1. A Guide to Unix Using Linux Fourth Edition
Chapter 10
Developing UNIX/Linux Applications in C and C++

2. Objectives Understand basic elements of C programming Debug C programs
Create, compile, and test C programs
Use the make utility to revise and maintain source files
A Guide to Unix Using Linux, Fourth Edition

3. Objectives (continued)
Identify differences between C and C++ programming
Create a simple C++ program
Create a C++ program that reads a text file
Create a C++ program that demonstrates how C++ enhances C functions
A Guide to Unix Using Linux, Fourth Edition

4. Introducing C Programming
Unix was developed and refined in the C language
Original UNIX OS was written in assembly language
Assembly language: low-level language; provides maximum access to all the computer’s devices
Ritchie and Kernighan from AT&T Bell Labs rewrote most of UNIX using C in early 1970s
C is native to UNIX/Linux
Works best as an application development tool
Example: daemons are written in C
C is a compiled language
A Guide to Unix Using Linux, Fourth Edition

5. Creating a C Program
A Guide to Unix Using Linux, Fourth Edition

6. C Keywords Keywords have special meanings
Cannot be used as names for variables or functions
A Guide to Unix Using Linux, Fourth Edition

7. The C Library The C language is very small
No input or output facilities
All I/O is performed through the C library
C library: consists of functions that perform file, screen, and keyboard operations
Other tasks: functions to perform string operations, memory allocation, control, etc.
To perform one of these operations in program, place a function call
Linker joins code of library function with program’s object code to create executable file
A Guide to Unix Using Linux, Fourth Edition

8. Program Format C programs are made up of one or more functions
Every function must have a name
Every C program must have a main() function:
int main() { }
A Guide to Unix Using Linux, Fourth Edition

9. Including Comments /* denotes the beginning of a comment
*/ denotes the end of a comment
Compiler ignores everything between the symbols:
/* Here is a program that does nothing. */
int main() { }
A Guide to Unix Using Linux, Fourth Edition

10. Using the Preprocessor #include Directive
The following program creates output:
/* A simple C program */
#include <stdio.h>
int main() {
printf("C is a programming language.\n");
printf("C is very compatible with UNIX/Linux.\n"); }
stdio.h is a header file
A preprocessor directive
A Guide to Unix Using Linux, Fourth Edition

11. Using the Preprocessor #include Directive (continued)
A Guide to Unix Using Linux, Fourth Edition

12. Specifying Data Types
Variables and constants represent data used in a C program
You must declare variables and state type of data that variable can hold
A variable’s data type determines upper and lower limits of its range of values
Exact limits of ranges vary among compilers and hardware platforms
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13. Specifying Data Types (continued)
A Guide to Unix Using Linux, Fourth Edition

14. Character Constants
Characters are represented internally in a single byte of the computer’s memory
Stored according to the character’s code in the host character set
Example: if machine uses ASCII codes, letter A is stored in memory as the number 65
In a program, a character constant must be enclosed in single quotation marks
‘A’
‘c’
A Guide to Unix Using Linux, Fourth Edition

15. Using Strings A string is a group of characters, such as a name
Strings are stored in memory in consecutive memory locations
String constants are enclosed in double quotation marks
"Linux is a great operating system."
C does not provide a data type for character strings
Use a character array instead: char name[20];
Can hold a string of 19 characters, terminated with the null character
A Guide to Unix Using Linux, Fourth Edition

16. Including Identifiers
Identifiers are names given to variables and functions
First character must be a letter or an underscore
For other characters, use letters, underscores, or digits
Variable names can be limited to 31 characters
Some compilers require first eight characters to be unique
Uppercase and lowercase characters are distinct
Tip: use meaningful identifiers
Examples: radius, customer_name, my_name
A Guide to Unix Using Linux, Fourth Edition

17. Declaring Variables
You must declare all variables before you use them in a program
int days;
You can declare multiple variables of the same type on the same line
int days, months, years;
You can initialize variables with values at the time they are declared
int days = 5, months = 2, years = 10;
A Guide to Unix Using Linux, Fourth Edition

18. Understanding the Scope of Variables
Scope of a variable: part of the program in which the variable is defined and accessible
/* This program declares a local variable
in function main. The program does nothing
else. */
int main() {
int days; }
An automatic variable
/* This program declares a global variable
The program does nothing else. */
int days;
int main() { }
A global or external variable
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19. Using Math Operators Example:
x = y + 3;
Increment and decrement operators are unary
Examples: count--; or x = ++j;
A Guide to Unix Using Linux, Fourth Edition

20. Generating Formatted Output with printf()
Examples:
printf("Hello");
printf("Your age is %d", 30);
printf("Your age is %d", age);
printf("The values are %d %d", num1, num2);
printf("You have worked %d minutes", hours*60);
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21. Generating Formatted Output with printf() (continued)
A Guide to Unix Using Linux, Fourth Edition

22. Using the C Compiler
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23. Using the if Statement Example: if (weight > 1000) {
printf("Warning!\n");
printf("You have exceeded the limit.\n");
printf("Please remove some weight.\n"); }
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24. Using the if Statement (continued)
if-else construct allows program to do one thing if a condition is true and another if it is false
Example:
if (hours > 40)
printf("You can go home now.");
else printf("Keep working!");
A Guide to Unix Using Linux, Fourth Edition

25. Using C Loops Three looping mechanisms in C: for loop while loop
for (count = 0; count < 100; count++)
printf("Hello\n");
while loop
x = 0;
while (x++ < 100)
printf("x is equal to %d\n", x);
do-while loop do
while (x++ < 100);
A Guide to Unix Using Linux, Fourth Edition

26. Defining Functions
To define a function, declare its name and create the function’s block of code
#include <stdio.h>
void message();
int main() {
message(); }
void message()
printf("Greetings from the function message.");
printf("Have a nice day.");
~] $ ./func1
Greetings from the function message.
Have a nice day.
Function prototype
This function does not return a value
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27. Using Function Arguments
Argument: a value passed to a function
Stored in special automatic variables
#include <stdio.h>
void print_square(int val)
main() {
int num = 5;
print_square(num); }
printf("\nThe square is %d\n", val*val);
~]$ ./func2
The square is 25
A Guide to Unix Using Linux, Fourth Edition

28. Using Function Return Values
#include <stdio.h>
int triple(int num);
int main() {
int x = 6, y;
y = triple(x);
printf("%d tripled is %d.\n", x, y); }
int triple(int num)
return (num * 3);
~]$ ./func3
6 tripled is 18.
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29. Working with Files in C
C file input/output is designed to use file pointers
FILE *fp;
Before you can use a file, it must be opened
if ((fp = fopen("myfile.dat", "r")) == NULL) {
printf("Error opening myfile.dat\n"); }
When a file is closed, its buffers are flushed
if (feof(fp))
fclose(fp);
C provides many functions for reading/writing files
ch = fgetc(fp);
fputc(ch, fp);
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30. Using the make Utility to Maintain Program Source Files
You might often work with a program that has many files of source code
Advantage: break down code into smaller modules that can be reused
Problem: avoid recompiling all files if only one changed
make utility tracks what needs to be recompiled by using a time stamp field for each source file
Must create a control file called makefile
Must exist in current directory
A Guide to Unix Using Linux, Fourth Edition

31. Using the make Utility to Maintain Program Source Files (continued)
Example of a makefile:
abs_main.o: abs_main.c
gcc -c abs_main.c
abs_func.o: abs_func.c
gcc -c abs_func.c
abs2: abs_main.o abs_func.o
gcc abs_main.o abs_func.o -o abs2
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32. Using the make Utility to Maintain Program Source Files (continued)
A Guide to Unix Using Linux, Fourth Edition

33. Debugging Your Program
Typical errors for new C programmers include using incorrect syntax
Example: forgetting to terminate a statement with a semicolon (;)
Example of a compiler error output:
simple.c:10: unterminated string or character constant
simple.c:10: possible real start of unterminated constant
simple.c:4:10: missing terminating " character
simple.c:5: error: syntax error before ’}’ token
Compiler generally produces more error lines than the number of mistakes it finds in the code
A Guide to Unix Using Linux, Fourth Edition

34. Debugging Your Program (continued)
Steps to correct syntax errors within your programs:
Write down the line number of each error and a brief description
Edit your source file
Start with the first line number the compiler reports
Within the source file, correct the error
Then, continue with the next line number
After correcting errors, save and recompile
A Guide to Unix Using Linux, Fourth Edition

35. Creating a C Program to Accept Input
A Guide to Unix Using Linux, Fourth Edition

36. Creating a C Program to Accept Input (continued)
Examples:
scanf("%d", &age);
scanf("%s", city);
scanf("%d %f %d", &x, &y, &z);
A Guide to Unix Using Linux, Fourth Edition

37. Introducing C++ Programming
C++ is a programming language developed by Bjarne Stroustrup at AT&T Bell Labs
Adds object-oriented programming capabilities
Object-oriented programming uses objects for handling data
C++ programs introduce objects as a new data class
Object: collection of data and methods, which manipulate the data
Function overloading is an additional useful feature
Compiler: g++
Example: g++ myprogram.C -o myprogram
Typical file extensions: .C or .cpp
A Guide to Unix Using Linux, Fourth Edition

38. Creating a Simple C++ Program
//======================================================
// Program Name: simple.C
// By: MP
// Purpose: First program in C++ showing how to
// produce output
#include <iostream>
using namespace std;
int main(void) {
cout << "C++ is a programming language.\n";
cout << "Like C, C++ is compatible with UNIX/Linux.\n"; }
A Guide to Unix Using Linux, Fourth Edition

39. Creating a C++ Program that Reads a Text File
//======================================================
// Program Name: fileread.C
// By: MP
// Purpose: C++ program that reads contents of a file
#include <iostream>
#include <fstream>
using namespace std;
int main(void) {
ifstream file("testfile");
char record_in[256];
if (file.fail())
cout << "Error opening file.\n";
else
while (!file.eof())
file.getline(record_in, sizeof(record_in));
if (file.good())
cout << record_in << endl; }
A Guide to Unix Using Linux, Fourth Edition

40. How C++ Enhances C Functions
//======================================================
// Program Name: datestuf.C
// By: MP
// Purpose: Shows you two ways to access the
// system date
#include <iostream>
#include <ctime>
using namespace std;
void display_time(const struct tm *tim) {
cout << "1. It is now " << asctime(tim); }
void display_time(const time_t *tim)
cout << "2. It is now " << ctime(tim);
int main(void)
time_t tim = time(NULL);
struct tm *ltim = localtime(&tim);
display_time(ltim);
display_time(&tim);
A Guide to Unix Using Linux, Fourth Edition

41. Summary
C program modules are compiled separately into object code and linked to make up a program
C programs first execute instructions in main()
make is used to maintain an application’s source files
C follows procedural principles, while C++ primarily follows object-oriented programming principles
The standard stream library used by C++ is iostream
C++ provides two statements for standard input and standard output: cin and cout, respectively
Function overloading allows functions to handle multiple sets of criteria
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42. Command Summary
A Guide to Unix Using Linux, Fourth Edition