#ifdef #ifndef extern "c"与头文件

Remarks
The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it's defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default.

Example
 
// specifying_linkage1.cpp
int i = 1;
void other();

int main() {
   // Reference to i, defined above:
   extern int i;
}

void other() {
   // Address of global i assigned to pointer variable:
   static int *external_i = &i;

   // i will be redefined; global i no longer visible:
   // int i = 16;
}
 

In C++, when used with a string, extern specifies that the linkage conventions of another language are being used for the declarator(s). C functions and data can be accessed only if they are previously declared as having C linkage. However, they must be defined in a separately compiled translation unit.

Microsoft C++ supports the strings "C" and "C++" in the string-literal field. All of the standard include files use the extern "C" syntax to allow the run-time library functions to be used in C++ programs.

The following example shows alternative ways to declare names that have C linkage:

  Copy Code
// specifying_linkage2.cpp
// compile with: /c
// Declare printf with C linkage.
extern "C" int printf( const char *fmt, ... );

//  Cause everything in the specified header files
//   to have C linkage.
extern "C" {
   // add your #include statements here
   #include <stdio.h>
}

//  Declare the two functions ShowChar and GetChar
//   with C linkage.
extern "C" {
   char ShowChar( char ch );
   char GetChar( void );
}

//  Define the two functions ShowChar and GetChar
//   with C linkage.
extern "C" char ShowChar( char ch ) {
   putchar( ch );
   return ch;
}

extern "C" char GetChar( void ) {
   char ch;

   ch = getchar();
   return ch;
}

// Declare a global variable, errno, with C linkage.
extern "C" int errno;
 

Microsoft Specific

All Microsoft C standard include files use conditional compilation directives to detect C++ compilation. When a C++ compilation is detected, the prototypes are enclosed in an extern "C" directive as follows:

 
// Sample.h
#if defined(__cplusplus)
extern "C"
{
#endif

// Function declarations

#if defined(__cplusplus)
}
#endif
 

__cplusplus:
Predefined Macros
The compiler recognizes predefined ANSI C macros and the Microsoft C++ implementation provides several more.
the __cplusplus defined for C++ programs only.
 
头文件一般定义方式如下：
// MyHeaderFile.h
#ifndef  __MyHeaderFile__
#define  __MyHeaderFile__      // 方便管理头文件

#if defined(__cplusplus)
extern "C"
{
#endif

// Function declarations

#if defined(__cplusplus)
}
#endif
#endif  // __MyHeaderFile__