Question

I normally wouldn’t do this, but the project I’m currently working on requires a couple functions that are in a c source file.

extern "C" {
    int words(char sentence[]);
    int match(char str[], char sentence[], int n);
}

I simply want to know, where is the best place to add these prototypes to the linking functions in c?

Should it be added to the c++ source file (in my case, command.cpp), or the c/c++ header? (command.h)

Answer 1

The least error-prone way is to put the declarations in a shared C & C++ header file, and #ifdef the extern "C" { to only be used by the C++ compiler (it’s a syntax error in C):

#ifdef __cplusplus
extern "C" {
#endif
  int words(char sentence[]);
  int match(char str[], char sentence[], int n);
#ifdef __cplusplus
}
#endif

I personally also like this variant of pre-#defining a macro for the unscoped extern syntax:

#ifdef __cplusplus
#define CFUN extern "C"
#else
#define CFUN
#endif

CFUN int words(char sentence[]);
CFUN int match(char str[], char sentence[], int n);

This keeps the messy #ifdef stuff localised. You can place it in one of your project-wide header files and use the CFUN specifier anywhere. You may want to prefix the name of the CFUN macro if it’s likely to collide with an existing definition.