Thread: Problem linking to external library

Hello all,

I've been having issues trying to compile the basic example for thepaho MQTT library. I've asked this question on stackoverflow before, where it hasn't gotten any traction, so I'm hoping someone here can help me.

I downloaded the pre-built binaries for my system (Windows 10 64) from their projects page. I unpacked the zip file to a folder in the documents folder, where I also created a .c file with the example at the bottom of the Paho product page. My editor is atom and my compiler is gcc. When I tried to compile it in Atom, I got this error:

Code:

undefined reference to MQTTClient_create' 

So I went searching and found plenty of topics, but I still couldn't figure out, how to resolve this issue. From this stackoverflow topic I gather that it's a linker problem and that I need to link the files during compile, so here's what I tried:

Code:

gcc MQTT.c -L "C:\Users\Pete\Documents\MQTT on C\Examples\Paho\lib"-l paho-mqtt3c

Which still gives me the same undefined reference error. When I try to link to the dll of the same name, the compiler does not find the file.

Can anyone point me in the right direction, please?
Any help is appreciated!

Here goes:

Code:

C:\Users\Pete\Documents\MQTT on C\Examples>gcc MQTT.c -L"C:/Users/Pete/Documents/MQTT on C/Examples/Paho/lib" -lpaho-mqtt3c
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0xc1): undefined reference to `MQTTClient_create'
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0xe8): undefined reference to `MQTTClient_connect'
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0x163): undefined reference to `MQTTClient_publishMessage'
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0x1ae): undefined reference to `MQTTClient_waitForCompletion'
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0x1e0): undefined reference to `MQTTClient_disconnect'
c:/mingw/bin/../lib/gcc/mingw32/8.2.0/../../../../mingw32/bin/ld.exe: C:\Users\Pete\AppData\Local\Temp\ccFGYzxk.o:MQTT.c:(.text+0x1ef): undefined reference to `MQTTClient_destroy'
collect2.exe: error: ld returned 1 exit status

Yeah, basically it complains about all the functions. I've tried linking the different files before, with no change in result. As far as I understand it the ...a libs are for the asynchronous functions, so I wouldn't expect these functions to be in there anyway.
Is there anything else I can try?

If I add the .lib extension I get an error: cannot find -lpaho-mqtt3c.lib

Thanks for all your input so far! it's been good getting some confirmation that I'm not completely wrong in what I tried.

Hi Tim,

thanks for the tip! I have tried that before and it's the same error: cannot find -lpaho-mqtt3c.dll

Which other two are you referring to?

-l paho-mqtt3c.dll --> cannot find
-l paho-mqtt3c.lib --> cannot find
-l paho-mqtt3c --> undefined reference MQTTClient_Create
-l paho-mqtt3cs.dll --> cannot find
-l paho-mqtt3a.dll --> cannot find

Sorry about that!

Both of these give me the cannot find error. Which makes sense as those files do not exist. Should I have them?