User's Guide

It is the use of these features, as opposed to whether the class keyword is used rather than
struct, that introduces incompatibilities with C structs.
HP aC++ Calling HP C
Calling between C and C++ is a normal operation, since C++ is for the most part a superset of
C. You should, however, be aware of the following:
• Using the extern "C" Linkage Specification
• Differences in Argument Passing Conventions
• The main() Function
Using the extern "C" Linkage Specification
To handle overloaded function names the HP aC++ compiler generates new, unique names for all
functions declared in a C++ program. To geneate these names, the compiler uses a function-name
encoding scheme that is implementation dependent. A linkage directive tells the compiler to inhibit
this default encoding of a function name for a particular function.
To call a C function from a C++ program, you must disable the usual encoding scheme when you
declare the C function. When you do not disable the usual encoding scheme, the function name
declared in your C++ program will not match the function name in your C module defining the
function.
When the names do not match, the linker cannot resolve them. To avoid these linkage problems,
use a linkage directive when you declare the C function in the C++ program.
Syntax of extern "C"
All HP aC++ linkage directives must have either of the following formats:
extern “C” function_declaration
extern “C”
{
function_declaration1
function_declaration2
...
function_declarationN
}
Examples of extern "C"
The following declarations are equivalent:
extern “C” char* get_name(); // declare the external C module
and
extern “C”
{
char* get_name(); // declare the external C module
}
You can also use a linkage directive with all the functions in a file, as shown in the following
example. This is useful when you use C library functions in a C++ program.
extern “C”
{
#include “myclibrary.h”
}
NOTE: Do not use extern "C" when you include standard C header files. These header files
already contain extern "C" directives.
Data Compatibility between C and C++ 189