Linker error calling C function from C++ despite using EXTERN "C" - c++

Question

I am using this library
http://rtmpdump.mplayerhq.hu/librtmp.3.html which is written in C.
It already uses extern "C" on all of its definitions. But still when I call a function from my [main.cpp] file,, compiler shows these errors:
[1>main.obj : error LNK2028: unresolved token (0A00000F) "extern "C" struct RTMP *
__cdecl RTMP_Alloc(void)" (?RTMP_Alloc##$$J0YAPAURTMP##XZ) referenced in function "int
__cdecl main(void)" (?main##$$HYAHXZ)]
[1>main.obj : error LNK2019: unresolved external symbol "extern "C" struct RTMP *
__cdecl RTMP_Alloc(void)" (?RTMP_Alloc##$$J0YAPAURTMP##XZ) referenced in function "int
__cdecl main(void)" (?main##$$HYAHXZ)]
I am using Visual Studio 2008. And there is no compilation error.
What am I missing?

Answer 1

Here's a scenario that fits the symptoms. Two source files and a header.
first.c
#include "first.h"
struct RTMP *RTMP_Alloc()
{
return (struct RTMP *) 0;
}
second.cpp
#include "first.h"
int main (int argc, char **argv)
{
RTMP *result;
result = RTMP_Alloc();
return 0;
}
first.h
#ifdef __cplusplus
extern "C"
#endif
struct RTMP
{
int val1;
int val2;
} ;
struct RTMP *RTMP_Alloc();
Notice that the extern "C" only applies to the struct definition. To correct it, either it needs to be inserted before the RTMP_Alloc definition, or even easier for large blocks, put inside an extern "C" { ... } construct (notice the addition of the curly braces).