How do I mix C and C++ source code?

There are two ways to combine C and C++ code:

Separate C and C++ source and header files.

Include C header files in your C++ code by wrapping them in an extern "C" block. The C++ compiler will automatically take care of the name mangling and calling conventions differences between C and C++. The functions and variables from the C source code can be used in the C++ code.
Compile the C and C++ source files separately.
Link all object files together.

Example

c_hdr.h: Declaration of c_foo C function
c_src.c: Definition of c_foo C function
main.cpp

extern "C" {
    // Include C header file
    #include "c_hdr.h"
}

int main() {
    // Call the C function from the C++ code
    c_foo(42);
    return 0;
}

C functions/variables in the C++ source files

Declare the C functions/variables in an extern "C" block. The functions declared within it will use the C linkage, which means they will have C naming and calling conventions and will be callable/accessible from C++ code.

Example

extern "C" {
    // Declaration of a C function
    void c_foo(int arg);
}

// Definition of the C function
void c_foo(int arg) {
    // Do not use arg parameter
    (void)arg;
}

int main() {
    // Call the C function from C++ code
    c_foo(42);
    return 0;
}