ECE 264 Advanced C Programming

2009/01/14

What is computer programming? Characteristics of programs?

Computer programs are everywhere in our daily lives. Think of Google, Facebook, Amazon... cellular phone, text messaging ... What are typical functionalities of a program?

• Iterations: When you search a book, the database goes through, i.e. iterates, many books to check whether any book matches your keyword (or keywords).
• Comparison / Condition: This "checking" compares your keyword and the keywords associated with the books.
• Collection of Information: The list of books matching your search is created and displayed.

The program also has to handle networking and formatting an HTML page that is shown on your browser.  In fact, there are probably several programs, one for receiving your request, one for searching the database, and one for creating and formatting the output. 

What is C?

C was developed in early 1970s in Bell Lab for programming the UNIX operating system. At that time, OS was written primarily in Assembly. From the beginning, high performance is a major goal in C. C is one of the mostly widely used programming languages now. You can find C in high performance supercomputers as well as tiny embedded systems.  In fact, there are many variations of C. Some applications extend the functionalities of C and some others restrict C. C is different from some other types of languages, especially script languages.  In C, you need to declare a variable before using it. This is different from MATLAB and shell programming.  You will learn shell programming in ECE 364. 

A brief history of programming languages.

Advanced C Programming

This class is Advaned C Programming.  We will review some basic concepts about programming but we will move quickly to some advanced concepts. If you have never written a program in any language, this course may be too fast for you.  Please talk to the instructor as soon as possible. We have a textbook but it is actually a reference. We will cover many advanced topics that are not in the textbook. All exams are open-book and open-note. You can bring any book you want to the exam.

Type System

C has a type system to prevent programmers from doing something wrong. For example, you can write

          int a = 4; /* a is an integer */

          int b = 7;

          int c = a + b; /* c is 11. This is a comment */

You can also mix types when the mixing makes sense:

          int d = 2;

          float e = 11.05; /* e is a single-precision floating point number */

          float f = d + e; /* f is 13.05 */

However, you may not mix types when it makes no sense. For example

          Book bo; /* Book is a programmer-defined type */

          Bike bi; /* Bike is another programmer-defined type */

          int g = bo + bi; /* What does this mean? */

You need to three steps to execute a C program: compile, link, and execute. Compiler translates a C source program (a text file we can read) to an object file. Linker takes several object files and creates an executable. Finally, the program is executed. C checks types at compilation to prevent run-time type errors. We will learn more about this concept later.

First Example

Let's see a simple C program.

#include < stdio.h > 
/* remove space after < and before >. They are needed to be shown in HTML. */
int main(int argc, char * argv[])
{
  int cnt;
  for (cnt = 0; cnt < argc; cnt ++)
    {
      printf("%s\n", argv[cnt]);
    }
  return 0;
}

The first line tells C compiler (in fact, it is the "preprocessor" but we can ignore the difference for the time being) to include the standard IO library. Why do we need to do this? Because we are going to use a function that will output some information. We will talk abut this in just a moment. The program has a function called main. A C program usually has several functions. The main function is special because it is the initial point of the program. This function returns an integer value and takes two input arguments.  The first argument has the type of an integer. The second is an array of character arrays. C does not have strings; instead, each string is an array of characters.  We usually call the first argument argc and the second argv to represent the counter of arguments and the values of arguments. Each function is implemented by the code inside a pair of curly brackets. Inside the function body, we declare an integer variable called cnt to mean a counter. This counter iterates through the input arguments and prints one argument on each line. This printf function is a function provided in C library. That is the reason we need to include stdio.h at the top. This is called a header file. Finally, the program returns 0. In C, if a function completes successfully, it usually returns 0. If the function fails, it usually returns -1. In ECE 264, the return value of main may not seem important. When you do script programming in ECE 364, you may need to test whether a program terminates successfully. In that scenario, the return value becomes important.

You probably have many questions about why C is written in this way. A programming language has many rules and also many conventions. These rules are different from the rules of physics. A programming language is invented by some people and they decide the language should have these rules. They certainly have their own reasons. A programming language is similar to a natural language, such as English or Chinese. Why do you put a subject before a verb in English? Meanwhile, a programming language also has convention. For example, the input arguments of main are called argc and argv. Can you change them? Yes. You can call them dog and cat and the program does exactly the same thing. C has a collection of words that are reserved for special meanings. These are called keywords (page 77 in ABOC).  As long as you do not use the keywords, the compiler does not care whether you call it argc or dog. However, you have to be careful when you violate the convention. Your program will be more difficult to read by a person, possibly yourself as well.

Rules and convention can be good. Consider this example

Everyone knows exactly what you mean when you put a note at a particular location. These are the rules. The rules make communciation between musicians easier. Composers also follow certain convention when they write music. For example, a symphony usually has four movements. Do these rules restrict the creativity of musicians? I don't think so. Can you say Mozart not creative?

When you write a program, you have to follow certain rules and convention.

Compile and Execute C Program

How do we compile, link, and execute the program? Let's call the program ex1.c.

              gcc -Wall ex1.c -o ex1

This program has only one file so we can compile and link in one step. gcc is a C compiler available for many types of computers.  It is open-source and free of charge. -Wall means to turn on all warning messages. You should use this because warnings usually indicate errors. The file name ex1.c is provided and the output is ex1. If you do not give -o and the output name, the default output is a.out.

To execute this program, simple type

              ./ex1

here ./ means the current directory. This is important because it is possible to have another program called ex1 somewhere else on your computer. You want to execute the program at this directory. A common error among beginning programmers is to call their program as "test".  It turns out "test" is also a program provided by UNIX. If you do not use ./, you will call the wrong one (usually in /usr/bin/test).  If we execute the program by typing

              ./ex1 ece 264 spring 2009

the output is

./ex1

ece

264

spring

2009.

The program prints the arguments one-by-one, as promised, starting from the program's name ./ex1.

Course Logistics and Grading

Blackboard

Second Example

This example shows how to create and function and how to call it.

#include < stdio.h >
#include < stdlib.h >
/* remove space after < and before >. They are needed to be shown in HTML. */
int add(int a, int b)
{
  return (a + b);
}

int main(int argc, char * argv[])
{
  int val1;
  int val2;
  if (argc < 3)
    {
      fprintf(stderr, "need two numbers\n");
      return -1;
    }
  val1 = atoi(argv[1]);
  val2 = atoi(argv[2]);
  printf("%d + %d = %d\n", val1, val2, add(val1, val2));
  return 0;
}

We also include another header file called stdlib.h for standard C library. This program creates a function called add. It takes two integer arguments and returns the sum of the two arguments. The main function requires three arguments: the first one is the program's name (this is always available), the second and the third are two numbers. If the program is not given the three arguments, the program prints an error message and returns -1. When the main function returns, this program terminates.  If there are three arguments, val1 and val2 are obtained by converting the arguments from characters to integers. This is achieved by calling the atoi function. This function is provided by the standard C library. Next, we are going to print the values of val1, val2, and the sum of the two values.  Let's call this program ex2 and execute it.

               ./ex2 4 9

The output is

                 4 + 9 = 13    

We have successfully written two C programs. This is a good accomplishment in the first lecture.