Including C Files In C++ Files ..

Including C files in C++ Code

1st method:-

If you are including a C header file that isn't provided by the system, you may need to wrap the #include line in an extern "C" { /*...*/ } construct. This tells the C++ compiler that the functions declared in the header file are C functions.
// This is C++ code

extern "C" {
// Get declaration for f(int i, char c, float x)
#include "my-C-code.h"
}

int main()
{
f(7, 'x', 3.14); // Note: nothing unusual in the call
...
}

2nd Method:-

If you are including a C header file that isn't provided by the system, and if you are able to change the C header, you should strongly consider adding the extern "C" {...} logic inside the header to make it easier for C++ users to #include it into their C++ code. Since a C compiler won't understand the extern "C" construct, you must wrap the extern "C" { and } lines in an #ifdef so they won't be seen by normal C compilers.

Step #1: Put the following lines at the very top of your C header file (note: the symbol __cplusplus is #defined if/only-if the compiler is a C++ compiler):
#ifdef __cplusplus
extern "C" {
#endif

Step #2: Put the following lines at the very bottom of your C header file:
#ifdef __cplusplus
}
#endif
Now you can #include your C header without any extern "C" nonsense in your C++ code:
// This is C++ code

// Get declaration for f(int i, char c, float x)
#include "my-C-code.h" // Note: nothing unusual in #include line

int main()
{
f(7, 'x', 3.14); // Note: nothing unusual in the call
...
}

Functions to be called in C++ code:-

If you have an individual C function that you want to call, and for some reason you don't have or don't want to #include a C header file in which that function is declared, you can declare the individual C function in your C++ code using the extern "C" syntax. Naturally you need to use the full function prototype:

extern "C" void f(int i, char c, float x);

A block of several C functions can be grouped via braces:

extern "C" {
void f(int i, char c, float x);
int g(char* s, char const* s2);
double sqrtOfSumOfSquares(double a, double b);
}

After this you simply call the function just as if it were a C++ function:
int main()
{
f(7, 'x', 3.14); // Note: nothing unusual in the call
...
}

Some Precautions:-
If a function has more than one linkage specification, they must agree; it is an error to declare functions as having both C and C++ linkage. Furthermore, if two declarations for a function occur in a program — one with a linkage specification and one without — the declaration with the linkage specification must be first. Any redundant declarations of functions that already have linkage specification are given the linkage specified in the first declaration. For example:
extern "C" int CFunc1();
...
int CFunc1(); // Redeclaration is benign; C linkage is
// retained.

int CFunc2();
...
extern "C" int CFunc2(); // Error: not the first declaration of
// CFunc2; cannot contain linkage
// specifier.