使用Eclipse Paho MQTT编译Visual C ++ Win32项目时出现LNK2019错误 [英] LNK2019 error when compiling a Visual C++ Win32 project with Eclipse Paho MQTT

问题描述

我正在尝试构建一个使用Eclipse Paho MQTT库的Visual C ++ Win32项目-请参见 https ://eclipse.org/paho/clients/c/ 该库采用2个文件的形式:

I'm trying to build a Visual C++ Win32 project which uses the Eclipse Paho MQTT library - see https://eclipse.org/paho/clients/c/ The library takes the form of 2 files:

• paho-mqtt3c.lib
• paho-mqtt3c.dll

该库最初是用C编写的-我已经尝试过使用预编译的二进制文件并在自己编译时使用.

The library is originally written in C - I've tried with the precompiled binary and when compiling it myself.

在我的客户端Visual C ++项目中，在链接器选项下，我在其他库目录"字段中包含了正确的库路径，在其​​他依赖项"字段中直接包含了paho-mqtt3c.lib文件.但是，当我尝试构建时，链接器会因使用的每个库函数而出错.所有错误都具有相似的格式:

In my client Visual C++ project, under the linker options, I've included the correct library path in the "Additional Library Directories" field and the paho-mqtt3c.lib file directly in the "Additional Dependencies" field. However when I try to build, the linker bins out with an error for each library function used. All errors share a similar format:

错误2错误LNK2019:未解析的外部符号"int __cdecl MQTTClient_connect(void *，struct MQTTClient_connectOptions *)"(?MQTTClient_connect @@ YAHPAXPAUMQTTClient_connectOptions @@@ Z)在函数_main c:\ Project \ scrapbook \ MQTT_Example \ MQTT_Example中引用main.obj MQTT_Example

Error 2 error LNK2019: unresolved external symbol "int __cdecl MQTTClient_connect(void *,struct MQTTClient_connectOptions *)" (?MQTTClient_connect@@YAHPAXPAUMQTTClient_connectOptions@@@Z) referenced in function _main c:\Project\scrapbook\MQTT_Example\MQTT_Example\main.obj MQTT_Example

我的代码如下:

#include <MQTTClient.h>
#include <MQTTClientPersistence.h>

:    :    :

// Start MQTT connection
MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
MQTTClient_message pubmsg = MQTTClient_message_initializer;
MQTTClient_deliveryToken token;

// Convert CString to char*
int connectResponse;
char* szBuf = strBroker.GetBuffer(strBroker.GetLength()) ;

// Create client
MQTTClient_create(&client, szBuf, "MQTTTestClient", MQTTCLIENT_PERSISTENCE_NONE, NULL);

conn_opts.keepAliveInterval = 20;
conn_opts.cleansession = 1;

MQTTClient_setCallbacks(client, NULL, NULL, msgarrvd, NULL);

// Connect to broker
if ((connectResponse = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
{
    // Oops.. that didn't work!
}

有人对如何解决这个问题有任何想法吗? (我知道有数以百万计的LNK2019问题，但到目前为止我找不到能解决我问题的任何东西...)

Does anyone have any ideas about how to solve this? (I know there are millions of LNK2019 questions but I cannot find anything that has solved my problem so far...)

推荐答案

所以事实证明，问题是我试图在C ++项目中包括一个C库​​(具有许多C风格声明的函数).

So it turns out the problem was that I was attempting to include a C library (with lots of C style declared functions) in a C++ project.

解决方案是将函数声明(即包含头文件的位置)包装在extern"C"块中:

The solution is to wrap the function declarations (i.e. where the header files are included) in an extern "C" block:

extern "C"{
#include <MQTTClient.h>
#include <MQTTClientPersistence.h>
}

这告诉链接器链接C而不是C ++函数.

This tells the linker to link against C rather than C++ functions.

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