Header
Files
When we divide a
program into several source files, problems arise: How can a function in one
file call a function that's defined in another file? How can a function access an external variable in another file? How can
two files share the same macro definition or type definition? The answer lies
with the #include directive, which makes it possible to share
information—function prototypes, macro definitions. type definitions, and
more—among any number of source files.
The
#include directive tells the preprocessor to open a specified file and insert
its contents into the current file. Thus, if we want several source files to
have access to the same information, we'll put that information in a file and
then use #include to bring the file's contents into each of the source files.
Files that are included in this fashion are called header files (or
sometimes include files): I'll discuss them in more detail later in this
section. By convention, header files have the extension . h.
Note: The C standard uses the term "source file" to
refer to all files written by the programmer, including both .c and .h files.
I'll use "source file" to refer to . c files only.
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