Linker error when using a struct variable - c

Question

When I try to compile the program below, I get a linker error:
/tmp/ccAikIsV.o undefined reference to uvlc_encode_blockline
I have all the structures video_controller_t .. etc declared and typedefed
int uvlc_encode_blockline(struct video_controller_t* controller, const struct vp_api_picture_t* blockline, bool_t picture_complete );
int uvlc_decode_blockline(struct video_controller_t* controller,struct vp_api_picture_t* picture, bool_t* got_image );
int uvlc_update( struct video_controller_t* controller );
int uvlc_cache( struct video_controller_t* controller,struct video_stream_t* ex_stream);
const uvlc_codec_t uvlc_codec = {
uvlc_encode_blockline,
uvlc_decode_blockline,
uvlc_update,
uvlc_cache,
{ 0 }
};
Please let me know if anyone has any suggestions.
Update
I agree that we should define the function to tell linker to get it and use it. When I did something similar, I did not get any error. I declared function pointer:
typedef C_RESULT (encode_blockline_fc)(struct video_controller_t controller,
const struct vp_api_picture_t* blockline,
bool_t picture_complete);
and used it in struct below:
typedef struct _video_codec_t
{
encode_blockline_fc encode_blockline;
decode_blockline_fc decode_blockline;
update_fc update;
cache_stream_fc cache_stream;
} video_codec_t;
Please let me know if there is anything wrong in the syntax below:
const uvlc_codec_t uvlc_codec = {
uvlc_encode_blockline,
uvlc_decode_blockline,
uvlc_update,
uvlc_cache,
{ 0 }
};
I agree that we should define the function to tell linker to get it and use it. when I did something similar, I did not get any error. I declared function pointer
typedef C_RESULT (encode_blockline_fc)( struct video_controller_t controller, const struct vp_api_picture_t* blockline, bool_t picture_complete );
and used it in struct below
typedef struct _video_codec_t { encode_blockline_fc encode_blockline; decode_blockline_fc decode_blockline; update_fc update; cache_stream_fc cache_stream; }video_codec_t
Please let me know if there is anything wrong in the syntax below:
const uvlc_codec_t uvlc_codec = {
uvlc_encode_blockline,
uvlc_decode_blockline,
uvlc_update,
uvlc_cache,
{ 0 }
};

Answer 1

The linker complains about an unresolved reference to a function, not about the struct. You should pass the linker an object file or a library that contains this function, or define it in the current compilation unit.

Answer 2

There are few possibility of Linker Error-
1- If you fail to include all of the necessary object files in the linking process.
2- If there are more than one definition for a class, function, or variable.
So check it.

Answer 3

You've told the compiler that the function uvlc_encode_blockline() exists somewhere, but when the linker goes to look for it, it can't actually find it — so it complains that it can't find it.
The declaration is enough to let the source file compile to object code. To link the program, you actually have to define the function and link the object file that contains the definition in with the object file that references it.