A Guide to Unix Using Linux Fourth Edition

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 A Guide to Unix Using Linux, Fourth Edition

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 A Guide to Unix Using Linux, Fourth Edition

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); A Guide to Unix Using Linux, Fourth Edition

21. Generating Formatted Output with printf() (continued) A Guide to Unix Using Linux, Fourth Edition

22. Using the C Compiler A Guide to Unix Using Linux, Fourth Edition

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"); } A Guide to Unix Using Linux, Fourth Edition

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 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 x = 0; do printf("x is equal to %d\n", x); while (x++ < 100); A Guide to Unix Using Linux, Fourth Edition

26. #include <stdio.h> • void message(); • int main() • { • message(); • } • void message() • { • printf("Greetings from the function message."); • printf("Have a nice day."); • } Function prototype This function does not return a value • [stephen@localhost ~] $ ./func1 • Greetings from the function message. • Have a nice day. Defining Functions • To define a function, declare its name and create the function’s block of code A Guide to Unix Using Linux, Fourth Edition

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); • } • void print_square(int val) • { • printf("\nThe square is %d\n", val*val); • } • [stephen@localhost ~]$ ./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); • } • [stephen@localhost ~]$ ./func3 • 6 tripled is 18. A Guide to Unix Using Linux, Fourth Edition

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); A Guide to Unix Using Linux, Fourth Edition

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 A Guide to Unix Using Linux, Fourth Edition

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 overloadingis 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 A Guide to Unix Using Linux, Fourth Edition

42. Command Summary A Guide to Unix Using Linux, Fourth Edition