What is C Header Linguistically?

Question

I need to know what is meant by "Header" in C language? is it:

• an area in the code, like header area in HTML code (a title> or declaration area? or definition area?, or kind of)?
• or it is like a function (can be: function, or sub routine, or a scope, or kind of) that can be called.

Answer 1

A header is a convention generally accepted by C programmers.

It is a usually a .h file which is included into C source files which provides several benefits.

1.- Provides declaration of data types, global variables, constants and functions.

So you don't have to rewrite them time and again. And if they need being changed you just need to change it in a single file.

In example this program composed of two compliation units (two .c files) compiles and runs just fine.

// File funcs.c
#include <stdio.h>
struct Position {
  int x;
  int y;
};

void print( struct Position p ) {
  printf("%d,%d\n", p.x, p.y );
}

// File main.c
struct Position {
  int x;
  int y;
};

int main(void) {
  struct Position p;
  p.x = 1; p.y = 2;
  print(p);
}

But it is more mantainable to have the declaration for the struct Position in a header file and just #include it everywhere it is needed, like this :

// File funcs.h
#ifndef FUNCS_H
#define FUNCS_H
struct Position {
  int x;
  int y;
};
#endif // FUNCS_H

//File funcs.c
#include <stdio.h>
#include "funcs.h"

void print( struct Position p ) {
  printf("%d,%d\n", p.x, p.y );
}

//File main.c
#include "funcs.h"
int main(void) {
  struct Position p;
  p.x = 1; p.y = 2;
  print(p);

2.- Provides some type safety.

C features implicit declaration of functions. A "feature" (or rather an arguable language design mistake) which was fixed in C99.

Consider this program composed of two .c files :

//File funcs.c
#include<stdio.h>
int f(long n)
{
  n = n+1;
  printf("%ld\n", n );  
}

// File main.c
int main(void)
{
  f("a");
  return 0;
}

With gcc this program compiles without warnings or errors. But does not behave as we could reasonable expect and desire :

jose@cpu ~/t/tt $ gcc -o test *.c
jose@cpu ~/t/tt $ ./test 
4195930

Using a header file like this :

//File funcs.h
#ifndef FUNCS_H
#define FUNCS_H
int f(long n);
#endif // FUNCS_H

//File funcs.c
#include<stdio.h>
int f(long n) {
  n = n+1;
  printf("%ld\n", n );  
}

// File main.c
#include"funcs.h"
int main(void) {
  f("a");
  return 0;
}

The program still compiles and works wrong but at least we get a warning :

jose@cpu ~/t $ gcc -o test *.c
main.c: In function 'main':
main.c:5:5: warning: passing argument 1 of 'f' makes integer from pointer without a cast
   f("a");
     ^
In file included from main.c:2:0:
funcs.h:3:5: note: expected 'long int' but argument is of type 'char *'
 int f(long n);
     ^
jose@cpu ~/t $ ./test
4195930

3.- Provide a public interface while letting the implementation details remain hidden.

When you design your program it is desirable to make it modular. That is to ensure that different parts of it (modules) are as independient as possible. So that when you need to make a change to one module you need not be worried about such change affecting other modules

Headers help in doing this because you put in the header of a modules the data structures, function prototypes and constants that will be needed by the users of such module.

The implementation details go into .c files.

That is how libraries work. The API interface is specified and distributed in header files. But the API code is in .c files which don't need to be distributed. As an user of the library you just need the headers and the compiled library, not its source code.

Answer 2

What is C header Linguistically?

Referring linguistically I'd say a C header file describes the interface as provided by a translation unit, namely the accompanying C file.

"interface" by means of

• types
• constants
• (global) variables
• functions

Not all of the above need to be part of a C header.

C headers are optional from the C language's perspective, but by convention they are expected to exist.

Answer 3

Usually when talking about "headers" in C, what is meant is the files you include with the #include preprocessor directive. Those are called header files.

It could also be other things, like a comment at the top of a source (or header file) could be header. A comment before a function could be a function header.

Or when reading data files or data over the internet, many protocols have the data split into a header and the actual data (see e.g. HTTP).

Answer 4

a header is a file with the .h extension. It is meant to help the compiler to understand the symbol (method, function, struct, variable...) that are used within the code. See of it like an glossary at the end of a book. It is used solely for development purpose. It helps develloper knowing what function are available without having to look in all the implementation file. Like a man page.

A search on google? http://www.tutorialspoint.com/cprogramming/c_header_files.htm :)