Class 05: C Basics

Held Wednesday, February 2, 2000

Overview

Today, we go ``back to the basics'' for the C programming language. I will assume that you know some Java in this overview.

Notes

• If you haven't bought K&R, it may be worth getting copies of the Stanford handouts (see the links page for more details.
• For the next few classes, I'll try to mix short lectures with lab exercises.
• Assignments:
  • HW 1: A Chat Program (due Monday)

Contents

• Some Basics
  • Hello World
  • Functions
  • Command Line
  • From Strings to Integers
  • Program Input
• Basic C Exercises
  • Exercise: Compiling Programs
  • Exercise: Functions
  • Exercise: Separate Compilation
  • Exercises: Command Line
  • Exercise: String Conversion
  • Exercise: User Input

Summary

• Hello World
• Some C basics
• Exercises: Compiling and writing simple C programs

Some Basics

• We'll start our consideration of C by looking at details of some of the standard ``first programs'' and related basic capabilities.
• The examples should help us with both the language and our implementation.

Hello World

• We start with a program that prints a greeting, typically called the ``Hello World'' application.
• Here's the program, in its overly-commented entirity

  /* 
     Print a greeting.  A standard first program. 
     Author: Too many programmers.
     Version: 1.1 of 2 February 2000
   */
  
  /* 
     Since we're doing output, we include the header that
     defines the standard I/O functions.
   */
  #include <stdio.h>
  
  /*
     Programs start with main. 
   */
  main()
  {
    printf("Hello world!\n");
  } /* main */
  

• Some important lessons:
  • C provides only one kind of comment. Comments start with /* and end with */.
  • You don't need to include return types for functions.
  • You cannot specify protection levels. Everything is public (more or less).
  • You must explicitly write the return character, which is \n.

Functions

• As you might guess, you can write new functions in C. The typical format is

  type name(parameters) {
    body
  }
  

• You need not specify the return type. If you don't specify a return type, C assumes that it is int.
• You can store functions in separate files.

Command Line

• C also gives you access to the command line.
• You declare your main method in a slightly different form.

  main(int argc, char *argv[])
  

  • argc is the number of command line arguments
  • argv is an array of the arguments
• In the exercises, you'll use these to print a greeting and to list all the arguments to a command.

From Strings to Integers

• As you know from Java, it is often useful to convert strings to integers.
• In Java, the function to use is atoi, for ``atom to integer''.

Program Input

• Just as you print output with printf, you read input with scanf.
• The typical usage for scanf is

  scanf(pattern, address-of-variable);
  

• For example, you read an integer with

  int i;
  scanf("%d", &i);
  

• Similarly (or perhaps not so similarly), you read a string with

  char str[128];
  scanf("%s", str);
  

• We'll come back to the differences in the next few days.

Basic C Exercises

Exercise: Compiling Programs

• Make a copy of the first program, preferably in a new empty directory. Save the program as hello.c.
• Try compiling the program with % gcc hello.c
  • What files are created? (Those of you who've read K&R should know.)
  • Run the executable you find.
• Try compiling the program with % gcc -c hello.c
  • What files are created?
• Try executing the command % gcc -o hello hello.o
  • What program is created?
  • Run that program
• Try executing the command % gcc -o goodbye hello.o
  • What program is created?
  • Run that program
• What does the -o flag do?
• Execute the command % hello > junk
  • What do you expect to happen?
  • What did happen?
• Remove the #include from the beginning of hello.c
  • What effect do you expect this to have?
  • Recompile to verify your expectation.

Exercise: Functions

• Add a second function, greet(char *name), to hello.c. The function should print a simple greeting.
• Update main to use the function. Use some default name, such as your own.

Exercise: Separate Compilation

• Move greet to a separate file, helloextras.c.
• Try compiling hello.c with

  % gcc hello.c
  

  • What happens?
  • Why?
• Compile hello.c wtih

  % gcc -o hello.c
  

  • What happens?
• Compile helloextras.c with

  % gcc -c helloextras.c
  

• Combine the two object files into one program with

  % gcc -o newhello hello.o helloextras.o
  

• What do you expect to happen if you try

  % gcc -o hello hello.o
  

  • Confirm your answer
• Add a main function to helloextras.c.
• Try to execute the following command

  % gcc -o newhello hello.o helloextras.o
  

  • What does the error suggest?

Exercises: Command Line

• Write a program, greet, whose behavior is given by the following examples

  % greet SamR
  Hi SamR!
  % greet Vivek
  Hi Vivek!
  

• Write a program, arguments, which prints out the arguments preceded by the argument number. Here's some sample output

  % arguments Hello "S. Rebelsky"
  0: arguments
  1: Hello
  2: S. Rebelsky
  

Exercise: String Conversion

• Write a program, factorial that computes the factorial of a value given on the command line.

Exercise: User Input

• Write a program, silly, that asks for a name and a number, and prints out some pithy saying that uses the name and the number.