@crispyoz thanks a lot again.

I'll post the result here after executing the above code.

@crispyoz OK, I got it.
I'll proceed it.

I want to make the code that publish/subscribe using SSL/TLS(mosquitto).
anyone know that, please let me know.
thanks.

@crispyoz thanks for your advice.
Do you know anything about sites or codes to refer to for SSL?
if you know that, let me know.

@crispyoz First of all, I would like to express my deep gratitude for your interest.
I am even more grateful for your sincere help with my poor English skills.

I found the reason for the error.
Linux for cross-compile was executed on a virtual environment in Windows. (like VMware s/w)
However, an error occurred due to the lack of size given to Linux.

It was found that size of at least 30G or more was required when the installation was performed in the above way.

I succeeded to execute the simple mqtt code.

Once again, I want to say thank you so much.

I hope this article helps others when they have similar problems.

So I want to modify the title of this article to more appropriate,
so please advise if there is a title to recommend.

Thanks to you, I am so grateful to solve this problem.

@crispyoz thanks for your reply again.
I got a late answer because this community site went down.

As a result of the above method you told me, I finally found the libmosquitto in menu>libraries.
thanks again.
but, I had an error during making the build system.

error : recipe for target 'world' failed

so, I'm trying to run the command('make j1 V=s') to see the exact error.
if I succeed with the simple mqtt code, I'll post it to you.

@crispyoz Thank you for your consideration.
I'd like to install the libmosquitto. but, there is not libmosquitto in Libraries tap.
so, I checked the feeds.conf file in the /source folder for verifying OpenWrt version.

list of feeds.conf file

src-git packages https://git.openwrt.org/feed/packages.git;openwrt-18.06
src-git luci https://git.openwrt.org/project/luci.git;openwrt-18.06
src-git routing https://git.openwrt.org/feed/routing.git;openwrt-18.06
src-git telephony https://git.openwrt.org/feed/telephony.git;openwrt-18.06
src-git onion https://github.com/OnionIoT/OpenWRT-Packages.git;openwrt-18.06

contents of build_info.json

version: 0.3.3
build: 251

Can I get the new version of build system?
or should I install the mosquitto package manually?

thanks.

@crispyoz thanks for the reply.
yes, I want to make a C program that can publish/subscribe to topics.

I had build system installed on ubuntu18.04(ubuntu).
and I succeeded in printing 'hello world' on omega2.

I installed the package of mosquitto and libmosquitto-dev on ubuntu.
(apt-get install mosquitto libmosquitto-dev)

mosquitto version : 2.0.14
mosquitto command list : mosquitto, mosquitto_pub(sub), mosquitto_passwd etc.

so I have found the mosquitto.h in '/usr/include/mosquitto.h'
but, there isn't mosquitto.h file in the folder related to 'mipsel-openwrt-linux'.
(ex, /home/source/build_dir/toolchain-mipsel_24kc_gcc-7.3.0_musl/gcc-7.3.0/../../include/mosquitto.h)
if I want to complie the c code with mosquitto function, mosquitto.h file have to be in the folder related 'mipsel-openwrt-linux'? is it right?

or is it possible, I can compile the c code with mosquitto using '/usr/include/mosquitto.h'?

if there is anything I misunderstood, please let me know.

I'm going to review the site you linked.
once again, thank you for your answer.

@crispyoz thanks for the reply.
I understood your intention of the previous reply.
I would like to handle the command according to the receiving messages I received.
so I would like to compile it in c language and put it in omega2.
(like esp32/8266 mqtt communication way)
thanks.

add :
The main question is, did anyone succeed in compiling the mosquitto code into the c language for omega2?
I want to compile the mosquitto code in c language for omega2.
thanks.