Call a C function from C++ code

Compile the C code like this:

gcc -c -o somecode.o somecode.c

Then the C++ code like this:

g++ -c -o othercode.o othercode.cpp

Then link them together, with the C++ linker:

g++ -o yourprogram somecode.o othercode.o

You also have to tell the C++ compiler a C header is coming when you include the declaration for the C function. So othercode.cpp begins with:

extern "C" {
#include "somecode.h"
}

somecode.h should contain something like:

 #ifndef SOMECODE_H_
 #define SOMECODE_H_

 void foo();

 #endif

---

(I used gcc in this example, but the principle is the same for any compiler. Build separately as C and C++, respectively, then link it together.)

Let me gather the bits and pieces from the other answers and comments, to give you an example with cleanly separated C and C++ code:

The C Part:

foo.h:

#ifndef FOO_H
#define FOO_H

void foo(void);

#endif 

foo.c

#include "foo.h"

void foo(void)
{
    /* ... */
}

Compile this with gcc -c -o foo.o foo.c.

The C++ Part:

bar.cpp

extern "C" {
  #include "foo.h" //a C header, so wrap it in extern "C" 
}

void bar() {
  foo();
}

Compile this with g++ -c -o bar.o bar.cpp

And then link it all together:

g++ -o myfoobar foo.o bar.o

Rationale: The C code should be plain C code, no #ifdefs for "maybe someday I'll call this from another language". If some C++ programmer calls your C functions, it's their problem how to do that, not yours. And if you are the C++ programmer, then the C header might not be yours and you should not change it, so the handling of unmangled function names (i.e. the extern "C") belongs in your C++ code.

You might, of course, write yourself a convenience C++ header that does nothing except wrapping the C header into an extern "C" declaration.