---
title: "fd_allocate"
description: "Allocate extra space for a file descriptor."
url: "https://wasix.org/docs/api-reference/wasi/fd_allocate/"
markdown: "https://wasix.org/docs/api-reference/wasi/fd_allocate.md"
---

# `fd_allocate`

Allocate extra space for a file descriptor.

## Description

The `fd_allocate` function is used to allocate additional space for a file descriptor. It allows extending the size of a file or buffer associated with the file descriptor.

## Syntax

```ebnf
  ;;; Note: This is similar to `posix_fallocate` in POSIX.
  ;;; Force the allocation of space in a file.
  (@interface func (export "fd_allocate")
    (param $fd $fd)
    ;;; The offset at which to start the allocation.
    (param $offset $filesize)
    ;;; The length of the area that is allocated.
    (param $len $filesize)
    (result $error (expected (error $errno)))
  )
```

## Parameters

- `ctx`: A mutable reference to the function environment.
- `fd`: The file descriptor to allocate space for.
- `offset`: The offset from the start marking the beginning of the allocation.
- `len`: The length from the offset marking the end of the allocation.

## Return Value

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

## Logging

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

- `%fd`: The file descriptor being used.
- `%offset`: The offset from the start of the allocation.
- `%len`: The length of the allocation.

## Note

The `fd_allocate` function allows for allocating additional space for a file descriptor. It extends the size of the file or buffer associated with the file descriptor based on the specified offset and length.

---

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