298 Part IV: C Level

Search through the file for some old friends, such as printf(), to see how it’s defined and prototyped inside the header file.

Writing your own dot-H file

You have absolutely no need to write your own header file. Not now, not ever. Of course, if you dive deeply into C, you may one day write some multimodule monster and need your own custom header, one that you can proudly show your fellow C Masters and one that they will ponder and say, jealously and guardedly, “I wish I could do that.”

To get a feel for it, and because this chapter would be unduly short other­ wise, create the following header file, HEAD.H, in your editor. Type the lines exactly as written, matching lowercase for lowercase and uppercase for uppercase:

/* This is my wee li’l header file */

#define HAPPY 0x01

#define BELCH printf #define SPIT { #define SPOT }

Save the file to disk as HEAD.H. You have to type the dot-H, lest your editor save it to disk with a C, TXT, or DOC extension.

You don’t compile or “run” a header file. Instead, you use it by #include-ing it in your source code. This type of program is listed next. Granted, it may not look at all like a C program. In fact, you may just shake your head in dis­ gust and toss this book down right now. Please, I beg, give me a few moments before you run off all huffy to Mr. Software and buy a Visual Basic book:

#include <stdio.h> #include “head.h”

int main()

SPIT

BELCH(“This guy is happy: %c\n”,HAPPY); return(0);

SPOT

Start over on a new slate in your editor. Type the preceding source code exactly. The second #include brings in your header file; note how it has head.h in double quotes. That’s how the compiler finds your header file instead of looking for it with the other, traditional headers. Also, don’t let the SPIT, BELCH, SPOT stuff toss you. It’s explained later.

Chapter 23: The Stuff That Comes First 299

Save the file to disk and name it HTEST.C. Compile and run. Don’t be stunned if you find no errors. Believe it or not, everything should work out just fine. Here’s a peek at what the output should look like:

This guy is happy: J

Mr. HAPPY is happy. You may be happy too, after understanding what went on. Here’s the blow-by-blow:

The second #include (Line 2) brings into your source code the HEAD.H file you may have created earlier in this chapter. All the instructions in that file are magically included with your source code when HTEST1.C is compiled (as are the instructions from the standard I/O header file, STDIO.H).

If you recall, inside the HEAD.H header file are a few of those #define direc­ tives, which tell the compiler to substitute certain characters or C language words with happy euphemisms. (Refer to your HEAD.H file, if you need to.)

For example, the word SPIT was defined as equal to the left curly brace. So, SPIT is used in the program rather than the first curly brace in Line 5:

SPIT

The word BELCH was defined to be equal to the word printf, so it serves as a substitute for that function as well in Line 6, and in Line 8 you see the word SPOT used rather than the final curly brace.

Just about anything in a C program can wend its way into your own, personal header files. #define statements are the most popular. Also allowed are com­ ments, variable definitions, advanced things called structures, and even source code (though that’s rare). Remember that because it’s eventually copied into your source code, anything that normally would go there can also go into a header file.

Mostly, any header file you write yourself contains a lot of #defines. A doozy I wrote for one program, for example, listed all the #defines for strange key combinations on the keyboard. For example:

#define F1 0x3B00

This line allows me to use the characters F1 rather than have to remem­ ber (or constantly look up) the ugly value the computer understands as the F1 key.

Can’t remember the code for the happy face? Just #define it in your own header file and use it instead, which is what is done here.

There’s no reason to redefine C language words and functions (unless you want to goof someone up). However, the first #define in HEAD.H sets the word HAPPY equal to the value of the PC’s happy-face character,

0x01.

300 Part IV: C Level

A final warning about header files

The typical header file used in C contains lots of information. What it doesn’t contain, however, is programming code. By that, I mean that although the printf() function may be defined, prototyped, massaged, and oriented inside the STDIO.H file, the programming code that carries out printf()’s task isn’t in the header file.

I mention this because occasionally a reader writes to me and says something along the lines of “Dan! I need the DOS.H header file in order to compile an older DOS program. Can you find it for me?” My answer is that even if I could find it, the header file alone does not help. You also need the library file in order to do the job.

Library files are like object code: They contain instructions for the micro­ processor, telling it how to go about doing certain tasks. The instructions for printf() to do its thing are contained in a standard C language library file, not in the header file.

Figure 23-3 illustrates the process. First, you take your source code, named HOLLOW.C in the figure. STDIO.H is #included, so it’s combined with the source code by the compiler. The result is an object file named HOLLOW.O (or HOLLOW.OBJ).

To turn the object file into a program file, a library is added. The library con­ tains the instructions for the C language functions. These are carefully linked into your source code so that a final program, HOLLOW (or HOLLOW.EXE) is produced.

It’s the library that contains the code, not the header file! If you want to com­ pile an older DOS program, you need an older DOS library to link into your source code. Without it, the header file really means nothing.

Library files live in the LIB folder, which is usually found right next to the INCLUDE folder. For example, in Unix, the /usr/lib folder dwells right next-door to the /usr/include folder.

More information on using library files is in C All-in-One Desk Reference For Dummies (Wiley).

Source code

(gcc)

Object code

(gcc)

Figure 23-3:

How library files fit into

the big Executable file picture.

Chapter 23: The Stuff That Comes First 301

HOLLOW.C

STDIO.H

Compiler

HOLLOW.O

1010101010100

1101001111010

1001010010110

LIBC.A

1010101010100

1101001111010

1001010010110

Linker

HOLLOW

1010101010100

1101001111010

1001010010110