Question

Another unresolved external symbol __cdecl C++

Asked by: php-newbie

The fun never ends...

I've got one VC++ solution A that I compiled to a lib.
I've got an existing C program B which I'm compiling as C++ solution. I added the lib A into project B and set the lib as a dependency. When I build using functions from A I get the errors below.

In the eap_example_peer.c (project B):
#include <eap_peer/eap.h> (project A)
 
	eap_ctx.eap = eap_peer_sm_init(&eap_ctx, &eap_cb, &eap_ctx, &eap_conf);
	if (eap_ctx.eap == NULL)
		return -1;
 
NOTE: eap_peer_sm_init is in project A
 
Error Message:
eap_example_peer.obj : error LNK2019: unresolved external symbol "struct eap_sm * __cdecl eap_peer_sm_init(void *,struct eapol_callbacks *,void *,struct eap_config *)" (?eap_peer_sm_init@@YAPAUeap_sm@@PAXPAUeapol_callbacks@@0PAUeap_config@@@Z) referenced in function "int __cdecl eap_example_peer_init(void)" (?eap_example_peer_init@@YAHXZ)
eap_example_peer.obj : error LNK2019: unresolved external symbol "int __cdecl eap_peer_register_methods(void)" (?eap_peer_register_methods@@YAHXZ) referenced in function "int __cdecl eap_example_peer_init(void)" (?eap_example_peer_init@@YAHXZ)
eap_example_peer.obj : error LNK2019: unresolved external symbol "void __cdecl wpabuf_free(struct wpabuf *)" (?wpabuf_free@@YAXPAUwpabuf@@@Z) referenced in function "void __cdecl eap_example_peer_deinit(void)" (?eap_example_peer_deinit@@YAXXZ)

                                  
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Asked On
2009-09-13 at 11:41:38ID24728096
Topics

Microsoft Visual C++.Net

,

C Programming Language

Participating Experts
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Points
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Answers

 

by: ShayanOHPosted on 2009-09-13 at 12:40:04ID: 25321444

try adding  extern "C" around the include directive, like the code snippet below.
Looks to me like incompatible naming conventions.
(The names in error are using C++ conventions, but you seem to have compiled the lib using C conventions)

extern "C" {
#include <eap_peer/eap.h>
}

                                              
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by: pgnatyukPosted on 2009-09-13 at 12:45:09ID: 25321459

These 3 functions:

eap_peer_sm_init
eap_peer_register_methods
wpabuf_free

should be declared with extern "C":

extern "C"

{

struct eap_sm * eap_peer_sm_init(void *,struct eapol_callbacks *,void *,struct eap_config *);
int eap_peer_register_methods();
void wpabuf_free();

}

probably in eap_example_peer.h file (whatever you use in include in your program).

You can read about it here:

"Linking C and C++ code"  http://en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B

Or in MSDN: http://msdn.microsoft.com/en-us/library/0603949d(VS.80).aspx

Shortly, extern "C" says to a C++ compiler that the following functions are compiled in C style.

 

by: pgnatyukPosted on 2009-09-13 at 12:57:27ID: 25321500

Take for example the last error in your original post - the compiler looks for function void wpabuf_free() but it uses so strange name for it  - ?wpabuf_free@@YAXPAUwpabuf@@@Z

C++ compiler decorated the function name in this style.

C Compiler in this case does just _wpabuf_free. Just with symbol '_' in the beginning. When for C++ compiler you say extern "C" before a function prototype:

extern "C" void Nothing();

it will compile this void Nothing() in "C style" - __cdecl calling convention is used. Today it's already a deep theory. Maybe even a useless theory - who's interested now to know what is this. :(

 

by: Infinity08Posted on 2009-09-13 at 23:48:33ID: 25323250

>> I've got an existing C program B which I'm compiling as C++ solution.

As was mentioned in your other question, did you forget to rename the source file to .cpp before compiling it ? Otherwise you might THINK that it was compiled with a C++ compiler, but actually a C compiler would have been used (the compiler looks at the extension to decide what type of file it is).

 

by: cupPosted on 2009-09-14 at 04:45:33ID: 25324491

Another alternative,  under project properties, C/C++, Advanced, Compile As

Set it as "Compile as C++ Code (/TP)"

This will force all C and C++ code to compile as C++.  Alternatively, if you set /TC, everything compiles as C.

20120131-EE-VQP-002

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