Write data to the file descriptor.


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.


  ;;; 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)))


  • 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.


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:

#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.