> For the complete documentation index, see [llms.txt](https://guide.cronuszen.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://guide.cronuszen.com/gpcscripting/gpc-script-guide/learn-gpc/user-created-functions.md).

# User-created functions

{% code title="scale-percent-function.gpc" %}

```cpp
int output;

main {
    output = scale_percent(get_ival(PS5_RX), 125);
    set_val(PS5_RX, output);
}

function scale_percent(value, percent) {
    return (value * percent) / 100;
}
```

{% endcode %}

Rules:

* Declare with `function name(parameter, ...) { ... }`.
* Parameters are optional: `function reset_mode() { ... }` is valid.
* Parameters are local integer values.
* Global runtime variables remain accessible.
* A reached `return expression;` immediately exits.
* If no return is reached, the result is `0`.
* Do not mix valued returns and bare returns.
* Recursion compiles with a warning because worst-case stack use cannot be proven; avoid it in real-time scripts.
* A user-created function's return value may be ignored. The unused-return compiler error applies to built-ins whose return value is marked as required, not to user functions.

A parameterless function used only for its side effect can be called as a statement:

{% code title="reset-mode-function.gpc" %}

```cpp
int mode;

main {
    if(event_press(PS5_CIRCLE)) {
        reset_mode();
    }
}

function reset_mode() {
    mode = 0;
}
```

{% endcode %}

Functions make repeated logic readable, but deep call chains and large expressions increase stack pressure.


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://guide.cronuszen.com/gpcscripting/gpc-script-guide/learn-gpc/user-created-functions.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.