Undefined symbols when using C header-only implementation in C++ file -

is there rule i'm unaware of regarding how c header-only file, contains actual implementations, gets linked c++?

i have header-only c file includes full definitions of several functions. when include file in .cpp file, , compile, linker reports several of functions named in file undefined.

the error looks this:

undefined symbols architecture i386:   "par_shapes_free_mesh(par_shapes_mesh_s*)", referenced from:       app::testgeo() in app.o 

the c file has in it:

void par_shapes_free_mesh(par_shapes_mesh*); //forward declaration  void par_shapes_free_mesh(par_shapes_mesh* mesh) {     free(mesh->points);     free(mesh->triangles);     free(mesh->normals);     free(mesh->tcoords);     free(mesh); } 

clearly, function in fact defined. include file @ top of app.cpp , use par_shapes_free_mesh produces error. i'm confounded.

i tried following trick made no difference:

#ifdef __cplusplus extern "c" { #endif  //and corresponding end brace 

this had interesting effect of providing same error messages, underscore in name:

"_par_shapes_free_mesh", referenced from:

there typedef in file:

typedef struct par_shapes_mesh_s {     float* points;     int npoints;     uint16_t* triangles;     int ntriangles;     float* normals;     float* tcoords; } par_shapes_mesh; 

in order definitions included during compilation need add #define par_shapes_implementation source file before including par_shapes.h.

#define par_shapes_implementation #include "par_shapes.h"