---
name: "io_uring_prep_msg_ring_fd_alloc"
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_msg_ring_fd_alloc"
---

# io\_uring\_prep\_msg\_ring\_fd\_alloc(3)

send a direct descriptor to another ring

## NAME

io\_uring\_prep\_msg\_ring\_fd - send a direct descriptor to another ring

## SYNOPSIS

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

**void io\_uring\_prep\_msg\_ring\_fd(struct io\_uring\_sqe \****sqe***,**                         **int** *fd***,**                         **int** *source\_fd***,**                         **int** *target\_fd***,**                         **\_\_u64** *data***,**                         **unsigned int** *flags***);**

**void io\_uring\_prep\_msg\_ring\_fd\_alloc(struct io\_uring\_sqe \****sqe***,**                         **int** *fd***,**                         **int** *source\_fd***,**                         **\_\_u64** *data***,**                         **unsigned int** *flags***);**

## DESCRIPTION

[**io\_uring\_prep\_msg\_ring\_fd**(3)](/io_uring_prep_msg_ring_fd/3.md) prepares an SQE to send a direct file descriptor to another ring. The submission queue entry *sqe* is setup to use the file descriptor *fd*, which must identify a target io\_uring context, to send the locally registered file descriptor with value *source\_fd* to the destination ring into index *target\_fd* and passing *data* as the user data in the target CQE with the request modifier flags set by *flags*. Currently there are no valid flag modifiers, this field must contain **0**.

[**io\_uring\_prep\_msg\_ring\_fd\_alloc**(3)](/io_uring_prep_msg_ring_fd_alloc/3.md) is similar to [**io\_uring\_prep\_msg\_ring\_fd**(3)](/io_uring_prep_msg_ring_fd/3.md)**,** but doesn't specify a target index for the direct descriptor. Instead, this index is allocated in the target ring and returned in the CQE *res* field.

## RETURN VALUE

None

## ERRORS

These are the errors that are reported in the CQE *res* field.

- **-ENOMEM**

  The kernel was unable to allocate memory for the request.

- **-EINVAL**

  One of the fields set in the SQE was invalid.

- **-EINVAL**

  Target ring is identical to the source ring.

- **-EBADFD**

  The descriptor passed in *fd* does not refer to an io\_uring file descriptor, or the ring is in a disabled state.

- **-EOVERFLOW**

  The kernel was unable to fill a CQE on the target ring. This can happen if the target CQ ring is in an overflow state and the kernel wasn't able to allocate memory for a new CQE entry.

- **-ENFILE**

  The direct descriptor table in the target ring was full, no new descriptors could be successfully allocated.