Undefined reference (but nm says the function exists)

Question

I have a library from a camera manufacturer, and in the demo code provided, a function called metadata_init() works fine, in my code though, I get an undefined reference error.

The make output from the demo code:

/opt/linaro-multilib-2013.09-gcc4.8/bin/arm-linux-gnueabihf-gcc -lm -g   -L"/home/aro/Downloads"  -o hicore  demoS2.c  -lpthread -lyuvlib  -lrt

and thats it, the build succeeds fine, I will have a working ./hicore application in there.

My project is a little bit more complicated, and I compile using eclipse. The console output is:

11:33:23 **** Build of configuration Camera-R4-Debug for project Camera ****
make all 
Building file: ../src/Camera.cpp
Invoking: Cross G++ Compiler
arm-linux-gnueabihf-g++ -I/opt/Camerasdk/R4/include -I"/cameraBuilds/Wrapper/include" -I"/opt/ExternalLibraries/curl/Camera/R4/include" -I"/opt/ExternalLibraries/libxml2/Camera/R4/include" -I"/opt/ExternalLibraries/OpenCV24/Camera/R4/include" -I"/home/aro/cameraBuilds/Camera" -I/opt/InternalLibraries/include -I/opt/InternalLibraries/include_linux -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/Camera.d" -MT"src/Camera.o" -o "src/Camera.o" "../src/Camera.cpp"
../src/Camera.cpp: In function ‘int metadata_construct_http_message(char*, METADATA_HTTP_MESSAGE_TYPE, void*, int*)’:
../src/Camera.cpp:379:24: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
                        "</svg>\r\n";
                        ^
../src/Camera.cpp: In function ‘int main()’:
../src/Camera.cpp:426:97: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
  (void)metadata_init("stream.cgi", strlen("stream.cgi"), metadata_construct_http_message);
                                                                                                 ^
Finished building: ../src/Camera.cpp

Building file: ../src/CameraFrameGrabber.cpp
Invoking: Cross G++ Compiler
arm-linux-gnueabihf-g++ -I/opt/Camerasdk/R4/include -I"/cameraBuilds/Wrapper/include" -I"/opt/ExternalLibraries/curl/Camera/R4/include" -I"/opt/ExternalLibraries/libxml2/Camera/R4/include" -I"/opt/ExternalLibraries/OpenCV24/Camera/R4/include" -I"/home/aro/cameraBuilds/Camera" -I/opt/InternalLibraries/include -I/opt/InternalLibraries/include_linux -I/opt/InternalLibraries/ipslib/include -I/opt/InternalLibraries/ipsstream/include -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/CameraFrameGrabber.d" -MT"src/CameraFrameGrabber.o" -o "src/CameraFrameGrabber.o" "../src/CameraFrameGrabber.cpp"
Finished building: ../src/CameraFrameGrabber.cpp

Building file: ../src/CameraLogger.cpp
Invoking: Cross G++ Compiler
arm-linux-gnueabihf-g++ -I/opt/Camerasdk/R4/include -I"/home/aro/cameraBuilds/Wrapper/include" -I"/opt/ExternalLibraries/curl/Camera/R4/include" -I"/opt/ExternalLibraries/libxml2/Camera/R4/include" -I"/opt/ExternalLibraries/OpenCV24/Camera/R4/include" -I"/home/aro/cameraBuilds/Camera" -I/opt/InternalLibraries/include -I/opt/InternalLibraries/include_linux -I/opt/InternalLibraries/ipslib/include -I/opt/InternalLibraries/ipsstream/include -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/CameraLogger.d" -MT"src/CameraLogger.o" -o "src/CameraLogger.o" "../src/CameraLogger.cpp"
Finished building: ../src/CameraLogger.cpp

Building file: ../src/CameraParameter.cpp
Invoking: Cross G++ Compiler
arm-linux-gnueabihf-g++ -I/opt/Camerasdk/R4/include -I"/home/aro/cameraBuilds/Wrapper/include" -I"/opt/ExternalLibraries/curl/Camera/R4/include" -I"/opt/ExternalLibraries/libxml2/Camera/R4/include" -I"/opt/ExternalLibraries/OpenCV24/Camera/R4/include" -I"/home/aro/cameraBuilds/Camera" -I/opt/InternalLibraries/include -I/opt/InternalLibraries/include_linux -I/opt/InternalLibraries/ipslib/include -I/opt/InternalLibraries/ipsstream/include -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/CameraParameter.d" -MT"src/CameraParameter.o" -o "src/CameraParameter.o" "../src/CameraParameter.cpp"
Finished building: ../src/CameraParameter.cpp

Building target: Camera
Invoking: Cross G++ Linker
arm-linux-gnueabihf-g++ -L/opt/Camerasdk/R4/lib -L"/cameraBuilds/Wrapper/Camera-R4-Debug" -L"/opt/ExternalLibraries/curl/Camera/R4/lib" -L"/opt/ExternalLibraries/libxml2/Camera/R4/lib" -L"/opt/ExternalLibraries/OpenCV24/Camera/R4/lib" -L"/opt/DetectionModules/Camera/R4/lib" -L/home/aro/Downloads -o "Camera"  ./src/Camera.o ./src/CameraFrameGrabber.o ./src/CameraLogger.o ./src/CameraParameter.o   -lWrapper -lxml2 -lopencv_highgui -lopencv_imgproc -lopencv_core -lpthread -lyuvlib -lrt -llibjasper -llibjpeg -llibpng -llibtiff -lzlib -lcurl
./src/Camera.o: In function `main':
/home/aro/cameraBuilds/Camera/Camera-R4-Debug/../src/Camera.cpp:426: undefined reference to `metadata_init(char*, int, int (*)(char*, METADATA_HTTP_MESSAGE_TYPE, void*, int*))'
collect2: error: ld returned 1 exit status
make: *** [Camera] Error 1

11:33:24 Build Finished (took 1s.212ms)

The code itself is the very same thing, I just copied it over.

Metadata.h:

#ifndef _HIK_METADATA_H_
#define _HIK_METADATA_H_

const int max_http_body_len = 100 * 1024;

typedef enum
{
    CMD_ADD_TYPE = 1,
    CMD_OTHER,
} METADATA_CTRL_TYPE;

typedef struct
{
    int length;
    int fd;
    METADATA_CTRL_TYPE cmd_type;
} METADATA_HEADER;

typedef struct
{
    char option[128];
    int  share_socket;
} METADATA_ADD_CFG;

typedef enum
{
    HTTP_HEADER_TYPE = 1,
    HTTP_BODY_TYPE,
} METADATA_HTTP_MESSAGE_TYPE;

typedef struct
{
    char boundary[64];
    char http_content_type[64];
    char multipart_content_type[64];
} METADATA_MULTIPART_TYPE;


typedef int (*p_metadata_construct_http_msg_callback_f)(char *p_option, METADATA_HTTP_MESSAGE_TYPE cmd_type, void *p_data, int *p_data_len);

int metadata_init(char *p_metadata_url, int url_len, p_metadata_construct_http_msg_callback_f p_callback_f);

#endif

In Camera.cpp:

#include "Metadata.h"    
int metadata_construct_http_message(char *p_option, METADATA_HTTP_MESSAGE_TYPE cmd_type, void *p_data, int *p_data_len)
    {
        // Removed for SO
        return 0;
    }

    int main()
    {
        (void)metadata_init("stream.cgi", strlen("stream.cgi"), metadata_construct_http_message);
    ...
    }

What causes this, and how can I debug this issue to narrow down how to fix it?

And where *is* `metadata_init` defined? Where is its implementation? — Some programmer dude, Jul 12 '17 at 09:44
Also, I strongly suggest you take a closer look at those warnings. They are there for a reason. — Some programmer dude, Jul 12 '17 at 09:45
It is in the libyuvlib.a provided by the camera manufacturer. I can find it with nm, its listed there, but I can't get it to work on my project. — SinisterMJ, Jul 12 '17 at 09:45
Yeah, the warnings are all in the new demo code. I will remove them once the code is working. — SinisterMJ, Jul 12 '17 at 09:46

geza · Accepted Answer · 2017-07-12T10:05:36.960

It think the problem could be that metadata_init is a C function, but you use it from a C++ code.

In case of this, extern "C" must be used in the header file, like this:

#ifdef  __cplusplus
extern "C" {
#endif

// embed the whole contents of the header file here, I just put the function here for brevity

int metadata_init(char *p_metadata_url, int url_len, p_metadata_construct_http_msg_callback_f p_callback_f);

#ifdef  __cplusplus
}
#endif

It is because of name mangling rules. Names are mangled differently in C and C++. With extern "C", you tell the compiler that the names inside should be used with "C" mangling.

C++ has to do a complex name mangling compared to C, because it has to embed almost all signature information to a name (all parameters type), while in C, the mangled name usually is the same as the function name, or there is a _ prepended.

Yes, that did it. Thanks for the help. – SinisterMJ Jul 12 '17 at 10:00 — SinisterMJ, Jul 12 '17 at 10:00

Undefined reference (but nm says the function exists)

1 Answers1