未定义的引用（但 nm 表示该函数存在）[重复]

Posted 2023-02-22

【中文标题】未定义的引用（但 nm 表示该函数存在）[重复]

【英文标题】：Undefined reference (but nm says the function exists) [duplicate]

【发布时间】：2017-07-12 09:42:17

【问题描述】：

我有一个相机制造商提供的库，在提供的演示代码中，一个名为 metadata_init() 的函数工作正常，但在我的代码中，我得到一个未定义的引用错误。

演示代码的 make 输出：

/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

就是这样，构建成功了，我将有一个工作的 ./hicore 应用程序。

我的项目稍微复杂一点，我用eclipse编译。 控制台输出为：

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)

代码本身是一样的，我只是复制过来的。

元数据.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

在 Camera.cpp 中：

#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);
    ...
    

是什么原因造成的，我该如何调试这个问题来缩小解决方法的范围？

【问题讨论】：

在哪里定义了 metadata_init？它的实现在哪里？

另外，我强烈建议您仔细查看这些警告。他们在那里是有原因的。

在相机厂商提供的libyuvlib.a中。我可以用 nm 找到它，它在那里列出，但我无法让它在我的项目中工作。

是的，警告都在新的演示代码中。一旦代码运行，我将删除它们。

【参考方案1】：

它认为问题可能在于metadata_init 是一个 C 函数，但您在 C++ 代码中使用它。

在这种情况下，必须在头文件中使用extern "C"，如下所示：

#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

这是因为名称修改规则。在 C 和 C++ 中，名称的处理方式不同。使用extern "C"，您告诉编译器内部的名称应该与“C”修饰一起使用。

与 C 相比，C++ 必须进行复杂的名称修改，因为它必须将几乎所有签名信息嵌入到名称中（所有参数类型），而在 C 中，修改后的名称通常与函数名称相同，或者前面有一个_。

【讨论】：

是的，做到了。感谢您的帮助。