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You need to use extern "C" in C++ when declaring a function that was implemented/compiled in C. The use of extern "C" tells the compiler/linker to use the C naming and calling conventions, instead of the C++ name mangling and C++ calling conventions that would be used otherwise. For functions provided by other libraries, you will almost never need to use extern "C", as well-written libraries will already have this in there for the public APIs that it exports to both C and C++. If, however, you write a library that you want to make available both in C and in C++, then you will have to conditionally put that in your headers.

1. If you have a function implemented in C and want to call it from C++.
1.1). if you can modify C header files
Typically the declarations in a C header file are surrounded with

#ifdef __cplusplus
extern "C" {
#endif
 
[... C declarations ...]
 
#ifdef __cplusplus
}
#endif
to make it usable from C++.

1.2). if you cannot modify C header files

include the header files in C++ source files
extern "C" {
/* include c library header */
}

or wrap the necessary c headers in a seperate header file then include it
#ifdef __cplusplus
extern "C" {
#endif
 
/* include c library header */
 
#ifdef __cplusplus
} // extern
#endif

1.3). If you only want to call several C functions, 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.

extern "C" {
   [... function declarations...]
 }
After this you simply call the function just as if it were a C++ function

2. If you have a function implemented in C++ and want to call it from C.

 // This is C++ code
 
 // Declare  using extern "C":
 extern "C" void cplusplus_func_name(paramlist);
  ...
  // Define function in some C++ module:
 void cplusplus_func_name(paramlist)
 {
   ...
 }

The extern "C" line tells the compiler that the external information sent to the linker should use C calling conventions and name mangling (e.g., preceded by a single underscore). Since name overloading isn't supported by C, you can't make several overloaded functions simultaneously callable by a C program.