers, databases,
spreadsheets, Java, IDEs/RAD tools, Alias Systems
Corporation (Alias|Wavefront), renderfarms, search engines…
Courtesy of Hugh Anderson
8. 8
UNIX (1/2)
You may wish to try the following
commands:
cat "food in cans"
[ Where is my brain?
(-
mkdir yellow_pages; cat > yellow_pages
ping elvis.org | awk '{print substr($1,1,5), $2,$2}'
UNIX is a multi-user multi-tasking
operating system. It is relatively stable,
and as a result, it is the most common OS
for servers on the Internet.
Courtesy of Hugh Anderson
9. 9
UNIX (2/2)
It is written almost entirely in C, and the
sources for various versions are available
for copying and inspection, making it the
most portable OS.
UNIX distributions:
http://www.mirrorservice.org/packages/unix/
Mini distributions: http://www.cotse.com/miniunix.htm
Courtesy of Hugh Anderson
10. 10
C (1/2)
C has been described as:
“a language that combines all the elegance
and power of assembly language…
with all the readability and maintainability of
assembly language” (!)
Courtesy of Hugh Anderson
11. 11
C (2/2)
C is procedural, not OO
OO means you concern yourself with the
components (objects) of your system
Procedural means… what do I do next…
(instead of what do I do it to…)
Library do anything, but are generally
much less safe than Java
Courtesy of Hugh Anderson
12. 12
C: Horrible code
Here is an example of correct, but
extremely hard to read code:
Courtesy of Hugh Anderson
13. 13
C: Obfuscated code
This code comes from an interesting website
devoted to the worst-of-the-worst-of-the-worst
C code.
The International Obfuscated C Code Contest
at http://www.ioccc.org/.
C programmers from around the world compete
to write the worst C code every year.
You may find it interesting to find out how not to
write C.
Courtesy of Hugh Anderson
15. 15
C: Comments
The first six lines of the program are an
(optional) comment. Comments are
placed between /* and */ symbols, or
(Java like) after a //.
You are put header comments something
like the above one in every program you
submit during this course.
You might want to spend a little time with
an editor, and create a master header
that you can use in all your C programs.
Courtesy of Hugh Anderson
16. 16
C: main()
The main part of the program to be run is
always called main. This is just a tradition.
It could have been called ‘program’ or
‘starting-place’ or just about anything, but
the designers decided to use the word
‘main’.
Not ‘Main’, ‘MAIN’, or ‘MaIn’.
Case-sensitive in C.
Courtesy of Hugh Anderson
17. 17
C: body
The body of the program contains a
single statement which prints out the
message Hello Singapore! Followed by a
newline character n.
The printf is called a statement or a
command.
Courtesy of Hugh Anderson
18. 18
C: printf (1/3)
The C statement printf can be used in a
few different ways, but this way is the
simplest.
The string within the quotes is printed out
on the console.
The “n” indicates that a new line is added
to the output – shifting suceeding outputs
onto the next line.
Courtesy of Hugh Anderson
19. 19
C: printf (2/3)
The format given here for printf is:
printf ( <string> );
On a UNIX system, the online help can
give you detailed information about each
C programming construct.
Courtesy of Hugh Anderson
20. 20
C: printf (3/3)
If you use the UNIX command “man
printf”, or “man –s 3c printf”, you get:
Courtesy of Hugh Anderson
25. 25
Safety
C can be ‘unsafe’
In Java:
In C:
Courtesy of Hugh Anderson
String str1, str2;
. . .
str1 = str2;
char str1[10], str2[10];
. . .
strcpy(str1, str2);
26. 26
String copy (1/2)
char str1[10] – allocates 10 characters for a
string
strcpy – copies the second string to the first
printf - %s specifies the format used to print the
argument
Courtesy of Hugh Anderson
int main() {
char str1[10], str2[10];
strcpy(str1, "Important");
printf("Contents of str1: %sn", str1);
}
27. 27
String copy (2/2)
Courtesy of Hugh Anderson
int main() {
char str1[10], str2[10];
strcpy(str1, "Important");
strcpy(str2, "Not quite so important");
printf("Contents of str1: %sn", str1);
}
28. 28
Warnings to the compiler
We get warnings – so fix these first.
Courtesy of Hugh Anderson
#include <stdio.h>
int main() {
char str1[10], str2[10];
strcpy(str1, "Important");
strcpy(str2, "Not quite so important");
printf("Contents of str1: %sn", str1);
return 0;
}
Compile: gcc -Wall
29. 29
No compilation error, but…
Courtesy of Hugh Anderson
Output:
ortant
Aaaaaaarrgggghhhh!
The storage locations for str1 and str2 are…
31. 31
#include <stdio.h>
Include a standard file.
Any C line that begins with # is pre-
processed with a program called the C
pre-processor (cpp).
#include is textual replacement.
Courtesy of Hugh Anderson
33. 33
Compilation (2/2)
gcc x.c is…
Courtesy of Hugh Anderson
cpp – C pre-processor
cc1 – compiler to assembly language
as – assembles to an object module
ld – links object modules to executable
C C .s .o
U U U U
cpp cc1 as ld
(C source) (C source) (Assembler) (Object
module)
(Executable)
34. 34
C: Tokens
The compiler splits the program into
tokens. For our first program, the tokens
would be:
Courtesy of Hugh Anderson
main ( ) { printf ( "Hello Singapore!n" ) ; }
35. 35
Writing clear code
We could write our program just like this:
main(){printf("Hello Singapore!n");}
and the program would compile and run
accurately.
However, we do not do this. It is a
common programming practice to try as
much as possible to make our programs
readable.
Courtesy of Hugh Anderson
36. 36
An aside into UNIX
Look at with ps or ps –f or ps –fl or
ps –efl:
Courtesy of Hugh Anderson
Note parent process…
38. 38
UNIX process hierarchy
The first process is init (process id 1).
All other processes descend from it, so
we have a tree.
We will see this later, but processes
create new processes using a fork()
system call.
Courtesy of Hugh Anderson
39. 39
UNIX system calls (1/2)
When a process runs some functions are
critical, and are only performed by the OS
kernel.
In our “Hello Singapore” program, all
output (I/O) is considered critical, and so
the printf() call in the program will end up
performing a system call.
Similarly for fork() – creating new
processes.
Courtesy of Hugh Anderson
41. 41
Back to C compilation
Courtesy of Hugh Anderson
42. 42
Typical Unix command
& run in backgrund (simultaneously)
; command separator
() bracketed expression
Courtesy of Hugh Anderson
gcc ic.c & (/usr/ucb/ps –a; /usr/ucb/ps –a; /usr/ucb/ps –a)
The interpretation of these symbols is done
by the shell program, in this case bash.
