Header Files (.bi)

Provides an interface for a module.

A header file is a special kind of source file that typically only contains 
preprocessor statements, defines, declarations, prototypes, constants, 
enumerations, or similar types of statements, however, a header file can 
contain any valid source code if the purpose suits.  What makes them 
different from other module (.bas) source files, is instead of being 
compiled directly, they are included by another source file (module or 
header) using the #include preprocessor directive.  All compiled libraries 
typically have one or more header files that can be included in another 
source file and will introduce to the compiler all the names of the 
procedures usable in a particular library.

FreeBASIC Header Files
   Some of the keywords, constants, and procedures documented in this 
   manual are not normally available when compiling a source code unless a 
   specific header file is included in the source first.
   * inc/fbc-int/array.bi
   * inc/datetime.bi
   * inc/dir.bi
   * inc/fbgfx.bi
   * inc/fbio.bi
   * inc/fblimits.bi
   * inc/fbprng.bi
   * inc/fbthread.bi
   * inc/file.bi
   * inc/fbc-int/symbol.bi
   * inc/crt/string.bi
   * inc/vbcompat.bi

Case Sensitivity
   Although the FreeBASIC language itself is not case-sensitive, the file 
   system on which it is running might be.  If a header file can not be 
   found, check that FreeBASIC is searching for it the correct location and 
   ensure that name of both the directory and filename of the header file 
   specified in the #include statement is using the correct upper and lower 
   case letters.

Path Separators
   FreeBASIC will automatically switch backslash ( \ ) and forward slash ( 
   / ) characters as needed for a given platform.  This allows source code 
   to be easily cross compatible.

Including a header only once
   It is common that header files need to #include other header files to 
   compile correctly.  FreeBASIC offers three methods for guarding against 
   including a header file more than once.
   * #ifndef guards in the header file
   * #include Once where the file is included
   * #pragma Once in the header file itself

#ifndef guards in the header file
   The use of #ifndef and #define is a common practice in nearly any 
   language that supports preprocessing.  The first time a file is 
   included, a unique symbol is defined.  The next time the same header 
   file is included, the definition of the symbol is checked, and if it is 
   already defined, the contents of the header file are skipped.
   '' header.bi
   #ifndef __HEADER_BI__
   #define __HEADER_BI__

   #print These statements will only be included Once,
   #print even though header.bi might be included more 
   #print than Once in the same source file.

   #endif

#include once
   At the point in the source code where the header file is included, the 
   optional "once" specifier of the #include directive can tell the 
   compiler to only include the source file one time.
   '' header.bi
   #include Once "fbgfx.bi"

   '' module.bas
   #include Once "fbgfx.bi"
   #include Once "header.bi"

#pragma once
   #pragma Once can be used in a header file to indicate that the header 
   file should only be included once.  
   '' header.bi
   #pragma Once
   #print This header will only ever be included Once per module

Version
   * Since fbc 1.10.0, symbol.bi and fblimits.bi are added.
   * Before fbc 1.10.0 (for fbc 1.08.0 and fbc 1.09.0), fbprng.bi was 
     named fbmath.bi.
   * Since fbc 1.08.0, array.bi is added.
   * Before fbc 1.08.0, the standard fbmath.bi header did not exist.

See also
   * Source Files (.bas)
   * Header Files Index

