The Power of C in the Cloud

In my last post, I introduced you to the cloud IDE Codenvy. The primarily reason I used it is because it has a simply way to hook into Google App Engine. However, for a more robust cloud IDE, I would recommended Compilr. Compilr has support for many of the languages Codenvy does (Ruby, Java, PHP, Python) but it also has support for Objective-C and C# as well as low level languages like C and C++.

That last point leads to some interesting results. Because we have access to such low level languages, we can use the Compilr servers for general computing. From what I can tell, the Compilr servers that execute the code are running a Unix-like operating system on x86 hardware. So if you’re in a pinch and need access to a Unix like environment, Compilr can be very useful. I happened to run into such a situation while trying to make use of WoLF PSORT for a rather long sequence and lacking the necessary Linux PC to compile and run the program locally.

So, how do you execute commands on commands on Compilr’s servers? First, create your C++ project. Your program will be run in the “content” folder of your project so if you have any external files you need, place them there:

 

If you get an error message when uploading, but you also get “Upload complete”, you upload has finished. Refresh the page for the files to show up in the project panel.

The key to our program is the system function. Simply hand it commands to execute and it will do so. To untar our WoLF PSORT file execute the following:

system ("tar -zxf WoLFPSORT_package_v0.2.tar.gz");

Remember to mark the files for execution if nessessary:

system ("chmod 777 ./WoLFPSORT_package_v0.2/bin/psortModifiedForWolfFiles/psortModifiedForWoLF");

There are a few restrictions Compilr has placed on free accounts.

First, you are forced to make public project and you are restricted to 50 builds/runs. These aren’t huge problems and you can always create a new account and port code if you go over 50 builds. In fact I have noticed the build counter often reset after refreshing the page, so this never became an issue for me.

Second, you are limited to 4000 characters of output. A very simple solution, pipe output into a log file by appending > log.txt to your command:

system ("./WoLFPSORT_package_v0.2/bin/psortModifiedForWolfFiles/psortModifiedForWoLF -t ./WoLFPSORT_package_v0.2/bin/psortModifiedForWolfFiles/all.seq > log.txt");

You may also need to pipe errors and warning, in which case you can log to the same file by appending >log.txt 2>&1 to your command. Or you may wish to disable errors and warnings completely while still capturing output, in which you would append the following 1>log.txt 2>/dev/null .

Third, if your program has not accepted input in 40 seconds, it will be terminated. This requires a tricky work around. I ended up spawning a child process to accept input while the parent process was busy. The child process will send an alert for input every 30 seconds. If you choose to do something similar, remember to flush stdout before sleeping. The syntax is as follows:

fflush(stdout);

sleep(30);

Interestingly, this method triggers a security measure when one process exits before the other and will terminate the program. So make sure no process exits before your program has finished doing it’s work.

Some final points about Compilr:

Much like Codenvy, Compilr has issues with Internet Explorer. I managed to get it working in Firefox.

The first compile of every session will fail with “Lost connection to server”. Simply use a dummy application to trigger it and continue your work. If you run an application that takes a long time and it loses connection, it could block you from running further applications until it finishes.

Always backup your progress! I can’t stress this enough. If the server that stores the code loses connection to the server that runs the code while “Syncing application data”, you can lose large chunks of your work.

Finally, I’ve attached some sample code on the download website.