Debugging a C++ linker error

Question

First I'll give specific details about the problem I'm having, then I'll go into the interesting results I'm getting from nm when I try to debug the issue. Hopefully an nm guru will be able to interpret those results.

I've statically compiled the Casablanca C++ REST SDK to the library libcpprest.a. That works fine, but when I link against my code using the command:

/usr/bin/c++       CMakeFiles/dcodr_service.dir/dcodr_service.cc.o 
CMakeFiles/dcodr_service.dir/viterbi.cc.o CMakeFiles/dcodr_service.dir/utils.cc.o 
CMakeFiles/dcodr_service.dir/hmm.cc.o CMakeFiles/dcodr_service.dir/grammar.cc.o  
-o dcodr_service  -L/home/user/casablanca/Release/Binaries -rdynamic -lfftw3f
-lcpprest -lcommon_utilities -lboost_program_options -lboost_regex -lboost_system 
-lboost_filesystem -lboost_iostreams /opt/OpenBLAS/lib/libopenblas.a -Wl,
-rpath,/home/user/casablanca/Release/Binaries

I get the error:

CMakeFiles/dcodr_service.dir/dcodr_service.cc.o: In function 
`web::http::http_request::extract_json(bool) 
const::{lambda(unsigned long)#1}::operator()(unsigned long) const':
/home/user/casablanca/Release/include/cpprest/http_msg.h:829: 
undefined reference to `web::http::details::http_msg_base::_extract_json(bool)'

It appears that _extract_json(bool) is defined in http_msg.cpp, which also appears to be compiled when I look at the verbose output of making libcpprest.a. Now, running nm -e --demangle libcpprest.a I get the following two lines (separated by thousands of others):

00000000000015ee T web::http::details::http_msg_base::_extract_json(bool)
                 U web::http::details::http_msg_base::_extract_json(bool)

I'm new to using nm, but it appears to me that _extract_json(bool) is both in the code section and in the list of undefined symbols. Is there a better way of interpreting the nm results?

As an aside, when I compile cpprest as a dynamic library and link against it everything works fine, but for various reasons I have to create a static binary of dcodr_service.

EDIT: Following up on a comment from πάντα ῥεῖ, _extract_json(bool) is used in the following context:

        return pplx::create_task(_m_impl->_get_data_available())
.then([impl, force](utility::size64_t) { return impl->_extract_json(force); });

That is, it is used inside a lambda function. Could that cause a linker error?

I'd suspect you already took a look at [the manual](http://linux.die.net/man/1/nm)? — πάντα ῥεῖ, Sep 22 '14 at 20:15
Not sure, but could there be some inconsistencies due to inlined code? — πάντα ῥεῖ, Sep 22 '14 at 20:24
A static library is just an archive of totally independent object files. One object file has this function defined, another one references it. There is nothing unusual here. — n. m. could be an AI, Sep 22 '14 at 20:33
Is it possible a mangling error? Try "nm CMakeFiles/dcodr_service.dir/dcodr_service.cc.o | grep extract_json" and the same for libcpprest.a? — Henry Hu, Sep 22 '14 at 20:33
I would suspect you are not picking up the right `libcpprest.a` file. — jxh, Sep 22 '14 at 21:02
@n.m. Is it a problem that the reference occurs later in the library than the definition? — Dan Hook, Sep 23 '14 at 13:53

Debugging a C++ linker error

0 Answers0