Linkage to Non-C++ Functions

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.

Syntax

linkage-specification :

extern string-literal { declaration-list_opt }
extern string-literal declaration

declaration-list :

declaration
declaration-list declaration

Microsoft C++ supports the strings "C" and "C++" in the string-literal field. The following example shows alternative ways to declare names that have C linkage:

// Declare printf with C linkage.
extern "C" int printf( const char *fmt, ... );

//  Cause everything in the header file "cinclude.h"
//   to have C linkage.
extern "C"
{
#include <cinclude.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;