Called C static library with everything wrapped in extern "C" and it's still not working

Question

I'm calling a C static-link library in VS2010 by wrapping the header file with extern "C", but I'm still getting a LNK2001: unresolved external symbol. I'm calling the same .lib file somewhere else with no problems and I'm reusing old code from somewhere else that definitely works. The code I'm using can be found at github.com/toots/microsoft-audio-watermarking. I'm trying to use the "thrabs" function found in hthres.h. I can't think of any obvious reasons it's not working. Here's the example code. thrabs is the c external. Is it possible my variable _fm_lookup which is normally defined in C++ is the problem?

int i;
for(i = 0; i < NFREQ; i++) 
    _fm_lookup[i] = thrabs((0.5 + i) * (float) fs * 0.5 / (float) NFREQ);

the C static lib is being included by using

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

the error is

Error   37  error LNK2001: unresolved external symbol _fm_lookup    C:\Users\David\Documents\Projects\watermarking\Builds\VisualStudio2010\internal2003.lib(hthres.obj)

Provide a [MCVE] please! (This includes to post verbatim error messages in your question) — πάντα ῥεῖ, Jun 01 '16 at 18:47
Do NOT start user-defined identifiers with an underscore ! (if your lib has a function named fm_lookup inside it ,it might not get linked in) — engf-010, Jun 01 '16 at 19:12
I removed the underscore, but I'm still getting the same error — daithioronain, Jun 01 '16 at 19:15
So you use it during compilation ,but do you actually link the library ? — engf-010, Jun 01 '16 at 19:40
I've included the internal2003.lib file as an additional dependency into the project, yes. It also shows up under the command line — daithioronain, Jun 01 '16 at 19:44
what is the extension of the sourcefile with the function thrabs( ... ) ? if it's .cpp than your function is probably not a c-function but a c++-function. — engf-010, Jun 01 '16 at 19:51
I'm calling thrabs from a .cpp file, but thrabs is declared in a .h file. I've surrounded all the declarations with extern "c" since the static-library being used was created in c. Does that makes sense? — daithioronain, Jun 01 '16 at 19:55
Did you try it without extern C around the include directive ? In regard to the c or c++ issue : many compilers determine how to compile (c or c++) from the extension used by source files ( .c == c and .cpp == c++) — engf-010, Jun 01 '16 at 20:01
after reading your description of the problem again : In hthres.c you must have a reference to an externally defined symbol _fm_lookup ! If this symbol is to be defined by the user(s) of the library then this IMHO a design-flaw better to be corrected or documented better. — engf-010, Jun 01 '16 at 20:14
try extern C around the definition of _fm_lookup ( NOTE with the _ ) — engf-010, Jun 01 '16 at 20:21
Hmm, I'm not entirely sure if I follow. I need to define somewhere _fm_lookup? Where would I define this and how would the .lib file know it's there. — daithioronain, Jun 01 '16 at 20:21
you removed the undersore from _fm_lookup ,not ? put the underscore back (for now) and put extern C in front of the definition. If it builds well then that was a/the problem.. — engf-010, Jun 01 '16 at 20:26
The compiler doesn't like that. Thanks for your help. This is painful. — daithioronain, Jun 01 '16 at 20:44
I knew what you meant. It didn't like it because I was declaring inside a class, so I moved it outside and it still doesn't work. — daithioronain, Jun 01 '16 at 20:54
I wonder is it because the name of my .h and .cpp file is different. — daithioronain, Jun 01 '16 at 21:01
Difference in filenames doesn't have influence. Has the source-file of hthres.obj a .cpp or a .c extension ? — engf-010, Jun 01 '16 at 21:08
I have no idea how to find that out since hthres is part of a .lib that I have no access to the internals of. It's 3rd party. — daithioronain, Jun 01 '16 at 21:11
Almost out of ideas. Last idea : place your algorithm in a function in a .c file and call this new c-function instead. — engf-010, Jun 01 '16 at 21:28
It worked. Couldn't go to bed without one last try!!! I've learned a lot today. Thanks! — daithioronain, Jun 01 '16 at 22:41

score 2 · Answer 1 · answered Jun 01 '16 at 22:42

2

Needed to move the offending code into its own c file and call it from there while using extern "C". Painful lesson learned.

answered Jun 01 '16 at 22:42

daithioronain

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1
6

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Congrats ! Glad to see that my last suggestion worked. – engf-010 Jun 02 '16 at 10:23
how exactly was it done? how did you call it from C++? is it possible at all? – B''H Bi'ezras -- Boruch Hashem Oct 30 '20 at 05:47

Called C static library with everything wrapped in extern "C" and it's still not working

1 Answers1