This page contains notes about using various runtime environments in ReCodEx as well as their internal usage inside [isolate](https://github.com/ioi/isolate) sandbox. At present ReCodEx supports
Exercises may be configured for multiple environments, so the student may choose the language for the solution. However, Data-only, Haskell, and Prolog have somewhat unique configurations, therefore an exercise must use such environment exclusively.
Some languages does not have uniquely identified entry point (like `main` routine in C/C++, for instance). For these languages, we have defined entry point -- file which is used as application bootstrap or main compilation unit.
Namely, entry point is present in Haskell, JavaScript, PHP, Python, and Rust. The entry point may be specified in the exercise configuration (pointing to a file provided by the author), or left unspecified. In such case, the student may choose the entry point in case more than one file is submitted.
The C# solutions are executed with the help of [wrapper code](https://github.com/ReCodEx/utils/blob/master/runners/cs/Wrapper.cs). This code searches for `main` method in submitted source codes. There has to be only one class containing `main` method, otherwise error will be thrown. The name of the file containing wrapper code is `ReCodEx_Csharp_Wrapper.cs` and it contains namespace `CodEx` with class `Wrapper`, so be aware of filename and class collisions.
In C# language there are none of the handy functions to operate with textual inputs like `scanf` in C. Therefore for every C# build in ReCodEx there is [Reader](https://github.com/ReCodEx/utils/blob/master/runners/cs/Reader.cs) library included to make life of C# programmers easier. Please note that library is suppose to work only with ASCII files and some difficulties can be observed with modern Unicode (UTF-8) encodings. The `Reader` library is located in `ReCodEx_Csharp_Reader.cs` library contains namespace `CodEx` with class `Reader`, so again, be aware of filename and class collisions.
The solutions of Go has to be located only in one package, the standard `main` executable package. ReCodEx uses flat folder structure of submitted solution, therefore separate Go packages cannot be used. Multiple files are fine as long as they contain the same package.
By default, we use a script that looks for a class with a `main` method and executes it for running Java programs. There has to be only one class containing `main` method, otherwise error will be observed. The executed script is named `javarun.java` and can be found in the [utils](https://github.com/ReCodEx/utils/blob/master/runners/java/javarun.java) repository. In tested solutions we use its byte-compiled counterpart `javarun.class`, so be aware of name collisions.
## Python
We use a wrapper script that translates exceptions to error codes. This script can be found in the [utils](https://github.com/ReCodEx/utils/blob/master/runners/py/runner.py) repository. The file is named `runner.py` during execution of solutions, so be aware of filename collisions.
Due to a [Python bug](http://bugs.python.org/issue10496), the interpreter
requires the `$HOME` variable to be set to something. Otherwise, it tries to
look up the UID used by Isolate in `/etc/passwd` and fails. Setting `HOME=/box`
