Suppose you need a type available in two separate translation units:

struct point {
  int x, y;
};

Copying such a declaration manually into the two corresponding source files is not only tedious, but also error-prone, and doubly so if you decide later to alter it.

Instead, shared declarations should be placed in headers, usually text files with a suffix .h. For example:

// in point.h
struct point {
  int x, y;
};

This declaration is now available in any source file where it is needed, simply by using the directive #include:

#include "point.h"

double distance(const struct point *p)
{
  return sqrt(p->x * p->x + p->y * p->y);
}

The technique works equally well with function prototypes, in-line function declarations and macro definitions that need to be shared across source files. Related types, functions and macros are usually grouped together in a single header, rather than defining a header for each one. Extending our example, we could declare distance together with the type it operates on:

// in point.h

struct point {
  int x, y;
};

double distance(const struct point *p);

A header can also be included using this form:

#include <something.h>

This usually tells the preprocessor to look for the header in a preconfigured location, such as where external libraries are stored, rather than within the same directory as the source files. C provides a Standard Library, which is exposed through its Standard headers, which should always be included using the form <>.

An #include directive matches the grammar of control-line:

control-line

# include pp-tokens new-line

header-name

< h-char-sequence >

" q-char-sequence "

h-char-sequence

h-char

h-char-sequence h-char

q-char-sequence

q-char

q-char-sequence q-char

h-char

any member of the source character set except > and the new-line character

q-char

any member of the source character set except " and the new-line character

Its pp-tokens are subject to macro expansion, and the result must match header-name.

// in point.h
#ifndef POINT_HEADER_INCLUDED
#define POINT_HEADER_INCLUDED

struct point {
  int x, y;
};

double distance(const struct point *p);

#endif