---
name: "io_uring_register_sync_msg"
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_register_sync_msg"
---

# io\_uring\_register\_sync\_msg(3)

send a synchronous message to another ring

## NAME

io\_uring\_register\_sync\_msg - send a synchronous message to another ring

## SYNOPSIS

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

**int io\_uring\_register\_sync\_msg(struct io\_uring\_sqe \****sqe***);**

## DESCRIPTION

[**io\_uring\_register\_sync\_msg**(3)](/io_uring_register_sync_msg/3.md) issues a synchronous MSG\_RING request. The *sqe* parameter must have been cleared and initialized with [**io\_uring\_prep\_msg\_ring**(3)](/io_uring_prep_msg_ring/3.md)**.**

Normally message requests are sent from one ring to another ring. But there are also cases where a source ring is not available, yet it would be convenient to send a message to a destination ring. [**io\_uring\_register\_sync\_msg**(3)](/io_uring_register_sync_msg/3.md) exists for that purpose. A source ring is not required to send a message to another ring, instead the *sqe* parameter can be placed on the stack and filled in using the normal message helpers, and then [**io\_uring\_register\_sync\_msg**(3)](/io_uring_register_sync_msg/3.md) can be called. Since a source ring does not exist, the results of the operation is returned directly rather than via a CQE. On the destination/receiving end, a CQE is posted, as it would have been with a non-sync request.

Only data request are supported, sending files such as setup by [**io\_uring\_prep\_msg\_ring\_fd**(3)](/io_uring_prep_msg_ring_fd/3.md) is not supported. The given SQE should be initialized by [**io\_uring\_prep\_msg\_ring**(3)](/io_uring_prep_msg_ring/3.md) or [**io\_uring\_prep\_msg\_ring\_cqe\_flags**(3)](/io_uring_prep_msg_ring_cqe_flags/3.md)**,** or any other helper that sets up a non-fd message request.

The targeted ring may be any ring that the user has access to, even the ring itself. This request can be used for simple message passing to another ring, allowing 32+64 bits of data to be transferred through the *len* and *data* fields. The use case may be anything from simply waking up someone waiting on the targeted ring, or it can be used to pass messages between the two rings.

[**io\_uring\_prep\_msg\_ring\_cqe\_flags**(3)](/io_uring_prep_msg_ring_cqe_flags/3.md) is similar to [**io\_uring\_prep\_msg\_ring**(3)](/io_uring_prep_msg_ring/3.md)**.** But has an addition *cqe\_flags* parameter, which is used to set *flags* field on CQE side. That way, you can set the CQE flags field *cqe-\>flags* when sending a message. Be aware that io\_uring could potentially set additional bits into this field.

Available since kernel 6.13.

## RETURN VALUE

Returns 0 on success, or **-errno** on error.

## SEE ALSO

[**io\_uring\_prep\_msg\_ring\_cqe\_flags**(3)](/io_uring_prep_msg_ring_cqe_flags/3.md)**,** [**io\_uring\_prep\_msg\_ring**(3)](/io_uring_prep_msg_ring/3.md)