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.
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.
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.
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
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!
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 :
#This C program starts with #include
#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")
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
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 7from the user instead of using constants. Try this program instead:
#include
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
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
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
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:
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 1several 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
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
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
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
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
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;
}
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.
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
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
#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.
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;
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
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
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
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.
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
#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
#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
#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.
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
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
#include
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
#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.
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
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.
Pointers
Pointers are used everywhere in C, so if you want to use the C language fully you have to have a very good understanding of pointers. They have to become comfortable for you. The goal of this section and the next several that follow is to help you build a complete understanding of pointers and how C uses them. For most people it takes a little time and some practice to become fully comfortable with pointers, but once you master them you are a full-fledged C programmer.
C uses pointers in three different ways:
C uses pointers to create dynamic data structures -- data structures built up from blocks of memory allocated from the heap at run-time.
C uses pointers to handle variable parameters passed to functions.
Pointers in C provide an alternative way to access information stored in arrays. Pointer techniques are especially valuable when you work with strings. There is an intimate link between arrays and pointers in C.
In some cases, C programmers also use pointers because they make the code slightly more efficient. What you will find is that, once you are completely comfortable with pointers, you tend to use them all the time.
We will start this discussion with a basic introduction to pointers and the concepts surrounding pointers, and then move on to the three techniques described above. Especially on this article, you will want to read things twice. The first time through you can learn all the concepts. The second time through you can work on binding the concepts together into an integrated whole in your mind. After you make your way through the material the second time, it will make a lot of sense.
Pointers: Why?
Imagine that you would like to create a text editor -- a program that lets you edit normal ASCII text files, like "vi" on UNIX or "Notepad" on Windows. A text editor is a fairly common thing for someone to create because, if you think about it, a text editor is probably a programmer's most commonly used piece of software. The text editor is a programmer's intimate link to the computer -- it is where you enter all of your thoughts and then manipulate them. Obviously, with anything you use that often and work with that closely, you want it to be just right. Therefore many programmers create their own editors and customize them to suit their individual working styles and preferences.
So one day you sit down to begin working on your editor. After thinking about the features you want, you begin to think about the "data structure" for your editor. That is, you begin thinking about how you will store the document you are editing in memory so that you can manipulate it in your program. What you need is a way to store the information you are entering in a form that can be manipulated quickly and easily. You believe that one way to do that is to organize the data on the basis of lines of characters. Given what we have discussed so far, the only thing you have at your disposal at this point is an array. You think, "Well, a typical line is 80 characters long, and a typical file is no more than 1,000 lines long." You therefore declare a two-dimensional array, like this:
char doc[1000][80];
This declaration requests an array of 1,000 80-character lines. This array has a total size of 80,000 characters.
As you think about your editor and its data structure some more, however, you might realize three things:
Some documents are long lists. Every line is short, but there are thousands of lines.
Some special-purpose text files have very long lines. For example, a certain data file might have lines containing 542 characters, with each character representing the amino acid pairs in segments of DNA.
In most modern editors, you can open multiple files at one time.
Let's say you set a maximum of 10 open files at once, a maximum line length of 1,000 characters and a maximum file size of 50,000 lines. Your declaration now looks like this:
char doc[50000][1000][10];
That doesn't seem like an unreasonable thing, until you pull out your calculator, multiply 50,000 by 1,000 by 10 and realize the array contains 500 million characters! Most computers today are going to have a problem with an array that size. They simply do not have the RAM, or even the virtual memory space, to support an array that large. If users were to try to run three or four copies of this program simultaneously on even the largest multi-user system, it would put a severe strain on the facilities.
Even if the computer would accept a request for such a large array, you can see that it is an extravagant waste of space. It seems strange to declare a 500 million character array when, in the vast majority of cases, you will run this editor to look at 100 line files that consume at most 4,000 or 5,000 bytes. The problem with an array is the fact that you have to declare it to have its maximum size in every dimension from the beginning. Those maximum sizes often multiply together to form very large numbers. Also, if you happen to need to be able to edit an odd file with a 2,000 character line in it, you are out of luck. There is really no way for you to predict and handle the maximum line length of a text file, because, technically, that number is infinite.
Pointers are designed to solve this problem. With pointers, you can create dynamic data structures. Instead of declaring your worst-case memory consumption up-front in an array, you instead allocate memory from the heap while the program is running. That way you can use the exact amount of memory a document needs, with no waste. In addition, when you close a document you can return the memory to the heap so that other parts of the program can use it. With pointers, memory can be recycled while the program is running.
By the way, if you read the previous discussion and one of the big questions you have is, "What IS a byte, really?," then the article How Bits and Bytes Work will help you understand the concepts, as well as things like "mega," "giga" and "tera." Go take a look and then come back.
Pointer Basics
To understand pointers, it helps to compare them to normal variables.
A "normal variable" is a location in memory that can hold a value. For example, when you declare a variable i as an integer, four bytes of memory are set aside for it. In your program, you refer to that location in memory by the name i. At the machine level that location has a memory address. The four bytes at that address are known to you, the programmer, as i, and the four bytes can hold one integer value.
A pointer is different. A pointer is a variable that points to another variable. This means that a pointer holds the memory address of another variable. Put another way, the pointer does not hold a value in the traditional sense; instead, it holds the address of another variable. A pointer "points to" that other variable by holding a copy of its address.
Because a pointer holds an address rather than a value, it has two parts. The pointer itself holds the address. That address points to a value. There is the pointer and the value pointed to. This fact can be a little confusing until you get comfortable with it, but once you get comfortable it becomes extremely powerful.
The following example code shows a typical pointer:
#include
int main()
{
int i,j;
int *p; /* a pointer to an integer */
p = &i;
*p=5;
j=i;
printf("%d %d %d\n", i, j, *p);
return 0;
}
The first declaration in this program declares two normal integer variables named i and j. The line int *p declares a pointer named p. This line asks the compiler to declare a variable p that is a pointer to an integer. The * indicates that a pointer is being declared rather than a normal variable. You can create a pointer to anything: a float, a structure, a char, and so on. Just use a * to indicate that you want a pointer rather than a normal variable.
The line p = &i; will definitely be new to you. In C, & is called the address operator. The expression &i means, "The memory address of the variable i." Thus, the expression p = &i; means, "Assign to p the address of i." Once you execute this statement, p "points to" i. Before you do so, p contains a random, unknown address, and its use will likely cause a segmentation fault or similar program crash.
One good way to visualize what is happening is to draw a picture. After i, j and p are declared, the world looks like this:
In this drawing the three variables i, j and p have been declared, but none of the three has been initialized. The two integer variables are therefore drawn as boxes containing question marks -- they could contain any value at this point in the program's execution. The pointer is drawn as a circle to distinguish it from a normal variable that holds a value, and the random arrows indicate that it can be pointing anywhere at this moment.
After the line p = &I;, p is initialized and it points to i, like this:
Once p points to i, the memory location i has two names. It is still known as i, but now it is known as *p as well. This is how C talks about the two parts of a pointer variable: p is the location holding the address, while *p is the location pointed to by that address. Therefore *p=5 means that the location pointed to by p should be set to 5, like this:
Because the location *p is also i, i also takes on the value 5. Consequently, j=i; sets j to 5, and the printf statement produces 5 5 5.
The main feature of a pointer is its two-part nature. The pointer itself holds an address. The pointer also points to a value of a specific type - the value at the address the point holds. The pointer itself, in this case, is p. The value pointed to is *p.
Pointers: Understanding Memory Addresses
The previous discussion becomes a little clearer if you understand how memory addresses work in a computer's hardware. If you have not read it already, now would be a good time to read How Bits and Bytes Work to fully understand bits, bytes and words.
All computers have memory, also known as RAM (random access memory). For example, your computer might have 16 or 32 or 64 megabytes of RAM installed right now. RAM holds the programs that your computer is currently running along with the data they are currently manipulating (their variables and data structures). Memory can be thought of simply as an array of bytes. In this array, every memory location has its own address -- the address of the first byte is 0, followed by 1, 2, 3, and so on. Memory addresses act just like the indexes of a normal array. The computer can access any address in memory at any time (hence the name "random access memory"). It can also group bytes together as it needs to to form larger variables, arrays, and structures. For example, a floating point variable consumes 4 contiguous bytes in memory. You might make the following global declaration in a program:
float f;
This statement says, "Declare a location named f that can hold one floating point value." When the program runs, the computer reserves space for the variable f somewhere in memory. That location has a fixed address in the memory space, like this:
While you think of the variable f, the computer thinks of a specific address in memory (for example, 248,440). Therefore, when you create a statement like this:
f = 3.14;
The compiler might translate that into, "Load the value 3.14 into memory location 248,440." The computer is always thinking of memory in terms of addresses and values at those addresses.
There are, by the way, several interesting side effects to the way your computer treats memory. For example, say that you include the following code in one of your programs:
int i, s[4], t[4], u=0;
for (i=0; i<=4; i++)
{
s[i] = i;
t[i] =i;
}
printf("s:t\n");
for (i=0; i<=4; i++)
printf("%d:%d\n", s[i], t[i]);
printf("u = %d\n", u);
The output that you see from the program will probably look like this:
s:t
1:5
2:2
3:3
4:4
5:5
u = 5
Why are t[0] and u incorrect? If you look carefully at the code, you can see that the for loops are writing one element past the end of each array. In memory, the arrays are placed adjacent to one another, as shown here:
Therefore, when you try to write to s[4], which does not exist, the system writes into t[0] instead because t[0] is where s[4] ought to be. When you write into t[4], you are really writing into u. As far as the computer is concerned, s[4] is simply an address, and it can write into it. As you can see however, even though the computer executes the program, it is not correct or valid. The program corrupts the array t in the process of running. If you execute the following statement, more severe consequences result:
s[1000000] = 5;
The location s[1000000] is more than likely outside of your program's memory space. In other words, you are writing into memory that your program does not own. On a system with protected memory spaces (UNIX, Windows 98/NT), this sort of statement will cause the system to terminate execution of the program. On other systems (Windows 3.1, the Mac), however, the system is not aware of what you are doing. You end up damaging the code or variables in another application. The effect of the violation can range from nothing at all to a complete system crash. In memory, i, s, t and u are all placed next to one another at specific addresses. Therefore, if you write past the boundaries of a variable, the computer will do what you say but it will end up corrupting another memory location.
Because C and C++ do not perform any sort of range checking when you access an element of an array, it is essential that you, as a programmer, pay careful attention to array ranges yourself and keep within the array's appropriate boundaries. Unintentionally reading or writing outside of array boundaries always leads to faulty program behavior.
As another example, try the following:
#include
int main()
{
int i,j;
int *p; /* a pointer to an integer */
printf("%d %d\n", p, &i);
p = &i;
printf("%d %d\n", p, &i);
return 0;
}
This code tells the compiler to print out the address held in p, along with the address of i. The variable p starts off with some crazy value or with 0. The address of i is generally a large value. For example, when I ran this code, I received the following output:
0 2147478276
2147478276 2147478276
which means that the address of i is 2147478276. Once the statement p = &i; has been executed, p contains the address of i. Try this as well:
#include
void main()
{
int *p; /* a pointer to an integer */
printf("%d\n",*p);
}
This code tells the compiler to print the value that p points to. However, p has not been initialized yet; it contains the address 0 or some random address. In most cases, a segmentation fault (or some other run-time error) results, which means that you have used a pointer that points to an invalid area of memory. Almost always, an uninitialized pointer or a bad pointer address is the cause of segmentation faults.
Having said all of this, we can now look at pointers in a whole new light. Take this program, for example:
#include
int main()
{
int i;
int *p; /* a pointer to an integer */
p = &i;
*p=5;
printf("%d %d\n", i, *p);
return 0;
}
Here is what's happening:
The variable i consumes 4 bytes of memory. The pointer p also consumes 4 bytes (on most machines in use today, a pointer consumes 4 bytes of memory. Memory addresses are 32-bits long on most CPUs today, although there is a increasing trend toward 64-bit addressing). The location of i has a specific address, in this case 248,440. The pointer p holds that address once you say p = &i;. The variables *p and i are therefore equivalent.
The pointer p literally holds the address of i. When you say something like this in a program:
printf("%d", p);
what comes out is the actual address of the variable i.
Pointers: Pointing to the Same Address
Here is a cool aspect of C: Any number of pointers can point to the same address. For example, you could declare p, q, and r as integer pointers and set all of them to point to i, as shown here:
int i;
int *p, *q, *r;
p = &i;
q = &i;
r = p;
Note that in this code, r points to the same thing that p points to, which is i. You can assign pointers to one another, and the address is copied from the right-hand side to the left-hand side during the assignment. After executing the above code, this is how things would look:
The variable i now has four names: i, *p, *q and *r. There is no limit on the number of pointers that can hold (and therefore point to) the same address.
Pointers: Common Bugs
Bug #1 - Uninitialized pointers
One of the easiest ways to create a pointer bug is to try to reference the value of a pointer even though the pointer is uninitialized and does not yet point to a valid address. For example:
int *p;
*p = 12;
The pointer p is uninitialized and points to a random location in memory when you declare it. It could be pointing into the system stack, or the global variables, or into the program's code space, or into the operating system. When you say *p=12;, the program will simply try to write a 12 to whatever random location p points to. The program may explode immediately, or may wait half an hour and then explode, or it may subtly corrupt data in another part of your program and you may never realize it. This can make this error very hard to track down. Make sure you initialize all pointers to a valid address before dereferencing them.
Bug #2 - Invalid Pointer References
An invalid pointer reference occurs when a pointer's value is referenced even though the pointer doesn't point to a valid block.
One way to create this error is to say p=q;, when q is uninitialized. The pointer p will then become uninitialized as well, and any reference to *p is an invalid pointer reference.
The only way to avoid this bug is to draw pictures of each step of the program and make sure that all pointers point somewhere. Invalid pointer references cause a program to crash inexplicably for the same reasons given in Bug #1.
Bug #3 - Zero Pointer Reference
A zero pointer reference occurs whenever a pointer pointing to zero is used in a statement that attempts to reference a block. For example, if p is a pointer to an integer, the following code is invalid:
p = 0;
*p = 12;
There is no block pointed to by p. Therefore, trying to read or write anything from or to that block is an invalid zero pointer reference. There are good, valid reasons to point a pointer to zero, as we will see in later articles. Dereferencing such a pointer, however, is invalid.
All of these bugs are fatal to a program that contains them. You must watch your code so that these bugs do not occur. The best way to do that is to draw pictures of the code's execution step by step.
Using Pointers for Function Parameters
Most C programmers first use pointers to implement something called variable parameters in functions. You have actually been using variable parameters in the scanf function -- that's why you've had to use the & (the address operator) on variables used with scanf. Now that you understand pointers you can see what has really been going on.
To understand how variable parameters work, lets see how we might go about implementing a swap function in C. To implement a swap function, what you would like to do is pass in two variables and have the function swap their values. Here's one attempt at an implementation -- enter and execute the following code and see what happens:
#include
void swap(int i, int j)
{
int t;
t=i;
i=j;
j=t;
}
void main()
{
int a,b;
a=5;
b=10;
printf("%d %d\n", a, b);
swap(a,b);
printf("%d %d\n", a, b);
}
When you execute this program, you will find that no swapping takes place. The values of a and b are passed to swap, and the swap function does swap them, but when the function returns nothing happens.
To make this function work correctly you can use pointers, as shown below:
#include
void swap(int *i, int *j)
{
int t;
t = *i;
*i = *j;
*j = t;
}
void main()
{
int a,b;
a=5;
b=10;
printf("%d %d\n",a,b);
swap(&a,&b);
printf("%d %d\n",a,b);
}
To get an idea of what this code does, print it out, draw the two integers a and b, and enter 5 and 10 in them. Now draw the two pointers i and j, along with the integer t. When swap is called, it is passed the addresses of a and b. Thus, i points to a (draw an arrow from i to a) and j points to b (draw another arrow from b to j). Once the pointers are initialized by the function call, *i is another name for a, and *j is another name for b. Now run the code in swap. When the code uses *i and *j, it really means a and b. When the function completes, a and b have been swapped.
Suppose you accidentally forget the & when the swap function is called, and that the swap line accidentally looks like this: swap(a, b);. This causes a segmentation fault. When you leave out the &, the value of a is passed instead of its address. Therefore, i points to an invalid location in memory and the system crashes when *i is used.
This is also why scanf crashes if you forget the & on variables passed to it. The scanf function is using pointers to put the value it reads back into the variable you have passed. Without the &, scanf is passed a bad address and crashes.
Variable parameters are one of the most common uses of pointers in C. Now you understand what's happening!
Dynamic Data Structures
Dynamic data structures are data structures that grow and shrink as you need them to by allocating and deallocating memory from a place called the heap. They are extremely important in C because they allow the programmer to exactly control memory consumption.
Dynamic data structures allocate blocks of memory from the heap as required, and link those blocks together into some kind of data structure using pointers. When the data structure no longer needs a block of memory, it will return the block to the heap for reuse. This recycling makes very efficient use of memory.
No comments:
Post a Comment