io\_uring\_prep\_openat(3) liburing Manual io\_uring\_prep\_openat(3)

# [NAME](#NAME)

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

# [SYNOPSIS](#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](#DESCRIPTION)

The [**io\_uring\_prep\_openat**(3)](/man/os/liburing/io_uring_prep_openat.3) 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)](/man/os/liburing/io_uring_prep_open.3) 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)](/man/os/liburing/io_uring_register_files.3) 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)](/man/os/liburing/io_uring_register_files_update.3).

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)](/man/os/liburing/openat.2) or [**open**(2)](/man/os/liburing/open.2) request. See that man page for details.

# [RETURN VALUE](#RETURN_VALUE)

None

# [ERRORS](#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](#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)](/man/os/liburing/io_uring_queue_init_params.3).

# [SEE ALSO](#SEE_ALSO)

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

March 13, 2022 liburing-2.2

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Document: io_uring_prep_openat(3) from liburing 2.15-r5 (Chainguard OS)
Source: https://github.com/axboe/liburing.git