Keywords

Forget nouns, verbs, adjectives, and adverbs. The C language has keywords. Unlike human languages, where you need to know at least 2,000 or so words to be somewhat literate, the C language sports a scant vocabulary: Only a handful of keywords exist, and you may never use them all. Table 3-1 lists the 44 keywords of the C language.

The keywords shown in Table 3-1 represent the C language’s basic commands. These simple directions are combined in various interesting ways to do wondrous things. But the language doesn’t stop at keywords; continue reading in the next section.

• Don’t bother memorizing the list of keywords. Though I still know the 23 “to be” words in English (and in the same order as specified in my eighth grade English text), I’ve never memorized the C language keywords.
• The keywords are all case-sensitive, as shown in Table 3-1.
• Of the 44 keywords, 32 are original C language keywords. The C99 update (in 1999) added 5 more, and the more recent C11 (2011) update added 7. Most of the newer keywords begin with an underscore, as in _Alignas.

• Keywords are also known as reserved words, which means that you cannot name functions or variables the same as keywords. The compiler moans like a drunken, partisan political blogger when you attempt to do so.

Functions

Where you find only 44 keywords, there are hundreds (if not thousands) of functions in the C language, including functions you write yourself. Think of a function as a programming machine that accomplishes a task. Truly, functions are the workhorses of the C language.

The telltale sign of the function is the appearance of parentheses, as in puts() for the “put string” function, which displays text. String is programming lingo for text that’s longer than a single character.

Functions are used in several ways. For example, a beep() function may cause a computer’s speaker to beep:

beep();

Some functions are sent values, as in

puts("Greetings, human.");

Here, the string Greetings, human. (including the period) is sent to the puts() function, which sends the string to standard output (displayed on the screen). The double quotes define the string; they aren't sent to standard output. The information in the parentheses is said to be the function’s arguments, or values. They are passed to the function.

Functions can generate, or return, information as well:

value = rand();

The rand() function generates a random number, which is returned from the function and stored in the variable named value. Functions in C return only one value at a time. They can also return nothing. The function’s documentation explains what the function returns.

Functions can also be sent information and return something:

result = sqrt(256);

The sqrt() function is sent the value 256. It then calculates the square root of that value. The calculation returned is stored in the result variable.

• See the later section “Variables and values” for a discussion of what a variable is.

• A function in C must be defined before it's used. That definition is called a prototype. It’s necessary for the compiler to understand whether your code is properly using the function.

• Lists of, and documentation for, all the C language functions is found online, in what are called C library references.
• Library function prototypes are held in header files, which must be included in your source code. See the later section “Adding a function.”
• The mechanics of the functions are stored in C language libraries. A library is a collection of functions and the code that executes those functions. When you link your program, the linker incorporates the functions’ object code into the final program.
• As with keywords, function names are case-sensitive.

Operators

Mixed in with functions and keywords are various symbols collectively known as operators. Most of them are mathematic in origin, including traditional symbols like the plus (+), minus (–), and equal (=) signs.

Operators get thrown in with functions, keywords, and other parts of the C language; for example:

result = sqrt(value) + 5;

Here, the = and + operators are used to concoct some sort of mathematical mumbo jumbo.

Not all C language operators perform math. Appendix C lists the lot.

Variables and values

A program works by manipulating information stored in variables. A variable is a container into which you can stuff values, characters, or other forms of information. The program can also work on specific, unchanging values that I call immediate values:

result = sqrt(value) + 5;

In this example, result and value are variables; their content is unknown by looking at the code, and the content can change as the program runs. The number 5 is an immediate value.

C sports different types of variables, each designed to hold specific values or data types. Chapter 6 explains variables and values in more detail.

Statements and structure

As with human languages, programming languages feature syntax — it's the method by which the pieces fit together. Unlike English, where syntax can be determined by rolling dice, the method by which C puts together keywords, functions, operators, variables, and values is quite strict.

The core of the C language is the statement, which is similar to a sentence in English. A statement is an action, a direction that the program gives to the hardware. All statements end with a semicolon, the C language equivalent of a period:

beep();

Here, the single function beep() is a statement. It can be that simple. In fact, a single semicolon on a line can be a statement:

;

The preceding statement does nothing.

Statements in C are executed one after the other, beginning at the top of the source code and working down to the bottom. Ways exist to change that order as the program runs, which are presented elsewhere in this book.

The paragraph-level syntax for the C language involves blocks. These are defined by enclosing statements in a pair of curly brackets, or braces:

{
if( money < 0 ) getjob();
party();
sleep(24);
}

These three statements form a block, held within curly brackets, indicating that they belong together. They’re either part of a function or part of a loop or something similar. Regardless, they all go together and are executed one after the other.

You’ll notice that the statements held within a block are indented one tab stop. That’s a tradition in C, but it’s not required. The term whitespace is used to refer to tabs, empty lines, and other empty parts of the source code.

Generally, the C compiler ignores whitespace, looking instead for semicolons and curly brackets. For example, you can edit the source code from Listing 2-1 to read:

#include <stdio.h>
int main(){puts("Greetings, human.");return 0;}

That’s two lines of source code where before you saw several. The include directive must be on a line by itself, but the C code can be all scrunched up with no whitespace. The code compiles successfully.

Thankfully, most programmers use whitespace to make their code more readable.

• A common mistake made by beginning C language programmers is forgetting to place the semicolon after a statement. It’s also a common mistake made by experienced programmers!

• The compiler is the tool that finds missing semicolons. That’s because when you forget the semicolon, the compiler assumes that two statements are really one statement. The effect is that the compiler becomes confused and, therefore, in a fit of panic, flags those lines of source code with a warning or error.

Comments

Some items in your C language source code are parts of neither the language nor the structure. Those are comments, which can be information about the program, notes to yourself, or filthy limericks.

Traditional C comments begin with the /* characters and end with the */ characters. All text between these two markers is ignored by the compiler, shunned by the linker, and avoided in the final program.

Listing 3-1 shows an update to the code from project ex0201, where comments have been liberally added.

LISTING 3-1 Overly Commented Source Code

/* Author: Dan Gookin */
/* This program displays text on the screen */

#include <stdio.h> /* Required for puts() */

int main()
{
puts("Greetings, human."); /* Displays text */
return 0;
}

Comments in Listing 3-1 appear on a line by itself or at the end of a line. The first two lines can be combined for a multiline comment, as shown in Listing 3-2.

LISTING 3-2 Multiline Comments

/* Author: Dan Gookin
This program displays text on the screen */

#include <stdio.h> /* Required for puts() */

int main()
{
puts("Greetings, human."); /* Displays text */
return 0;
}

All text between the /* and the */ is ignored. Some source code editors, such as the one in Code::Blocks, display commented text in a unique color, which further confirms how the compiler sees and ignores the comment text. If you like, type the source code from Listing 3-2 into a text editor to see how comments work and whether they're highlighted.

A second comment style uses the double-slash (//) characters, which originated in the C++ language but is also available in C. This type of comment affects text on one line, from the // characters to the end of the line, as shown in Listing 3-3.

LISTING 3-3 Double-Slash Comments

#include <stdio.h>

int main()
{
puts("Greetings, human."); // Displays text
return 0;
}

Don’t worry about putting comments in your text at this point, unless you’re at a university somewhere and the professor is ridiculously strict about it. Comments are for you, the programmer, to help you understand your code and remember what your intentions are. They come in handy down the road, when you’re looking at your code and not fully understanding what you were doing. This situation happens frequently.

Understanding C program structure

To better understand how C programs come into being, you can create the simplest, most useless type of C program.

Exercise 3-1: Follow the steps in this section to create a new source code file, named ex0301.c. You can use Code::Blocks to create a new project or use a text editor and build the project in a terminal window at the command prompt.

Here are the specific steps:

1. Edit the source code to match Listing 3-5.

   That’s not a misprint. The source code is blank, empty. If you’re using Code::Blocks, erase the skeleton provided.

2. Save the source code.

3. Build.

   Nothing happens or you see a linker error message.

LISTING 3-5 A Simple Program That Does Nothing

Because the source code is empty, no object code is generated. Even if a program is created, it’s empty and does nothing. That’s what you told the compiler to do, and the resulting program did it well.

Setting the main() function

All C programs have a main() function. It’s the first function that’s run when a program starts. As a function, it requires parentheses for its arguments but also curly brackets to hold the function’s statements in a block, as shown in Listing 3-6.

Continue with Exercise 3-1: Rebuild the source code ex0301.c, as shown in Listing 3-6. Save the project. Build and run.

LISTING 3-6 The main() Function

main() {}

You may see a warning because the main() function lacks a data type. That’s okay. Otherwise, the program generates no output — which is great! You didn’t direct the code to do anything, and it did it well. What you see is the minimum C program. It’s also known as the dummy program.

• Main isn’t a keyword; it’s a function. It’s the required first function in all C language source code.
• Unlike other functions, main() doesn’t need to be prototyped. It does, however, use specific arguments, which is a topic covered in Chapter 15.

Returning something to the operating system

Proper protocol requires that when a program quits, it provides a value to the operating system. Call it a token of respect. That value is an integer (a whole number), usually zero, but sometimes other values are used, depending on what the program does and what the operating system expects.

Continue with Exercise 3-1: Update the source code for ex0301.c to reflect the changes shown in Listing 3-7.

LISTING 3-7 Adding the return Statement

int main()
{
return(1);
}

First, the main() function is declared to be an integer function. The int keyword tells the compiler that the function returns, or generates, an integer value.

The return statement sends the value 1 back to the operating system, effectively ending the main() function and, therefore, the program.

Continue with Exercise 3-1: Save, build, and run the project.

The results are similar to the previous run, but if you're using the Code::Blocks IDE, you see the return value of 1 specified in the summary text:

Process returned 1 (0x1)

If you like, edit the code again and change the return value to something else — say, 5. That value appears in the Code::Blocks output when you run the project.

• Traditionally, a return value of 0 is used to indicate that a program has completed its job successfully.
• Return values of 1 or greater often indicate some type of error, or perhaps they indicate the results of an operation.
• The keyword return can be used in a statement with or without parentheses. Here it is without them:

  return 1;

  In Listing 3-7, return is used with parentheses. The result is the same. I prefer to code return with parentheses, which is how it's shown throughout this book.

Adding a function

C programs should do something. Though you can use keywords and operators to have a program do marvelous things, the way to make these things useful is output. Continue working on this chapter’s example:

Continue with Exercise 3-1: Modify the project’s source code one final time to match Listing 3-8.

LISTING 3-8 More Updates for the Project

#include <stdio.h>

int main()
{
printf("4 times 5 is %d ",4*5);
return(0);
}

You’re adding three lines. First, add the include line, which brings in the printf() function’s prototype. Second, type a blank line to separate the processor directive from the main() function. Third, add the line with the printf() function. All functions must be declared before use, and the stdio.h file contains the declaration for printf().

Before proceeding, please note these two important items in your source code:

• Ensure that you type the include directive exactly as written:

  #include <stdio.h>

  This directive tells the precompiler to fetch the header file, stdio.h. The header file is required in order to use the printf() function.

• Ensure that you type the printf() statement exactly as written:

  The printf() function sends formatted text to the standard output device: the display. It also contains a math problem, 4*5. The result of that equation is calculated by the computer and then displayed in the formatted text:

  printf("4 times 5 is %d ",4*5);

  You'll find lots of important items in the printf() statement, each of which is required: quotes, comma, and semicolon. Don’t forget anything!

Later chapters cover the printf() function in more detail, so don’t worry if you’re not taking it all in at this point.

Finally, I’ve changed the return value from 1 to 0, the traditional value that’s passed back to the operating system.

Continue with Exercise 3-1: Save the project’s source code. Build and run.

If you get an error, double-check the source code. Otherwise, the result appears in the terminal window, looking something like this:

4 times 5 is 20

The basic C program is what you’ve seen presented in this section, as built upon over the past several sections. The functions you use will change, and you’ll learn how things work and become more comfortable as you explore the C language.