Introduction to the UNIX Operating System * Pima Community College * CIS137 -- 4.9 "C" A Programming Language

4.9 "C" A Programming Language

4.9 "C" A Programming Language

Most of the UNIX operating system is written in the "C" programming language. Most of the tools (and games) that run on the system are also written in "C". Many other utilities have a flavor of "C", such as the "C" shell csh, awk, and perl. The interface to the operating system is easily accessed with the "C" language. The operating system calls look like "C" function calls. The system calls are documented in section 2 of the manual pages. Library calls are documented in section 3 of the manual pages.

This lab will also be used to look at the effects of umask on protection bits when a file is created.

Using a text editor, create the file hello.c with the following "C" program:

/* Program to print "hello, world"                  */ 
/* Kernighan & Richie "The C Programming Language"  */ 
/* Copyright 1978  page 6                           */ 

#include <stdio.h> 

main() 
{ 
printf("hello, world\n"); 
}

Compile and run the program:
username@gort ~ $cc hello.c (cc is the standard C Compiler)
or
username@gort ~ $gcc hello.c (gcc is the gnu C Compiler)
The standard output of the compiler is a.out. You can use the option -o to put the executable in another file.
```
username@gort ~ $ ls -l a.out 
-rwxrwxrwx   1 ltaber    15165 Oct 13 14:30 a.out
```
username@gort ~ $a.out (run the program)
```
hello, world                (resulting output)
```
Now take a look at the effect of umask on file protection bits during file creation.
You may have noted above that the protection bits that you saw on your a.out file did not match my example. This is because you may have a different umask setting. To look at your current setting try:
```
username@gort ~ $ umask
```
Your output should be an octal number that is the current setting for umask. If a bit is set in your umask it will prevent that bit from being set when a file is created. Set your umask to zero.
```
username@gort ~ $ umask 0
```
Recompile your program and look at the protection bits. It should now match my example. Now try a more restrictive setting:
```
username@gort ~ $ umask 077
```
Recompile your program and look at the protection bits for a.out again. This will only allow you to read, write, or execute the program. Your results should look like this:
```
username@gort ~ $ ls -l a.out
-rwx------   1 ltaber    15165 Oct 13 14:30 a.out
```
Try a umask setting of 0777. Write on your lab what happens and explain why.

Create the file args.c with the following C program:


/* Program to print arguments to a program    */ 
/* L Taber   October 13, 1992   PCC           */ 

#include <stdio.h> 
 
main(argc, argv ) 
int argc;
char * argv[ ]; 

{
int i;

printf("argc = %d   File name = %s\n", argc, argv[0]); 
for ( i=1; i<argc; i++) 
    printf(" arg[%d] = %s\n" ,i,argv[i]); 
printf("\n"); 
}

Compile and test the program.

Create the following script file "test1":

#!/bin/bash
#
# L Taber  Argument passing
# test script 
a.out $@ 
a.out $* 
a.out "$@" 
a.out "$*" 
a.out '$@' 
a.out '$*'

Test the script file.

username@gort ~ $ test1 "A B" 'c d' e

The output should look as follows:

argc = 6   File name = a.out
 arg[1] = A
 arg[2] = B
 arg[3] = c
 arg[4] = d
 arg[5] = e

argc = 6   File name = a.out
 arg[1] = A
 arg[2] = B
 arg[3] = c
 arg[4] = d
 arg[5] = e

argc = 4   File name = a.out
 arg[1] = A B
 arg[2] = c d
 arg[3] = e

argc = 2   File name = a.out
 arg[1] = A B c d e

argc = 2   File name = a.out
 arg[1] = $@

argc = 2   File name = a.out
 arg[1] = $*

Turn in a copy of this output marked with:

your name 
TABER CIS137 
Lab 4.9: C Programming
and the results of a umask of zero

Please turn your lab to Louis Taber or to Pima Community College employee in room A-115 of the Santa Rita Building. Ask them to place it in the dark blue folder in Louis Taber's mailbox.

Instructor: Louis Taber, ltaber(at)uml.lt.Tucson.AZ.us (520) 206-6850
My new web Home page in Cleveland, OH
The Pima Community College web site

4.9 "C" A Programming Language