The #include Directive
Let's now move to line 2 in the C program of Listing 2.1:
#include <stdio.h>
You see that this line starts with a pound sign, #, which is followed by include. In C, #include forms a preprocessor directive that tells the C preprocessor to look for a file and place the contents of the file in the location where the #include directive indicates.
The preprocessor is a program that does some preparations for the C compiler before your code is compiled. More details about the C preprocessor are discussed in Hour 23, "The C Preprocessor."
Also in this line, you see that <stdio.h> follows #include. You may guess that the file the #include directive asks for is something called stdio.h. You are exactly right! Here, the #include directive does ask the C preprocessor to look for and place stdio.h where the directive is in the C program.
The name of the stdio.h file stands for standard input-output header file. The stdio.h file contains numerous prototypes and macros to perform input or output (I/O) for C programs. You'll see more program I/O in Hour 5, "Reading from and Writing to Standard I/O."
NOTE
The C programming language distinguishes between lowercase and uppercase characters. In other words, C is a case-sensitive language. For instance, stdio.h and STDIO.H are different filenames in C. Likewise, main() and Main() are two different function names.
Header Files
The files that are required by the #include directive, like stdio.h, are called header files because the #include directives are almost always placed at the head of C programs. Actually, the extension name of .h does mean "header."
Besides stdio.h, there are more header files, such as stdlib.h, string.h, math.h, and so on. Appendix A, "ANSI Standard Header Files," gives a list of all the ANSI standard header files.
Angle Brackets (< >) and Double Quotes (" ")
In the second line of Listing 2.1, there are two angle brackets, < and >, that are used to surround stdio.h. You may be wondering what the angle brackets do. In C, the angle brackets ask the C preprocessor to look for a header file in a directory other than the current one.
For instance, the current directory containing the 02L01.C file is called C:\code on my computer. Therefore, the angle brackets around <stdio.h> tell the C preprocessor to look for stdio.h in a directory other than C:\code.
If you want to let the C preprocessor look into the current directory first for a header file before it starts to look elsewhere, you can use double quotes to surround the name of the header file. For instance, when the C preprocessor sees "stdio.h", it looks in the current directory, which is C:\code on my machine, first before it goes elsewhere for the stdio.h header file.
Normally, the header files are saved in a subdirectory called include. For instance, I might install a Microsoft C compiler in the directory MSVC on my hard drive, which is labeled as the C drive. Then the path to access the header files becomes C:\MSVC\include.
