error LNK2019

[_Flaviu]

I have a xyz.h file where I have a function: function_new:

#ifdef __cplusplus
extern "C" {
#endif
....
void function_new(int nArg);
...
#ifdef __cplusplus
} /* closing brace for extern "C" */
#endif

I have a xyz.cpp file where is implemented function_new:

void function_new(int nArg)
{
...
}

and I get error:

1>somefile.obj : error LNK2019: unresolved external symbol _function_new referenced in function ...

and in somefile.cpp, in the top of the source code I have:

#include "xyz.h"

Why I still get link error LNK2019 ? What is wrong here ?