Page 140 - C-Language

Chapter 19: Declarations

Remarks

Declaration of identifier referring to object or function is often referred for short as simply a
declaration of object or function.

Examples

Calling a function from another C file

foo.h

#ifndef FOO_DOT_H /* This is an "include guard" */
#define FOO_DOT_H /* prevents the file from being included twice. */
/* Including a header file twice causes all kinds */
/* of interesting problems.*/

/**
* This is a function declaration.
* It tells the compiler that the function exists somewhere.
*/
void foo(int id, char *name);

#endif /* FOO_DOT_H */

foo.c

#include "foo.h" /* Always include the header file that declares something
* in the C file that defines it. This makes sure that the
* declaration and definition are always in-sync. Put this
* header first in foo.c to ensure the header is self-contained.
*/
#include <stdio.h>

/**
* This is the function definition.
* It is the actual body of the function which was declared elsewhere.
*/
void foo(int id, char *name)
{
fprintf(stderr, "foo(%d, \"%s\");\n", id, name);
/* This will print how foo was called to stderr - standard error.
* e.g., foo(42, "Hi!") will print `foo(42, "Hi!")`
*/
}

main.c

#include "foo.h"

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