---
name: "io_uring_prep_read_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_read_fixed"
---

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

prepare I/O read request with registered buffer

## NAME

io\_uring\_prep\_read\_fixed - prepare I/O read request with registered buffer

## SYNOPSIS

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

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

## DESCRIPTION

The [**io\_uring\_prep\_read\_fixed**(3)](/io_uring_prep_read_fixed/3.md) prepares an IO read request with a previously registered IO buffer. The submission queue entry *sqe* is setup to use the file descriptor *fd* to start reading *nbytes* into 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\_read**(3)](/io_uring_prep_read/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 read 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

This function accepts an unsigned number of bytes, but io\_uring\_cqe's result code is an \_\_s32 value, so in theory a short read with a large enough nbytes value could generate an ambiguous return.  But the number of bytes actually transferred has the same limit as [**read**(2)](/read/2.md) so this cannot happen in practice.

## SEE ALSO

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