io\_uring\_register\_files(3) liburing Manual io\_uring\_register\_files(3)

# [NAME](#NAME)

io\_uring\_register\_files - register file descriptors

# [SYNOPSIS](#SYNOPSIS)

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

```
int io_uring_register_files(struct io_uring *ring,
                            const int *files,
                            unsigned nr_files);
```

```
int io_uring_register_files_tags(struct io_uring *ring,
                                 const int *files,
                                 const __u64 *tags,
                                 unsigned nr);
```

```
int io_uring_register_files_sparse(struct io_uring *ring,
                                   unsigned nr_files);
```

```
int io_uring_register_files_update(struct io_uring *ring,
                                   unsigned off,
                                   const int *files,
                                   unsigned nr_files);
```

```
int io_uring_register_files_update_tag(struct io_uring *ring,
                                   unsigned off,
                                   const int *files,
                                   const __u64 *tags,
                                   unsigned nr_files);
```

# [DESCRIPTION](#DESCRIPTION)

The [**io\_uring\_register\_files**(3)](/man/os/liburing/io_uring_register_files.3) function registers *nr\_files* number of file descriptors defined by the array *files* belonging to the *ring* for subsequent operations.

The [**io\_uring\_register\_files\_tags**(3)](/man/os/liburing/io_uring_register_files_tags.3) function behaves the same as [**io\_uring\_register\_files**(3)](/man/os/liburing/io_uring_register_files.3) function but additionally takes *tags* parameter. See **IORING\_REGISTER\_BUFFERS2** for the resource tagging description.

The [**io\_uring\_register\_files\_sparse**(3)](/man/os/liburing/io_uring_register_files_sparse.3) function registers an empty file table of *nr\_files* number of file descriptors. These files must be updated before use, using eg [**io\_uring\_register\_files\_update\_tag**(3)](/man/os/liburing/io_uring_register_files_update_tag.3). Note that if the size of the sparse table exceeds what **RLIMIT\_NOFILE** allows, then [**io\_uring\_register\_files\_sparse**(3)](/man/os/liburing/io_uring_register_files_sparse.3) will attempt to raise the limit using [**setrlimit (2)**](/man/os/liburing/setrlimit.2) and retry the operation. If the registration fails after doing that, then an error will be returned. The sparse variant is available in kernels 5.19 and later.

Registering a file table is a prerequisite for using any request that uses direct descriptors.

Registered files have less overhead per operation than normal files. This is due to the kernel grabbing a reference count on a file when an operation begins, and dropping it when it's done. When the process file table is shared, for example if the process has ever created any threads, then this cost goes up even more. Using registered files reduces the overhead of file reference management across requests that operate on a file.

The [**io\_uring\_register\_files\_update**(3)](/man/os/liburing/io_uring_register_files_update.3) function updates existing registered files. The *off* is offset on which to start the update *nr\_files* number of files defined by the array *files* belonging to the *ring*.

The [**io\_uring\_register\_files\_update\_tag**(3)](/man/os/liburing/io_uring_register_files_update_tag.3) function behaves the same as [**io\_uring\_register\_files\_update**(3)](/man/os/liburing/io_uring_register_files_update.3) function but additionally takes *tags* parameter. See **IORING\_REGISTER\_BUFFERS2** for the resource tagging description.

# [RETURN VALUE](#RETURN_VALUE)

On success [**io\_uring\_register\_files**(3)](/man/os/liburing/io_uring_register_files.3), [**io\_uring\_register\_files\_tags**(3)](/man/os/liburing/io_uring_register_files_tags.3) and [**io\_uring\_register\_files\_sparse**(3)](/man/os/liburing/io_uring_register_files_sparse.3) return 0. [**io\_uring\_register\_files\_update**(3)](/man/os/liburing/io_uring_register_files_update.3) and [**io\_uring\_register\_files\_update\_tag**(3)](/man/os/liburing/io_uring_register_files_update_tag.3) return number of files updated. On failure they return **-errno**.

# [SEE ALSO](#SEE_ALSO)

[**io\_uring\_register**(2)](/man/os/liburing/io_uring_register.2), [**io\_uring\_get\_sqe**(3)](/man/os/liburing/io_uring_get_sqe.3), [**io\_uring\_unregister\_files**(3)](/man/os/liburing/io_uring_unregister_files.3)

November 15, 2021 liburing-2.1

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Document: io_uring_register_files_sparse(3) from liburing 2.15-r4 (Chainguard OS)
Source: https://github.com/axboe/liburing.git