C

How C Programming Works

Marshall Brain

Browse the article How C Programming Works

Introduction to How C Programming Works
The C programming language is a popular and widely used programming language for creating computer programs. Programmers around the world embrace C because it gives maximum control and efficiency to the programmer. If you are a programmer, or if you are interested in becoming a programmer, there are a couple of benefits you gain from learning C:
·You will be able to read and write code for a large number of platforms -- everything from microcontrollers to the most advanced scientific systems can be written in C, and many modern operating systems are written in C.
·The jump to the object oriented C++ language becomes much easier. C++ is an extension of C, and it is nearly impossible to learn C++ without learning C first.

In this article, we will walk through the entire language and show you how to become a C programmer, starting at the beginning. You will be amazed at all of the different things you can create once you know C!
What is C?
C is a computer programming language. That means that you can use C to create lists of instructions for a computer to follow. C is one of thousands of programming languages currently in use. C has been around for several decades and has won widespread acceptance because it gives programmers maximum control and efficiency. C is an easy language to learn. It is a bit more cryptic in its style than some other languages, but you get beyond that fairly quickly.

C is what is called a compiled language. This means that once you write your C program, you must run it through a C compiler to turn your program into an executable that the computer can run (execute). The C program is the human-readable form, while the executable that comes out of the compiler is the machine-readable and executable form. What this means is that to write and run a C program, you must have access to a C compiler. If you are using a UNIX machine (for example, if you are writing CGI scripts in C on your host's UNIX computer, or if you are a student working on a lab's UNIX machine), the C compiler is available for free. It is called either "cc" or "gcc" and is available on the command line. If you are a student, then the school will likely provide you with a compiler -- find out what the school is using and learn about it. If you are working at home on a Windows machine, you are going to need to download a free C compiler or purchase a commercial compiler. A widely used commercial compiler is Microsoft's Visual C++ environment (it compiles both C and C++ programs). Unfortunately, this program costs several hundred dollars. If you do not have hundreds of dollars to spend on a commercial compiler, then you can use one of the free compilers available on the Web. See http://delorie.com/djgpp/ as a starting point in your search.
We will start at the beginning with an extremely simple C program and build up from there. I will assume that you are using the UNIX command line and gcc as your environment for these examples; if you are not, all of the code will still work fine -- you will simply need to understand and use whatever compiler you have available.
Let's get started!
The Simplest C Program
Let's start with the simplest possible C program and use it both to understand the basics of C and the C compilation process. Type the following program into a standard text editor (vi or emacs on UNIX, Notepad on Windows or TeachText on a Macintosh). Then save the program to a file named samp.c. If you leave off .c, you will probably get some sort of error when you compile it, so make sure you remember the .c. Also, make sure that your editor does not automatically append some extra characters (such as .txt) to the name of the file. Here's the first program:
include <stdio.h>

int main()
{
printf("This is output from my first program!\n");
return 0;
}

When executed, this program instructs the computer to print out the line "This is output from my first program!" -- then the program quits. You can't get much simpler than that!

Position
When you enter this program, position #include so that the pound sign is in column 1 (the far left side). Otherwise, the spacing and indentation can be any way you like it. On some UNIX systems, you will find a program called cb, the C Beautifier, which will format code for you. The spacing and indentation shown above is a good example to follow.

To compile this code, take the following steps:
·On a UNIX machine, type gcc samp.c -o samp (if gcc does not work, try cc). This line invokes the C compiler called gcc, asks it to compile samp.c and asks it to place the executable file it creates under the name samp. To run the program, type samp (or, on some UNIX machines, ./samp).
·On a DOS or Windows machine using DJGPP, at an MS-DOS prompt type gcc samp.c -o samp.exe. This line invokes the C compiler called gcc, asks it to compile samp.c and asks it to place the executable file it creates under the name samp.exe. To run the program, type samp.
·If you are working with some other compiler or development system, read and follow the directions for the compiler you are using to compile and execute the program.
You should see the output "This is output from my first program!" when you run the program. Here is what happened when you compiled the program:

If you mistype the program, it either will not compile or it will not run. If the program does not compile or does not run correctly, edit it again and see where you went wrong in your typing. Fix the error and try again.
The Simplest C Program: What's Happening?
Let's walk through this program and start to see what the different lines are doing (Click here to open the program in another window):
This C program starts with#include <stdio.h>
·. This line includes the "standard I/O library" into your program. The standard I/O library lets you read input from the keyboard (called "standard in"), write output to the screen (called "standard out"), process text files stored on the disk, and so on. It is an extremely useful library. C has a large number of standard libraries like stdio, including string, time and math libraries. A library is simply a package of code that someone else has written to make your life easier (we'll discuss libraries a bit later).
The line int main()
· declares the main function. Every C program must have a function named main somewhere in the code. We will learn more about functions shortly. At run time, program execution starts at the first line of the main function.
·In C, the { and } symbols mark the beginning and end of a block of code. In this case, the block of code making up the main function contains two lines.
The printf
· statement in C allows you to send output to standard out (for us, the screen). The portion in quotes is called the format string and describes how the data is to be formatted when printed. The format string can contain string literals such as "This is output from my first program!," symbols for carriage returns (\n), and operators as placeholders for variables (see below). If you are using UNIX, you can type man 3 printf to get complete documentation for the printf function. If not, see the documentation included with your compiler for details about the printf function.
The return 0;
· line causes the function to return an error code of 0 (no error) to the shell that started execution. More on this capability a bit later.
Variables
As a programmer, you will frequently want your program to "remember" a value. For example, if your program requests a value from the user, or if it calculates a value, you will want to remember it somewhere so you can use it later. The way your program remembers things is by using variables. For example:
int b;
This line says, "I want to create a space called b that is able to hold one integer value." A variable has a name (in this case, b) and a type (in this case, int, an integer). You can store a value in b by saying something like:
b = 5;
You can use the value in b by saying something like:
printf("%d", b);
In C, there are several standard types for variables:
·int - integer (whole number) values
·float - floating point values
·char - single character values (such as "m" or "Z")
We will see examples of these other types as we go along.
Printf
The printf statement allows you to send output to standard out. For us, standard out is generally the screen (although you can redirect standard out into a text file or another command).
Here is another program that will help you learn more about printf:
#include <stdio.h>

int main()
{
int a, b, c;
a = 5;
b = 7;
c = a + b;
printf("%d + %d = %d\n", a, b, c);
return 0;
}

Type this program into a file and save it as add.c. Compile it with the line gcc add.c -o add and then run it by typing add (or ./add). You will see the line "5 + 7 = 12" as output.
Here is an explanation of the different lines in this program:
·The line int a, b, c; declares three integer variables named a, b and c. Integer variables hold whole numbers.
·The next line initializes the variable named a to the value 5.
·The next line sets b to 7.
·The next line adds a and b and "assigns" the result to c.
The computer adds the value in a (5) to the value in b (7) to form the result 12, and then places that new value (12) into the variable c. The variable c is assigned the value 12. For this reason, the = in this line is called "the assignment operator."
·The printf statement then prints the line "5 + 7 = 12." The %d placeholders in the printf statement act as placeholders for values. There are three %d placeholders, and at the end of the printf line there are the three variable names: a, b and c. C matches up the first %d with a and substitutes 5 there. It matches the second %d with b and substitutes 7. It matches the third %d with c and substitutes 12. Then it prints the completed line to the screen: 5 + 7 = 12. The +, the = and the spacing are a part of the format line and get embedded automatically between the %d operators as specified by the programmer.
Printf: Reading User Values
The previous program is good, but it would be better if it read in the values 5 and 7 from the user instead of using constants. Try this program instead:
#include <stdio.h>

int main()
{
int a, b, c;
printf("Enter the first value:");
scanf("%d", &a);
printf("Enter the second value:");
scanf("%d", &b);
c = a + b;
printf("%d + %d = %d\n", a, b, c);
return 0;
}

Here's how this program works when you execute it:

Make the changes, then compile and run the program to make sure it works. Note that scanf uses the same sort of format string as printf (type man scanf for more info). Also note the & in front of a and b. This is the address operator in C: It returns the address of the variable (this will not make sense until we discuss pointers). You must use the & operator in scanf on any variable of type char, int, or float, as well as structure types (which we will get to shortly). If you leave out the & operator, you will get an error when you run the program. Try it so that you can see what that sort of run-time error looks like.
Let's look at some variations to understand printf completely. Here is the simplest printf statement:
printf("Hello");

This call to printf has a format string that tells printf to send the word "Hello" to standard out. Contrast it with this:
printf("Hello\n");

The difference between the two is that the second version sends the word "Hello" followed by a carriage return to standard out.
The following line shows how to output the value of a variable using printf.
printf("%d", b);

The%d
is a placeholder that will be replaced by the value of the variable b when the printf statement is executed. Often, you will want to embed the value within some other words. One way to accomplish that is like this:
printf("The temperature is ");
printf("%d", b);
printf(" degrees\n");

An easier way is to say this:
printf("The temperature is %d degrees\n", b);

You can also use multiple %d placeholders in one printf statement:
printf("%d + %d = %d\n", a, b, c);

In the printf statement, it is extremely important that the number of operators in the format string corresponds exactly with the number and type of the variables following it. For example, if the format string contains three %d operators, then it must be followed by exactly three parameters and they must have the same types in the same order as those specified by the operators.
You can print all of the normal C types with printf by using different placeholders:
·int (integer values) uses %d
·float (floating point values) uses %f
·char (single character values) uses %c
·character strings (arrays of characters, discussed later) use %s
You can learn more about the nuances of printf on a UNIX machine by typing man 3 printf. Any other C compiler you are using will probably come with a manual or a help file that contains a description of printf.
Scanf
The scanf function allows you to accept input from standard in, which for us is generally the keyboard. The scanf function can do a lot of different things, but it is generally unreliable unless used in the simplest ways. It is unreliable because it does not handle human errors very well. But for simple programs it is good enough and easy-to-use.
The simplest application of scanf looks like this:
scanf("%d", &b);
The program will read in an integer value that the user enters on the keyboard (%d is for integers, as is printf, so b must be declared as an int) and place that value into b.
The scanf function uses the same placeholders as printf:
·int uses %d
·float uses %f
·char uses %c
·character strings (discussed later) use %s
You MUST put & in front of the variable used in scanf. The reason why will become clear once you learn about pointers. It is easy to forget the & sign, and when you forget it your program will almost always crash when you run it.
In general, it is best to use scanf as shown here -- to read a single value from the keyboard. Use multiple calls to scanf to read multiple values. In any real program, you will use the gets or fgets functions instead to read text a line at a time. Then you will "parse" the line to read its values. The reason that you do that is so you can detect errors in the input and handle them as you see fit.
The printf and scanf functions will take a bit of practice to be completely understood, but once mastered they are extremely useful.

Try This!

·Modify this program so that it accepts three values instead of two and adds all three together:
·#include <stdio.h>
·
·int main()
·{
· int a, b, c;
· printf("Enter the first value:");
· scanf("%d", &a);
· printf("Enter the second value:");
· scanf("%d", &b);
· c = a + b;
· printf("%d + %d = %d\n", a, b, c);
· return 0;
}
·
·Try deleting or adding random characters or words in one of the previous programs and watch how the compiler reacts to these errors.
For example, delete the b variable in the first line of the above program and see what the compiler does when you forget to declare a variable. Delete a semicolon and see what happens. Leave out one of the braces. Remove one of the parentheses next to the main function. Make each error by itself and then run the program through the compiler to see what happens. By simulating errors like these, you can learn about different compiler errors, and that will make your typos easier to find when you make them for real.


C Errors to Avoid
·Using the wrong character case - Case matters in C, so you cannot type Printf or PRINTF. It must be printf.
·Forgetting to use the & in scanf
·Too many or too few parameters following the format statement in printf or scanf
·Forgetting to declare a variable name before using it

Branching and Looping
In C, both if statements and while loops rely on the idea of Boolean expressions. Here is a simple C program demonstrating an if statement:
#include <stdio.h>

int main()
{
int b;
printf("Enter a value:");
scanf("%d", &b);
if (b < 0)
printf("The value is negative\n");
return 0;
}

This program accepts a number from the user. It then tests the number using an if statement to see if it is less than 0. If it is, the program prints a message. Otherwise, the program is silent. The (b < 0) portion of the program is the Boolean expression. C evaluates this expression to decide whether or not to print the message. If the Boolean expression evaluates to True, then C executes the single line immediately following the if statement (or a block of lines within braces immediately following the if statement). If the Boolean expression is False, then C skips the line or block of lines immediately following the if statement.

Here's slightly more complex example:
#include <stdio.h>

int main()
{
int b;
printf("Enter a value:");
scanf("%d", &b);
if (b < 0)
printf("The value is negative\n");
else if (b == 0)
printf("The value is zero\n");
else
printf("The value is positive\n");
return 0;
}

In this example, the else if and else sections evaluate for zero and positive values as well.
Here is a more complicated Boolean expression:
if ((x==y) && (j>k))
z=1;
else
q=10;

This statement says, "If the value in variable x equals the value in variable y, and if the value in variable j is greater than the value in variable k, then set the variable z to 1, otherwise set the variable q to 10." You will use if statements like this throughout your C programs to make decisions. In general, most of the decisions you make will be simple ones like the first example; but on occasion, things get more complicated.
Notice that C uses == to test for equality, while it uses = to assign a value to a variable. The && in C represents a Boolean AND operation.
Here are all of the Boolean operators in C:
equality ==
less than <
Greater than >
<= <=
>= >=
inequality !=
and &&
or ||
not !

You'll find that while statements are just as easy to use as if statements. For example:
while (a < b)
{
printf("%d\n", a);
a = a + 1;
}
This causes the two lines within the braces to be executed repeatedly until a is greater than or equal to b. The while statement in general works like this:
http://static.howstuffworks.com/gif/c-while.gif

C also provides a do-while structure:
do
{
printf("%d\n", a);
a = a + 1;
}
while (a < b);

The for loop in C is simply a shorthand way of expressing a while statement. For example, suppose you have the following code in C:
x=1;
while (x<10)
{
blah blah blah
x++; /* x++ is the same as saying x=x+1 */
}

You can convert this into a for loop as follows:
for(x=1; x<10; x++)
{
blah blah blah
}

Note that the while loop contains an initialization step (x=1), a test step (x<10), and an increment step (x++). The for loop lets you put all three parts onto one line, but you can put anything into those three parts. For example, suppose you have the following loop:
a=1;
b=6;
while (a < b)
{
a++;
printf("%d\n",a);
}

You can place this into a for statement as well:
for (a=1,b=6; a < b; a++,printf("%d\n",a));

It is slightly confusing, but it is possible. The comma operator lets you separate several different statements in the initialization and increment sections of the for loop (but not in the test section). Many C programmers like to pack a lot of information into a single line of C code; but a lot of people think it makes the code harder to understand, so they break it up.

=vs.==in Boolean expressions

The == sign is a problem in C because every now and then you may forget and type just = in a Boolean expression. This is an easy mistake to make, but to the compiler there is a very important difference. C will accept either = and == in a Boolean expression -- the behavior of the program changes remarkably between the two, however.
Boolean expressions evaluate to integers in C, and integers can be used inside of Boolean expressions. The integer value 0 in C is False, while any other integer value is True. The following is legal in C:
#include <stdio.h>

int main()
{
int a;

printf("Enter a number:");
scanf("%d", &a);
if (a)
{
printf("The value is True\n");
}
return 0;
}

If a is anything other than 0, the printf statement gets executed.
In C, a statement like if (a=b) means, "Assign b to a, and then test a for its Boolean value." So if a becomes 0, the if statement is False; otherwise, it is True. The value of a changes in the process. This is not the intended behavior if you meant to type == (although this feature is useful when used correctly), so be careful with your = and == usage.

Looping: A Real Example
Let's say that you would like to create a program that prints a Fahrenheit-to-Celsius conversion table. This is easily accomplished with a for loop or a while loop:
#include <stdio.h>

int main()
{
int a;
a = 0;
while (a <= 100)
{
printf("%4d degrees F = %4d degrees C\n",
a, (a - 32) * 5 / 9);
a = a + 10;
}
return 0;
}

If you run this program, it will produce a table of values starting at 0 degrees F and ending at 100 degrees F. The output will look like this:
0 degrees F = -17 degrees C
10 degrees F = -12 degrees C
20 degrees F = -6 degrees C
30 degrees F = -1 degrees C
40 degrees F = 4 degrees C
50 degrees F = 10 degrees C
60 degrees F = 15 degrees C
70 degrees F = 21 degrees C
80 degrees F = 26 degrees C
90 degrees F = 32 degrees C
100 degrees F = 37 degrees C

The table's values are in increments of 10 degrees. You can see that you can easily change the starting, ending or increment values of the table that the program produces.
If you wanted your values to be more accurate, you could use floating point values instead:
#include <stdio.h>

int main()
{
float a;
a = 0;
while (a <= 100)
{
printf("%6.2f degrees F = %6.2f degrees C\n",
a, (a - 32.0) * 5.0 / 9.0);
a = a + 10;
}
return 0;
}

You can see that the declaration for a has been changed to a float, and the %f symbol replaces the %d symbol in the printf statement. In addition, the %f symbol has some formatting applied to it: The value will be printed with six digits preceding the decimal point and two digits following the decimal point.
Now let's say that we wanted to modify the program so that the temperature 98.6 is inserted in the table at the proper position. That is, we want the table to increment every 10 degrees, but we also want the table to include an extra line for 98.6 degrees F because that is the normal body temperature for a human being. The following program accomplishes the goal:
#include <stdio.h>

int main()
{
float a;
a = 0;
while (a <= 100)
{
if (a > 98.6)
{
printf("%6.2f degrees F = %6.2f degrees C\n",
98.6, (98.6 - 32.0) * 5.0 / 9.0);
}
printf("%6.2f degrees F = %6.2f degrees C\n",
a, (a - 32.0) * 5.0 / 9.0);
a = a + 10;
}
return 0;
}

This program works if the ending value is 100, but if you change the ending value to 200 you will find that the program has a bug. It prints the line for 98.6 degrees too many times. We can fix that problem in several different ways. Here is one way:
#include <stdio.h>

int main()
{
float a, b;
a = 0;
b = -1;
while (a <= 100)
{
if ((a > 98.6) && (b < 98.6))
{
printf("%6.2f degrees F = %6.2f degrees C\n",
98.6, (98.6 - 32.0) * 5.0 / 9.0);
}
printf("%6.2f degrees F = %6.2f degrees C\n",
a, (a - 32.0) * 5.0 / 9.0);
b = a;
a = a + 10;
}
return 0;
}

Try This!

·Try changing the Fahrenheit-to-Celsius program so that it uses scanf to accept the starting, ending and increment value for the table from the user.
·Add a heading line to the table that is produced.
·Try to find a different solution to the bug fixed by the previous example.
·Create a table that converts pounds to kilograms or miles to kilometers.


C Errors to Avoid

·Putting = when you mean == in an if or while statement
·Forgetting to increment the counter inside the while loop - If you forget to increment the counter, you get an infinite loop (the loop never ends).
·Accidentally putting a ; at the end of a for loop or if statement so that the statement has no effect - For example:
·for (x=1; x<10; x++);
· printf("%d\n",x);
only prints out one value because the semicolon after the for statement acts as the one line the for loop executes.

Arrays

In this section, we will create a small C program that generates 10 random numbers and sorts them. To do that, we will use a new variable arrangement called an array.
http://static.howstuffworks.com/gif/c-array.gif
An array lets you declare and work with a collection of values of the same type. For example, you might want to create a collection of five integers. One way to do it would be to declare five integers directly:
int a, b, c, d, e;

This is okay, but what if you needed a thousand integers? An easier way is to declare an array of five integers:
int a[5];

The five separate integers inside this array are accessed by an index. All arrays start at index zero and go to n-1 in C. Thus, int a[5]; contains five elements. For example:
int a[5];

a[0] = 12;
a[1] = 9;
a[2] = 14;
a[3] = 5;
a[4] = 1;

One of the nice things about array indexing is that you can use a loop to manipulate the index. For example, the following code initializes all of the values in the array to 0:
int a[5];
int i;

for (i=0; i<5; i++)
a[i] = 0;

The following code initializes the values in the array sequentially and then prints them out:
#include <stdio.h>

int main()
{
int a[5];
int i;

for (i=0; i<5; i++)
a[i] = i;
for (i=0; i<5; i++)
printf("a[%d] = %d\n", i, a[i]);
}

Arrays are used all the time in C. To understand a common usage, start an editor and enter the following code:
#include <stdio.h>

#define MAX 10

int a[MAX];
int rand_seed=10;

/* from K&R
- returns random number between 0 and 32767.*/
int rand()
{
rand_seed = rand_seed * 1103515245 +12345;
return (unsigned int)(rand_seed / 65536) % 32768;
}

int main()
{
int i,t,x,y;

/* fill array */
for (i=0; i < MAX; i++)
{
a[i]=rand();
printf("%d\n",a[i]);
}

/* more stuff will go here in a minute */

return 0;
}

This code contains several new concepts. The #define line declares a constant named MAX and sets it to 10. Constant names are traditionally written in all caps to make them obvious in the code. The line int a[MAX]; shows you how to declare an array of integers in C. Note that because of the position of the array's declaration, it is global to the entire program.
The line int rand_seed=10 also declares a global variable, this time named rand_seed, that is initialized to 10 each time the program begins. This value is the starting seed for the random number code that follows. In a real random number generator, the seed should initialize as a random value, such as the system time. Here, the rand function will produce the same values each time you run the program.
The line int rand() is a function declaration. The rand function accepts no parameters and returns an integer value. We will learn more about functions later. The four lines that follow implement the rand function. We will ignore them for now.
The main function is normal. Four local integers are declared, and the array is filled with 10 random values using a for loop. Note that the array a contains 10 individual integers. You point to a specific integer in the array using square brackets. So a[0] refers to the first integer in the array, a[1] refers to the second, and so on. The line starting with /* and ending with */ is called a comment. The compiler completely ignores the line. You can place notes to yourself or other programmers in comments.
Now add the following code in place of the more stuff ... comment:
/* bubble sort the array */
for (x=0; x < MAX-1; x++)
for (y=0; y < MAX-x-1; y++)
if (a[y] > a[y+1])
{
t=a[y];
a[y]=a[y+1];
a[y+1]=t;
}
/* print sorted array */
printf("--------------------\n");
for (i=0; i < MAX; i++)
printf("%d\n",a[i]);

This code sorts the random values and prints them in sorted order. Each time you run it, you will get the same values. If you would like to change the values that are sorted, change the value of rand_seed each time you run the program.
The only easy way to truly understand what this code is doing is to execute it "by hand." That is, assume MAX is 4 to make it a little more manageable, take out a sheet of paper and pretend you are the computer. Draw the array on your paper and put four random, unsorted values into the array. Execute each line of the sorting section of the code and draw out exactly what happens. You will find that, each time through the inner loop, the larger values in the array are pushed toward the bottom of the array and the smaller values bubble up toward the top.

Try This!

·In the first piece of code, try changing the for loop that fills the array to a single line of code. Make sure that the result is the same as the original code.
·Take the bubble sort code out and put it into its own function. The function header will be void bubble_sort(). Then move the variables used by the bubble sort to the function as well, and make them local there. Because the array is global, you do not need to pass parameters.
·Initialize the random number seed to different values.


C Errors to Avoid

·C has no range checking, so if you index past the end of the array, it will not tell you about it. It will eventually crash or give you garbage data.
·A function call must include () even if no parameters are passed. For example, C will accept x=rand;, but the call will not work. The memory address of the rand function will be placed into x instead. You must say x=rand();.

More on Arrays
Variable Types

There are three standard variable types in C:

·Integer: int
·Floating point: float
·Character: char
An int is a 4-byte integer value. A float is a 4-byte floating point value. A char is a 1-byte single character (like "a" or "3"). A string is declared as an array of characters.
There are a number of derivative types:
·double (8-byte floating point value)
·short (2-byte integer)
·unsigned short or unsigned int (positive integers, no sign bit)
Operators and Operator Precedence
The operators in C are similar to the operators in most languages:

+ - addition
- - subtraction
/ - division
* - multiplication
% - mod
The / operator performs integer division if both operands are integers, and performs floating point division otherwise. For example:
void main()
{
float a;
a=10/3;
printf("%f\n",a);
}

This code prints out a floating point value since a is declared as type float, but a will be 3.0 because the code performed an integer division.
Operator precedence in C is also similar to that in most other languages. Division and multiplication occur first, then addition and subtraction. The result of the calculation 5+3*4 is 17, not 32, because the * operator has higher precedence than + in C. You can use parentheses to change the normal precedence ordering: (5+3)*4 is 32. The 5+3 is evaluated first because it is in parentheses. We'll get into precedence later -- it becomes somewhat complicated in C once pointers are introduced.
Typecasting
C allows you to perform type conversions on the fly. You do this especially often when using pointers. Typecasting also occurs during the assignment operation for certain types. For example, in the code above, the integer value was automatically converted to a float.

You do typecasting in C by placing the type name in parentheses and putting it in front of the value you want to change. Thus, in the above code, replacing the line a=10/3; with a=(float)10/3; produces 3.33333 as the result because 10 is converted to a floating point value before the division.
Typedef
You declare named, user-defined types in C with the
typedef statement. The following example shows a type that appears often in C code:
#define TRUE 1
#define FALSE 0
typedef int boolean;

void main()
{
boolean b;

b=FALSE;
blah blah blah
}

This code allows you to declare Boolean types in C programs.
If you do not like the word "float'' for real numbers, you can say:

typedef float real;
and then later say:

real r1,r2,r3;
You can place typedef statements anywhere in a C program as long as they come prior to their first use in the code.
Structures
Structures in C allow you to group variable into a package. Here's an example:

struct rec
{
int a,b,c;
float d,e,f;
};

struct rec r;

As shown here, whenever you want to declare structures of the type rec, you have to say struct rec. This line is very easy to forget, and you get many compiler errors because you absent-mindedly leave out the struct. You can compress the code into the form:
struct rec
{
int a,b,c;
float d,e,f;
} r;

where the type declaration for rec and the variable r are declared in the same statement. Or you can create a typedef statement for the structure name. For example, if you do not like saying struct rec r every time you want to declare a record, you can say:
typedef struct rec rec_type;
and then declare records of type rec_type by saying:
rec_type r;
You access fields of structure using a period, for example, r.a=5;.
Arrays
You declare arrays by inserting an array size after a normal declaration, as shown below:

int a[10]; /* array of integers */
char s[100]; /* array of characters
(a C string) */
float f[20]; /* array of reals */
struct rec r[50]; /* array of records */
Incrementing
Long Way Short Way
i=i+1; i++;
i=i-1; i--;
i=i+3; i += 3;
i=i*j; i *= j;

Try This!

·Try out different pieces of code to investigate typecasting and precedence. Try out int, char, float, and so on.
·Create an array of records and write some code to sort that array on one integer field.


C Error to Avoid

·As described above, using the / operator with two integers will often produce an unexpected result, so think about it whenever you use it.

Functions
Most languages allow you to create functions of some sort. Functions let you chop up a long program into named sections so that the sections can be reused throughout the program. Functions accept parameters and return a result. C functions can accept an unlimited number of parameters. In general, C does not care in what order you put your functions in the program, so long as a the function name is known to the compiler before it is called.
We have already talked a little about functions. The rand function seen previously is about as simple as a function can get. It accepts no parameters and returns an integer result:
int rand()
/* from K&R
- produces a random number between 0 and 32767.*/
{
rand_seed = rand_seed * 1103515245 +12345;
return (unsigned int)(rand_seed / 65536) % 32768;
}

The int rand() line declares the function rand to the rest of the program and specifies that rand will accept no parameters and return an integer result. This function has no local variables, but if it needed locals, they would go right below the opening { (C allows you to declare variables after any { -- they exist until the program reaches the matching } and then they disappear. A function's local variables therefore vanish as soon as the matching } is reached in the function. While they exist, local variables live on the system stack.) Note that there is no ; after the () in the first line. If you accidentally put one in, you will get a huge cascade of error messages from the compiler that make no sense. Also note that even though there are no parameters, you must use the (). They tell the compiler that you are declaring a function rather than simply declaring an int.
The return statement is important to any function that returns a result. It specifies the value that the function will return and causes the function to exit immediately. This means that you can place multiple return statements in the function to give it multiple exit points. If you do not place a return statement in a function, the function returns when it reaches } and returns a random value (many compilers will warn you if you fail to return a specific value). In C, a function can return values of any type: int, float, char, struct, etc.
There are several correct ways to call the rand function. For example: x=rand();.
The variable x is assigned the value returned by rand in this statement. Note that you must use () in the function call, even though no parameter is passed. Otherwise, x is given the memory address of the rand function, which is generally not what you intended.
You might also call rand this way:
if (rand() > 100)

Or this way:
rand();

In the latter case, the function is called but the value returned by rand is discarded. You may never want to do this with rand, but many functions return some kind of error code through the function name, and if you are not concerned with the error code (for example, because you know that an error is impossible) you can discard it in this way.
Functions can use a void return type if you intend to return nothing. For example:
void print_header()
{
printf("Program Number 1\n");
printf("by Marshall Brain\n");
printf("Version 1.0, released 12/26/91\n");
}

This function returns no value. You can call it with the following statement:
print_header();
You must include () in the call. If you do not, the function is not called, even though it will compile correctly on many systems.
C functions can accept parameters of any type. For example:
int fact(int i)
{
int j,k;

j=1;
for (k=2; k<=i; k++)
j=j*k;
return j;
}

returns the factorial of i, which is passed in as an integer parameter. Separate multiple parameters with commas:
int add (int i, int j)
{
return i+j;
}

C has evolved over the years. You will sometimes see functions such as add written in the "old style," as shown below:
int add(i,j)
int i;
int j;
{
return i+j;
}

It is important to be able to read code written in the older style. There is no difference in the way it executes; it is just a different notation. You should use the "new style," (known as ANSI C) with the type declared as part of the parameter list, unless you know you will be shipping the code to someone who has access only to an "old style" (non-ANSI) compiler.
Functions: Function Prototypes
It is now considered good form to use function prototypes for all functions in your program. A prototype declares the function name, its parameters, and its return type to the rest of the program prior to the function's actual declaration. To understand why function prototypes are useful, enter the following code and run it:
#include <stdio.h>

void main()
{
printf("%d\n",add(3));
}

int add(int i, int j)
{
return i+j;
}

This code compiles on many compilers without giving you a warning, even though add expects two parameters but receives only one. It works because many C compilers do not check for parameter matching either in type or count. You can waste an enormous amount of time debugging code in which you are simply passing one too many or too few parameters by mistake. The above code compiles properly, but it produces the wrong answer.
To solve this problem, C lets you place function prototypes at the beginning of (actually, anywhere in) a program. If you do so, C checks the types and counts of all parameter lists. Try compiling the following:
#include <stdio.h>

int add (int,int); /* function prototype for add */

void main()
{
printf("%d\n",add(3));
}

int add(int i, int j)
{
return i+j;
}

The prototype causes the compiler to flag an error on the printf statement.
Place one prototype for each function at the beginning of your program. They can save you a great deal of debugging time, and they also solve the problem you get when you compile with functions that you use before they are declared. For example, the following code will not compile:
#include <stdio.h>

void main()
{
printf("%d\n",add(3));
}

float add(int i, int j)
{
return i+j;
}

Why, you might ask, will it compile when add returns an int but not when it returns a float? Because older C compilers default to an int return value. Using a prototype will solve this problem. "Old style" (non-ANSI) compilers allow prototypes, but the parameter list for the prototype must be empty. Old style compilers do no error checking on parameter lists.

Try This!

·Go back to the bubble sort example presented earlier and create a function for the bubble sort.
·Go back to earlier programs and create a function to get input from the user rather than taking the input in the main function.

Libraries
Libraries are very important in C because the C language supports only the most basic features that it needs. C does not even contain I/O functions to read from the keyboard and write to the screen. Anything that extends beyond the basic language must be written by a programmer. The resulting chunks of code are often placed in libraries to make them easily reusable. We have seen the standard I/O, or stdio, library already: Standard libraries exist for standard I/O, math functions, string handling, time manipulation, and so on. You can use libraries in your own programs to split up your programs into modules. This makes them easier to understand, test, and debug, and also makes it possible to reuse code from other programs that you write.
You can create your own libraries easily. As an example, we will take some code from a previous article in this series and make a library out of two of its functions. Here's the code we will start with:
#include <stdio.h>

#define MAX 10

int a[MAX];
int rand_seed=10;

int rand()
/* from K&R
- produces a random number between 0 and 32767.*/
{
rand_seed = rand_seed * 1103515245 +12345;
return (unsigned int)(rand_seed / 65536) % 32768;
}

void main()
{
int i,t,x,y;

/* fill array */
for (i=0; i < MAX; i++)
{
a[i]=rand();
printf("%d\n",a[i]);
}

/* bubble sort the array */
for (x=0; x < MAX-1; x++)
for (y=0; y < MAX-x-1; y++)
if (a[y] > a[y+1])
{
t=a[y];
a[y]=a[y+1];
a[y+1]=t;
}

/* print sorted array */
printf("--------------------\n");
for (i=0; i < MAX; i++)
printf("%d\n",a[i]);
}

This code fills an array with random numbers, sorts them using a bubble sort, and then displays the sorted list.
Take the bubble sort code, and use what you learned in the previous article to make a function from it. Since both the array a and the constant MAX are known globally, the function you create needs no parameters, nor does it need to return a result. However, you should use local variables for x, y, and t.
Once you have tested the function to make sure it is working, pass in the number of elements as a parameter rather than using MAX:
#include <stdio.h>

#define MAX 10

int a[MAX];
int rand_seed=10;

/* from K&R
- returns random number between 0 and 32767.*/
int rand()
{
rand_seed = rand_seed * 1103515245 +12345;
return (unsigned int)(rand_seed / 65536) % 32768;
}

void bubble_sort(int m)
{
int x,y,t;
for (x=0; x < m-1; x++)
for (y=0; y < m-x-1; y++)
if (a[y] > a[y+1])
{
t=a[y];
a[y]=a[y+1];
a[y+1]=t;
}
}

void main()
{
int i,t,x,y;
/* fill array */
for (i=0; i < MAX; i++)
{
a[i]=rand();
printf("%d\n",a[i]);
}
bubble_sort(MAX);
/* print sorted array */
printf("--------------------\n");
for (i=0; i < MAX; i++)
printf("%d\n",a[i]);
}

You can also generalize the bubble_sort function even more by passing in a as a parameter:
bubble_sort(int m, int a[])
This line says, "Accept the integer array a of any size as a parameter." Nothing in the body of the bubble_sort function needs to change. To call bubble_sort, change the call to:
bubble_sort(MAX, a);
Note that &a has not been used in the function call even though the sort will change a. The reason for this will become clear once you understand pointers.
Making a Library
Since the rand and bubble_sort functions in the previous program are useful, you will probably want to reuse them in other programs you write. You can put them into a utility library to make their reuse easier.
Every library consists of two parts: a header file and the actual code file. The header file, normally denoted by a .h suffix, contains information about the library that programs using it need to know. In general, the header file contains constants and types, along with prototypes for functions available in the library. Enter the following header file and save it to a file named util.h.
/* util.h */
extern int rand();
extern void bubble_sort(int, int []);
These two lines are function prototypes. The word "extern" in C represents functions that will be linked in later. If you are using an old-style compiler, remove the parameters from the parameter list of bubble_sort.
Enter the following code into a file named util.c.
/* util.c */
#include "util.h"

int rand_seed=10;

/* from K&R
- produces a random number between 0 and 32767.*/
int rand()
{
rand_seed = rand_seed * 1103515245 +12345;
return (unsigned int)(rand_seed / 65536) % 32768;
}

void bubble_sort(int m,int a[])
{
int x,y,t;
for (x=0; x < m-1; x++)
for (y=0; y < m-x-1; y++)
if (a[y] > a[y+1])
{
t=a[y];
a[y]=a[y+1];
a[y+1]=t;
}
}

Note that the file includes its own header file (util.h) and that it uses quotes instead of the symbols < and> , which are used only for system libraries. As you can see, this looks like normal C code. Note that the variable rand_seed, because it is not in the header file, cannot be seen or modified by a program using this library. This is called information hiding. Adding the word static in front of int enforces the hiding completely.
Enter the following main program in a file named main.c.
#include <stdio.h>
#include "util.h"

#define MAX 10

int a[MAX];

void main()
{
int i,t,x,y;
/* fill array */
for (i=0; i < MAX; i++)
{
a[i]=rand();
printf("%d\n",a[i]);
}

bubble_sort(MAX,a);

/* print sorted array */
printf("--------------------\n");
for (i=0; i < MAX; i++)
printf("%d\n",a[i]);
}

This code includes the utility library. The main benefit of using a library is that the code in the main program is much shorter.
Compiling and Running with a Library
To compile the library, type the following at the command line (assuming you are using UNIX) (replace gcc with cc if your system uses cc):

gcc -c -g util.c

The -c causes the compiler to produce an object file for the library. The object file contains the library's machine code. It cannot be executed until it is linked to a program file that contains a main function. The machine code resides in a separate file named util.o.
To compile the main program, type the following:
gcc -c -g main.c

This line creates a file named main.o that contains the machine code for the main program. To create the final executable that contains the machine code for the entire program, link the two object files by typing the following:
gcc -o main main.o util.o

This links main.o and util.o to form an executable named main. To run it, type main.
Makefiles make working with libraries a bit easier. You'll find out about makefiles on the next page.
Makefiles
It can be cumbersome to type all of the gcc lines over and over again, especially if you are making a lot of changes to the code and it has several libraries. The make facility solves this problem. You can use the following makefile to replace the compilation sequence above:
main: main.o util.o
gcc -o main main.o util.o
main.o: main.c util.h
gcc -c -g main.c
util.o: util.c util.h
gcc -c -g util.c

Enter this into a file named makefile, and type maketo build the executable. Note that you must precede all gcc lines with a tab. (Eight spaces will not suffice -- it must be a tab. All other lines must be flush left.)
This makefile contains two types of lines. The lines appearing flush left are dependency lines. The lines preceded by a tab are executable lines, which can contain any valid UNIX command. A dependency line says that some file is dependent on some other set of files. For example, main.o: main.c util.h says that the file main.o is dependent on the files main.c and util.h. If either of these two files changes, the following executable line(s) should be executed to recreate main.o.
Note that the final executable produced by the whole makefile is main, on line 1 in the makefile. The final result of the makefile should always go on line 1, which in this makefile says that the file main is dependent on main.o and util.o. If either of these changes, execute the line gcc -o main main.o util.o to recreate main.
It is possible to put multiple lines to be executed below a dependency line -- they must all start with a tab. A large program may have several libraries and a main program. The makefile automatically recompiles everything that needs to be recompiled because of a change.
If you are not working on a UNIX machine, your compiler almost certainly has functionality equivalent to makefiles. Read the documentation for your compiler to learn how to use it.
Now you understand why you have been including stdio.h in earlier programs. It is simply a standard library that someone created long ago and made available to other programmers to make their lives easier.
Text Files
Text files in C are straightforward and easy to understand. All text file functions and types in C come from the stdio library.
When you need text I/O in a C program, and you need only one source for input information and one sink for output information, you can rely on stdin (standard in) and stdout (standard out). You can then use input and output redirection at the command line to move different information streams through the program. There are six different I/O commands in <stdio.h> that you can use with stdin and stdout:
·printf - prints formatted output to stdout
·scanf - reads formatted input from stdin
·puts - prints a string to stdout
·gets - reads a string from stdin
·putc - prints a character to stdout
·getc, getchar - reads a character from stdin
The advantage of stdin and stdout is that they are easy to use. Likewise, the ability to redirect I/O is very powerful. For example, maybe you want to create a program that reads from stdin and counts the number of characters:
#include <stdio.h>
#include <string.h>

void main()
{
char s[1000];
int count=0;
while (gets(s))
count += strlen(s);
printf("%d\n",count);
}

Enter this code and run it. It waits for input from stdin, so type a few lines. When you are done, press CTRL-D to signal end-of-file (eof). The gets function reads a line until it detects eof, then returns a 0 so that the while loop ends. When you press CTRL-D, you see a count of the number of characters in stdout (the screen). (Use man gets or your compiler's documentation to learn more about the gets function.)
Now, suppose you want to count the characters in a file. If you compiled the program to an executable named xxx, you can type the following:
xxx < filename

Instead of accepting input from the keyboard, the contents of the file named filename will be used instead. You can achieve the same result using pipes:
cat < filename | xxx

You can also redirect the output to a file:
xxx < filename > out

This command places the character count produced by the program in a text file named out.
Sometimes, you need to use a text file directly. For example, you might need to open a specific file and read from or write to it. You might want to manage several streams of input or output or create a program like a text editor that can save and recall data or configuration files on command. In that case, use the text file functions in stdio:
·fopen - opens a text file
·fclose - closes a text file
·feof - detects end-of-file marker in a file
·fprintf - prints formatted output to a file
·fscanf - reads formatted input from a file
·fputs - prints a string to a file
·fgets - reads a string from a file
·fputc - prints a character to a file
·fgetc - reads a character from a file
Text Files: Opening
You use fopen to open a file. It opens a file for a specified mode (the three most common are r, w, and a, for read, write, and append). It then returns a file pointer that you use to access the file. For example, suppose you want to open a file and write the numbers 1 to 10 in it. You could use the following code:
#include <stdio.h>
#define MAX 10

int main()
{
FILE *f;
int x;
f=fopen("out","w");
if (!f)
return 1;
for(x=1; x<=MAX; x++)
fprintf(f,"%d\n",x);
fclose(f);
return 0;
}

The fopen statement here opens a file named out with the w mode. This is a destructive write mode, which means that if out does not exist it is created, but if it does exist it is destroyed and a new file is created in its place. The fopen command returns a pointer to the file, which is stored in the variable f. This variable is used to refer to the file. If the file cannot be opened for some reason, f will contain NULL.

Main Function Return Values

This program is the first program in this series that returns an error value from the main program. If the fopen command fails, f will contain a NULL value (a zero). We test for that error with the if statement. The if statement looks at the True/False value of the variable f. Remember that in C, 0 is False and anything else is true. So if there were an error opening the file, f would contain zero, which is False. The ! is the NOT operator. It inverts a Boolean value. So the if statement could have been written like this:
if (f == 0)
That is equivalent. However, if (!f) is more common.
If there is a file error, we return a 1 from the main function. In UNIX, you can actually test for this value on the command line. See the shell documentation for details.

The fprintf statement should look very familiar: It is just like printf but uses the file pointer as its first parameter. The fclose statement closes the file when you are done.
Text Files: Reading
To read a file, open it with r mode. In general, it is not a good idea to use fscanf for reading: Unless the file is perfectly formatted, fscanf will not handle it correctly. Instead, use fgets to read in each line and then parse out the pieces you need.
The following code demonstrates the process of reading a file and dumping its contents to the screen:
#include <stdio.h>

int main()
{
FILE *f;
char s[1000];

f=fopen("infile","r");
if (!f)
return 1;
while (fgets(s,1000,f)!=NULL)
printf("%s",s);
fclose(f);
return 0;
}

The fgets statement returns a NULL value at the end-of-file marker. It reads a line (up to 1,000 characters in this case) and then prints it to stdout. Notice that the printf statement does not include \n in the format string, because fgets adds \n to the end of each line it reads. Thus, you can tell if a line is not complete in the event that it overflows the maximum line length specified in the second parameter to fgets.

C Errors to Avoid

·Do not accidentally type close instead of fclose. The close function exists, so the compiler accepts it. It will even appear to work if the program only opens or closes a few files. However, if the program opens and closes a file in a loop, it will eventually run out of available file handles and/or memory space and crash, because close is not closing the files correctly.