---
name: "io_uring_prep_read"
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"
---

# io\_uring\_prep\_read(3)

prepare I/O read request

## NAME

io\_uring\_prep\_read - prepare I/O read request

## SYNOPSIS

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

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

## DESCRIPTION

The [**io\_uring\_prep\_read**(3)](/io_uring_prep_read/3.md) prepares an IO read request. 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*.

On files that support seeking, if the offset is set to **-1**, the read operation commences at the file offset, and the file offset is incremented by the number of bytes read. See [**read**(2)](/read/2.md) for more details. Note that for an async API, reading and updating the current file offset may result in unpredictable behavior, unless access to the file is serialized. It is not encouraged to use this feature, if it's possible to provide the desired IO offset from the application or library.

On files that are not capable of seeking, the offset must be 0 or -1.

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\_get\_sqe**(3)](/io_uring_get_sqe/3.md), [**io\_uring\_prep\_readv**(3)](/io_uring_prep_readv/3.md), [**io\_uring\_prep\_readv2**(3)](/io_uring_prep_readv2/3.md), [**io\_uring\_submit**(3)](/io_uring_submit/3.md)