---
title: "fd_advise()"
description: "Advise the system about how a file will be used."
url: "https://wasix.org/docs/api-reference/wasi/fd_advise/"
markdown: "https://wasix.org/docs/api-reference/wasi/fd_advise.md"
---

# `fd_advise()`

Advise the system about how a file will be used.

## Description

The `fd_advise()` function is used to provide advice to the operating system about the intended usage of a file. This advice can help the system optimize performance or memory usage based on the specified parameters.

## Syntax

```ebnf
  ;;; Note: This is similar to `posix_fadvise` in POSIX.
  ;;; Provide file advisory information on a file descriptor.
  (@interface func (export "fd_advise")
    (param $fd $fd)
    ;;; The offset within the file to which the advisory applies.
    (param $offset $filesize)
    ;;; The length of the region to which the advisory applies.
    (param $len $filesize)
    ;;; The advice.
    (param $advice $advice)
    (result $error (expected (error $errno)))
  )
```

## Parameters

- `ctx`: A mutable reference to the function environment.
- `fd`: The file descriptor to which the advice applies.
- `offset`: The offset from which the advice applies.
- `len`: The length from the offset to which the advice applies.
- `advice`: The advice to be given to the operating system.

## Return Value

The function returns an `Errno` value indicating the success or failure of the operation.

## Note

The `fd_advise()` function allows applications to inform the operating system about the expected usage patterns of a file. The actual impact of the advice depends on the underlying operating system and filesystem implementation.

The implementation of this function currently returns `Errno::Success` to indicate a valid implementation, although the advice may not be actively utilized.

## Logging

This function has been instrumented with debug-level logging. It will log the following information:

- `fd`: The file descriptor provided.
- `offset`: The offset from which the advice applies.
- `len`: The length from the offset to which the advice applies.
- `advice`: The advice given.

---

[Documentation index](https://wasix.org/llms.txt) · [HTML version](https://wasix.org/docs/api-reference/wasi/fd_advise/)