---
name: "io_uring_prep_write_fixed"
section: "3"
platform: "linux"
package: "liburing"
version: "2.15-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/io_uring_prep_write_fixed"
---

# io\_uring\_prep\_write\_fixed(3)

prepare I/O write request with registered buffer

## NAME

io\_uring\_prep\_write\_fixed - prepare I/O write request with registered buffer

## SYNOPSIS

```
#include <liburing.h>
```

**void io\_uring\_prep\_write\_fixed(struct io\_uring\_sqe \****sqe***,**                         **int** *fd***,**                         **const void \****buf***,**                         **unsigned** *nbytes***,**                         **\_\_u64** *offset***,**                         **int** *buf\_index***);**

## DESCRIPTION

The [**io\_uring\_prep\_write\_fixed**(3)](/io_uring_prep_write_fixed/3.md) prepares an IO write request with a previously registered IO buffer. The submission queue entry *sqe* is setup to use the file descriptor *fd* to start writing *nbytes* from the buffer *buf* at the specified *offset* and with the buffer matching the registered index of *buf\_index*.

This works just like [**io\_uring\_prep\_write**(3)](/io_uring_prep_write/3.md) except it requires the use of buffers that have been registered with [**io\_uring\_register\_buffers**(3)](/io_uring_register_buffers/3.md). The *buf* and *nbytes* arguments must fall within a region specified by *buf\_index* in the previously registered buffer. The buffer need not be aligned with the start of the registered buffer.

After the write has been prepared it can be submitted with one of the submit functions.

## RETURN VALUE

None

## ERRORS

The CQE *res* field will contain the result of the operation. See the related man page for details on possible values. Note that where synchronous system calls will return **-1** on failure and set *errno* to the actual error value, io\_uring never uses *errno*. Instead it returns the negated *errno* directly in the CQE *res* field.

## NOTES

Despite accepting an unsigned number of bytes, this function can transfer at most INT\_MAX bytes per call (the maximum for the underlying syscall interface).

## SEE ALSO

[**io\_uring\_prep\_write**(3)](/io_uring_prep_write/3.md), [**io\_uring\_register\_buffers**(3)](/io_uring_register_buffers/3.md)