---
title: "fd_tell()"
description: "The fd_tell() function retrieves the current offset of the specified file descriptor relative to the start of the file."
url: "https://wasix.org/docs/api-reference/wasi/fd_tell/"
markdown: "https://wasix.org/docs/api-reference/wasi/fd_tell.md"
---

# `fd_tell()`

Get the offset of the file descriptor.

## Description

The `fd_tell()` function retrieves the current offset of the specified file descriptor relative to the start of the file.

## Syntax

```ebnf
  ;;; Note: This is similar to `lseek(fd, 0, SEEK_CUR)` in POSIX.
  ;;; Return the current offset of a file descriptor.
  (@interface func (export "fd_tell")
    (param $fd $fd)
    ;;; The current offset of the file descriptor, relative to the start of the file.
    (result $error (expected $filesize (error $errno)))
  )
```

## Parameters

- `ctx`: A mutable reference to the function environment.
- `fd`: The file descriptor to access.
- `offset`: A wasm pointer to a `Filesize` where the offset will be written.

## Return Value

The function returns an `Errno` value indicating the success or failure of the operation. If the operation is successful, it returns `Errno::Success`. Otherwise, it returns an appropriate `Errno` value.

## Logging

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

- `%fd`: The file descriptor being accessed.
- `offset`: The offset of the file descriptor.

## Note

The `fd_tell()` function retrieves the current offset of the specified file descriptor (`fd`) relative to the start of the file.

The function first retrieves the memory view and file system state from the WASI environment using the function context.

Next, it retrieves the file descriptor entry for the specified file descriptor from the file system state using the `get_fd()` method.

If the file descriptor does not have the `FD_TELL` right, the function returns `Errno::Access` to indicate an access error.

The function then retrieves the current offset of the file descriptor by loading it from the file descriptor entry using the `load()` method.

It records the offset value in the current span for logging purposes.

Finally, it writes the offset value to the memory location specified by the `offset` wasm pointer using the `write()` method.

The function returns `Errno::Success` to indicate a successful operation.

---

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