---
name: "io_uring_prep_open"
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_open"
---

# io\_uring\_prep\_open(3)

prepare an openat request

## NAME

io\_uring\_prep\_openat - prepare an openat request

## SYNOPSIS

```
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <liburing.h>
```

**void io\_uring\_prep\_open(struct io\_uring\_sqe \****sqe***,**                           **const char \****path***,**                           **int** *flags***,**                           **mode\_t** *mode***);**

**void io\_uring\_prep\_open\_direct(struct io\_uring\_sqe \****sqe***,**                                  **const char \****path***,**                                  **int** *flags***,**                                  **mode\_t** *mode***,**                                  **unsigned** *file\_index***);**

**void io\_uring\_prep\_openat(struct io\_uring\_sqe \****sqe***,**                           **int** *dfd***,**                           **const char \****path***,**                           **int** *flags***,**                           **mode\_t** *mode***);**

**void io\_uring\_prep\_openat\_direct(struct io\_uring\_sqe \****sqe***,**                                  **int** *dfd***,**                                  **const char \****path***,**                                  **int** *flags***,**                                  **mode\_t** *mode***,**                                  **unsigned** *file\_index***);**

## DESCRIPTION

The [**io\_uring\_prep\_openat**(3)](/io_uring_prep_openat/3.md) function prepares an openat request. The submission queue entry *sqe* is setup to use the directory file descriptor *dfd* to start opening a file described by *path* and using the open flags in *flags* and using the file mode bits specified in *mode*. Similarly [**io\_uring\_prep\_open**(3)](/io_uring_prep_open/3.md) prepares an open request.

If the direct variant is used, the application must first have registered a file table using [**io\_uring\_register\_files**(3)](/io_uring_register_files/3.md) of the appropriate size. Once registered, a direct accept request may use any entry in that table and is specified in *file\_index* , as long as it is within the size of the registered table. If a specified entry already contains a file, the file will first be removed from the table and closed. It's consistent with the behavior of updating an existing file with [**io\_uring\_register\_files\_update**(3)](/io_uring_register_files_update/3.md).

If **IORING\_FILE\_INDEX\_ALLOC** is used as the *file\_index* for a direct open, then io\_uring will allocate a free direct descriptor in the existing table. The allocated descriptor is returned in the CQE *res* field just like it would be for a non-direct open request. If no more entries are available in the direct descriptor table, **-ENFILE** is returned instead.

Direct descriptors are io\_uring private file descriptors. They avoid some of the overhead associated with thread shared file tables, and can be used in any subsequent io\_uring request that takes a file descriptor. To do so, **IOSQE\_FIXED\_FILE** must be set in the SQE *flags* member, and the SQE *fd* field should use the direct descriptor value rather than the regular file descriptor. Direct descriptors are managed like registered files.

The directory file descriptor *dfd* is always a regular file descriptor.

Note that old kernels don't check the SQE *file\_index* field, which is not a problem for liburing helpers, but users of the raw io\_uring interface need to zero SQEs to avoid unexpected behavior.

These functions prepare an async [**openat**(2)](/openat/2.md) or [**open**(2)](/open/2.md) request. See that man page for details.

## 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

As with any request that passes in data in a struct, that data must remain valid until the request has been successfully submitted. It need not remain valid until completion. Once a request has been submitted, the in-kernel state is stable. Very early kernels (5.4 and earlier) required state to be stable until the completion occurred. Applications can test for this behavior by inspecting the **IORING\_FEAT\_SUBMIT\_STABLE** flag passed back from [**io\_uring\_queue\_init\_params**(3)](/io_uring_queue_init_params/3.md).

## SEE ALSO

[**io\_uring\_get\_sqe**(3)](/io_uring_get_sqe/3.md), [**io\_uring\_submit**(3)](/io_uring_submit/3.md), [**io\_uring\_register**(2)](/io_uring_register/2.md), [**openat**(2)](/openat/2.md)