c - 如何安装 Eclipse Paho C

Question

我正在尝试在 Eclipse 中使用 Eclipse Paho C 运行一个简单的客户端程序。

我做了以下事情:
1)创建新项目
2) 将“include”文件夹和“lib”文件夹复制到项目目录
3) 将“include”文件夹目录添加到includes路径
4) 使用链接器链接库，如下图所示

但是当我构建以下代码时:

#include "stdio.h"
#include "stdlib.h"
#include "string.h"
#include "MQTTClient.h"

#define ADDRESS     "tcp://localhost:1883"
#define CLIENTID    "ExampleClientPub"
#define TOPIC       "MQTT Examples"
#define PAYLOAD     "Hello World!"
#define QOS         1
#define TIMEOUT     10000L

int main(int argc, char* argv[])
{
    MQTTClient client;
    MQTTClient_connectOptions conn_opts = MQTTClient_connectOptions_initializer;
    MQTTClient_message pubmsg = MQTTClient_message_initializer;
    MQTTClient_deliveryToken token;
    int rc;

    MQTTClient_create(&client, ADDRESS, CLIENTID,
        MQTTCLIENT_PERSISTENCE_NONE, NULL);
    conn_opts.keepAliveInterval = 20;
    conn_opts.cleansession = 1;

    if ((rc = MQTTClient_connect(client, &conn_opts)) != MQTTCLIENT_SUCCESS)
    {
        printf("Failed to connect, return code %d\n", rc);
        exit(-1);
    }
    pubmsg.payload = PAYLOAD;
    pubmsg.payloadlen = strlen(PAYLOAD);
    pubmsg.qos = QOS;
    pubmsg.retained = 0;
    MQTTClient_publishMessage(client, TOPIC, &pubmsg, &token);
    printf("Waiting for up to %d seconds for publication of %s\n"
            "on topic %s for client with ClientID: %s\n",
            (int)(TIMEOUT/1000), PAYLOAD, TOPIC, CLIENTID);
    rc = MQTTClient_waitForCompletion(client, token, TIMEOUT);
    printf("Message with delivery token %d delivered\n", token);
    MQTTClient_disconnect(client, 10000);
    MQTTClient_destroy(&client);
    return rc;
}

我得到以下信息:

19:31:16 **** Rebuild of configuration Debug for project MQTT_C_Client ****
Info: Internal Builder is used for build
gcc "-IE:\\WS\\MQTT_C_Client\\includes" -O0 -g3 -Wall -c -fmessage-length=0 -o "src\\main.o" "..\\src\\main.c" 
gcc "-LE:\\WS\\MQTT_C_Client\\libs" -o MQTT_C_Client.exe "src\\main.o" -lpaho-mqtt3a -lpaho-mqtt3as -lpaho-mqtt3c -lpaho-mqtt3cs 
src\main.o: In function `main':
E:\WS\MQTT_C_Client\Debug/../src/main.c:29: undefined reference to `MQTTClient_create'
E:\WS\MQTT_C_Client\Debug/../src/main.c:34: undefined reference to `MQTTClient_connect'
E:\WS\MQTT_C_Client\Debug/../src/main.c:43: undefined reference to `MQTTClient_publishMessage'
E:\WS\MQTT_C_Client\Debug/../src/main.c:47: undefined reference to `MQTTClient_waitForCompletion'
E:\WS\MQTT_C_Client\Debug/../src/main.c:49: undefined reference to `MQTTClient_disconnect'
E:\WS\MQTT_C_Client\Debug/../src/main.c:50: undefined reference to `MQTTClient_destroy'
collect2.exe: error: ld returned 1 exit status

19:31:29 Build Finished (took 12s.643ms)

知道我错过了什么吗？

Answer 1

这是错误的可能原因。 (可能不是正确的解决方案，具体取决于底层链接器)

这是 -L 相对于 -l 的位置给你带来了麻烦(我的观察有点粗心)。来自 gcc 联机帮助页:

You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant.

根据底层链接器的不同，-L 库搜索的范围可能会有所不同。尝试确保 -L 和 -l 彼此相邻，没有任何其他直接链接的对象路径将它们分开。

例如，

 gcc -o myprog main.o foo.o bar.o -L/some/path -L/some/other/path -la -lb 

还可以

但是，

gcc -o myprog main.o -L/some/path foo.o bar.o -L/some/other/path -la -lb 

不是。