I was helping my wife to scrape some website with a really noob code that I made and since the page was loading slowly the script did break several times and I had to be waiting this to happen since I didn’t have a way to check the process from other room of my house.

This issue gave me the idea to generate an android app that could show me notifications about some process that I was running on my machine but that it was long enough that I really didn’t wanted to wait it watching my pc.

Receiving notifications (idea 1)

An app written in something that could show notifications of the exit code of my code.

First issue: I don’t like mobile developing.

Second issue: in my last job I had to do something kind of similar and the result of this was having to send the information to google, to use something that I don’t remember too well but had to do with Firebase (this) and then have the application running in a way that it could get the notifications and show them… So it was too mucho for a simple “I want to know when the thing ended”.

Receiving notifications (idea 2)

Telegram bot. I remember an old coworker saying something about coding a bot for telegram to help pet owners when their pets get lost, so it should be easier, I don’t need to code another app and I don’t need a Firebase account.

Creating a telegram bot

Is really simple, you search for @BotFather and follow the instructions to generate a new bot, after that store the token and try it. You could generate a new chat with the bot or a new group. After having this you need to send a POST request to https://api.telegram.org/bot<token>/getUpdates , yes the “bot” string has to be before the token. You are going to get something like this:

{"ok":true,"result":[]}

If that is the case you need to send it a message, after that:

{"ok":true,"result":[{"update_id":<update_id>,
"message":{"message_id":<msg_id>,"from":{"id":<user_id>,"is_bot":false,"first_name":<user_name>,"language_code":"en"},"chat":{"id":<chat_id>,"first_name":<name>,"type":"private"},"date":<date_ts>,"text":"Hello"}}]}

That “Hello” was the message that I send to the bot, so we have the chat_id and we can do a GET request to https://api.telegram.org/bot<token>/sendMessage?chat_id=<chat_id>&text=Hello%20World giving you:

{"ok":true,"result":{"message_id":<message_id>,"from":{"id":<id>,"is_bot":true,"first_name":"<bot_name>","username":"<bot_username>"},"chat":{"id":<chat_id>,"first_name":"<you>","type":"private"},"date":<date_ts>,"text":"Hello World"}}

We have the bot ready

Sending the information

I want to do something like this: ./run | notification, this means I have to read stdout and stderror so I could send helpful messages, for the moment I will send the full message like in the terminal, I could beautify it but not for now.

I was able to generate a simple code using libcurl to send GET request to telegram and have the simple notification, no problem with that.

The problems

First: the code is reading 1000 chars in a buffer and re allocate memory when needed, this, I didn’t tested it, it could explote tomorrow, also for the moment I’m not sending partial messages because I think it will break 1/100 times so not a big issue.

Second: while I was finishing the code I used sprintf to format the final url string and started to have errors:

malloc(): unaligned tcache chunk detected

I didn’t understand why this was happening so I run it with valgrind and started working, I received notifications but the code still crashed. This took me a while but reading the gnu documentation of this function I noticed a little note

Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string s. Remember that the field width given in a conversion specification is only a minimum value. To avoid this problem, you can use snprintf or asprintf, described below.

This was exactly was happening I counted the amount of chars in the url really badly and was writing to memory that I didn’t ask for, and when I asked curl to generate the request It couldn’t since the memory was in a bad state. So I changed it to snprintf to verify and fix.

Regarding why valgrind magically fixed part of the issue I think it’s because valgrind adds code when running your code, when you check for errors you will see that the stack trace has calls to vg_ libraries like vg_replace_malloc.

Really nice project to understand better how Linux uses pipes and have a reason to read documentation. I also read wc and sort code from gnu coreutils to understand better how good code was written.

You can get the code here: github