---
title: "fd_write()"
description: "The fd_write() function writes data from one or more buffers to the specified file descriptor. It is similar to the POSIX write() function, but with additional support for writing multiple non-contiguous buffers in a single function call using the iovs parameter."
url: "https://wasix.org/docs/api-reference/wasi/fd_write/"
markdown: "https://wasix.org/docs/api-reference/wasi/fd_write.md"
---

# `fd_write()`

Write data to the file descriptor.

## Description

The `fd_write()` function writes data from one or more buffers to the specified file descriptor. It is similar to the POSIX `write()` function, but with additional support for writing multiple non-contiguous buffers in a single function call using the `iovs` parameter.

## Syntax

```ebnf
  ;;; interleaved while `write` is executed.
  ;;; for a regular file by other threads in the WASI process should not be
  ;;; Like POSIX, any calls of `write` (and other functions to read or write)
  ;;;
  ;;; Note: This is similar to `writev` in POSIX.
  ;;; Write to a file descriptor.
  (@interface func (export "fd_write")
    (param $fd $fd)
    ;;; List of scatter/gather vectors from which to retrieve data.
    (param $iovs $ciovec_array)
    (result $error (expected $size (error $errno)))
  )
```

## Parameters

- `ctx`: A mutable reference to the function environment.
- `fd`: The file descriptor (opened with writing permission) to write to.
- `iovs`: A wasm pointer to an array of `__wasi_ciovec_t` structures, each describing a buffer to write data from.
- `iovs_len`: The length of the `iovs` array.
- `nwritten`: A wasm pointer to an `M::Offset` value where the number of bytes written will be written.

## Return Value

The function returns a `Result` indicating the success or failure of the operation. If the operation is successful, it returns `Ok(Errno::Success)` along with the number of bytes written. If an error occurs, it returns an appropriate `Errno` value.

## Logging

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

- `%fd`: The file descriptor being written to.
- `nwritten`: The number of bytes written.

## POSIX Context

The `fd_write()` function is inspired by the POSIX `write()` function, which writes data to a file descriptor. However, the `fd_write()` function in the WASI environment provides additional support for writing multiple non-contiguous buffers in a single function call.

In the POSIX context, the `write()` function has the following signature:

```c
#include <unistd.h>

ssize_t write(int fd, const void *buf, size_t count);
```

- `fd`: The file descriptor to write to.
- `buf`: A pointer to the buffer containing the data to be written.
- `count`: The number of bytes to write from the buffer.

The `write()` function writes up to `count` bytes from the buffer pointed to by `buf` to the file referred to by the file descriptor `fd`. It returns the number of bytes actually written or -1 if an error occurs.

The `write()` function operates with raw bytes and doesn’t support the concept of vectors (`iovs`) like the `fd_write()` function in the WASI environment does.

It’s important to note that the `fd_write()` function in the WASI environment maps to the `write()` function in the POSIX context, but with additional support for vectors using the `iovs` parameter. The behavior of the `fd_write()` function follows the POSIX specification while accommodating the requirements of the WASI environment.

---

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