#include <linux/io_uring.h> int io_uring_register(unsigned int fd, unsigned int opcode, void *arg, unsigned int nr_args);
The io_uring_register() system call registers resources (e.g. user buffers, files, eventfd, personality, restrictions) for use in an io_uring(7) instance referenced by fd. Registering files or user buffers allows the kernel to take long term references to internal data structures or create long term mappings of application memory, greatly reducing per-I/O overhead.
fd is the file descriptor returned by a call to io_uring_setup(2). opcode can be one of:
arg points to a struct iovec array of nr_args entries. The buffers associated with the iovecs will be locked in memory and charged against the user's RLIMIT_MEMLOCK resource limit. See getrlimit(2) for more information. Additionally, there is a size limit of 1GiB per buffer. Currently, the buffers must be anonymous, non-file-backed memory, such as that returned by malloc(3) or mmap(2) with the MAP_ANONYMOUS flag set. It is expected that this limitation will be lifted in the future. Huge pages are supported as well. Note that the entire huge page will be pinned in the kernel, even if only a portion of it is used.
After a successful call, the supplied buffers are mapped into the kernel and eligible for I/O. To make use of them, the application must specify the IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED opcodes in the submission queue entry (see the struct io_uring_sqe definition in io_uring_enter(2)), and set the buf_index field to the desired buffer index. The memory range described by the submission queue entry's addr and len fields must fall within the indexed buffer.
It is perfectly valid to setup a large buffer and then only use part of it for an I/O, as long as the range is within the originally mapped region.
An application can increase or decrease the size or number of registered buffers by first unregistering the existing buffers, and then issuing a new call to io_uring_register() with the new buffers.
Note that registering buffers will wait for the ring to idle. If the application currently has requests in-flight, the registration will wait for those to finish before proceeding.
An application need not unregister buffers explicitly before shutting down the io_uring instance. Available since 5.1.
This operation takes no argument, and arg must be passed as NULL. All previously registered buffers associated with the io_uring instance will be released. Available since 5.1.
Register files for I/O. arg contains a pointer to an array of nr_args file descriptors (signed 32 bit integers).
To make use of the registered files, the IOSQE_FIXED_FILE flag must be set in the flags member of the struct io_uring_sqe, and the fd member is set to the index of the file in the file descriptor array.
The file set may be sparse, meaning that the fd field in the array may be set to -1. See IORING_REGISTER_FILES_UPDATE for how to update files in place.
Note that registering files will wait for the ring to idle. If the application currently has requests in-flight, the registration will wait for those to finish before proceeding. See IORING_REGISTER_FILES_UPDATE for how to update an existing set without that limitation.
Files are automatically unregistered when the io_uring instance is torn down. An application need only unregister if it wishes to register a new set of fds. Available since 5.1.
This operation replaces existing files in the registered file set with new ones, either turning a sparse entry (one where fd is equal to -1) into a real one, removing an existing entry (new one is set to -1), or replacing an existing entry with a new existing entry.
arg must contain a pointer to a struct io_uring_files_update, which contains an offset on which to start the update, and an array of file descriptors to use for the update. nr_args must contain the number of descriptors in the passed in array. Available since 5.5.
File descriptors can be skipped if they are set to IORING_REGISTER_FILES_SKIP. Skipping an fd will not touch the file associated with the previous fd at that index. Available since 5.12.
This operation requires no argument, and arg must be passed as NULL. All previously registered files associated with the io_uring instance will be unregistered. Available since 5.1.
It's possible to use eventfd(2) to get notified of completion events on an io_uring instance. If this is desired, an eventfd file descriptor can be registered through this operation. arg must contain a pointer to the eventfd file descriptor, and nr_args must be 1. Available since 5.2.
An application can temporarily disable notifications, coming through the registered eventfd, by setting the IORING_CQ_EVENTFD_DISABLED bit in the flags field of the CQ ring. Available since 5.8.
This works just like IORING_REGISTER_EVENTFD , except notifications are only posted for events that complete in an async manner. This means that events that complete inline while being submitted do not trigger a notification event. The arguments supplied are the same as for IORING_REGISTER_EVENTFD. Available since 5.6.
Unregister an eventfd file descriptor to stop notifications. Since only one eventfd descriptor is currently supported, this operation takes no argument, and arg must be passed as NULL and nr_args must be zero. Available since 5.2.
This operation returns a structure, io_uring_probe, which contains information about the opcodes supported by io_uring on the running kernel. arg must contain a pointer to a struct io_uring_probe, and nr_args must contain the size of the ops array in that probe struct. The ops array is of the type io_uring_probe_op, which holds the value of the opcode and a flags field. If the flags field has IO_URING_OP_SUPPORTED set, then this opcode is supported on the running kernel. Available since 5.6.
This operation registers credentials of the running application with io_uring, and returns an id associated with these credentials. Applications wishing to share a ring between separate users/processes can pass in this credential id in the sqe personality field. If set, that particular sqe will be issued with these credentials. Must be invoked with arg set to NULL and nr_args set to zero. Available since 5.6.
This operation unregisters a previously registered personality with io_uring. nr_args must be set to the id in question, and arg must be set to NULL. Available since 5.6.
This operation enables an io_uring ring started in a disabled state (IORING_SETUP_R_DISABLED was specified in the call to io_uring_setup(2)). While the io_uring ring is disabled, submissions are not allowed and registrations are not restricted.
After the execution of this operation, the io_uring ring is enabled: submissions and registration are allowed, but they will be validated following the registered restrictions (if any). This operation takes no argument, must be invoked with arg set to NULL and nr_args set to zero. Available since 5.10.
arg points to a struct io_uring_restriction array of nr_args entries.
With an entry it is possible to allow an io_uring_register() opcode, or specify which opcode and flags of the submission queue entry are allowed, or require certain flags to be specified (these flags must be set on each submission queue entry).
All the restrictions must be submitted with a single io_uring_register() call and they are handled as an allowlist (opcodes and flags not registered, are not allowed).
Restrictions can be registered only if the io_uring ring started in a disabled state (IORING_SETUP_R_DISABLED must be specified in the call to io_uring_setup(2)).
Available since 5.10.
On success, io_uring_register() returns 0. On error, -1 is returned, and errno is set accordingly.
The opcode field is not allowed due to registered restrictions.
One or more fds in the fd array are invalid.
IORING_REGISTER_ENABLE_RINGS or IORING_REGISTER_RESTRICTIONS was specified, but the io_uring ring is not disabled.
IORING_REGISTER_BUFFERS or IORING_REGISTER_FILES or IORING_REGISTER_RESTRICTIONS was specified, but there were already buffers, files, or restrictions registered.
buffer is outside of the process' accessible address space, or iov_len is greater than 1GiB.
IORING_REGISTER_BUFFERS or IORING_REGISTER_FILES was specified, but nr_args is 0.
IORING_REGISTER_BUFFERS was specified, but nr_args exceeds UIO_MAXIOV
IORING_UNREGISTER_BUFFERS or IORING_UNREGISTER_FILES was specified, and nr_args is non-zero or arg is non-NULL.
IORING_REGISTER_RESTRICTIONS was specified, but nr_args exceeds the maximum allowed number of restrictions or restriction opcode is invalid.
IORING_REGISTER_FILES was specified and nr_args exceeds the maximum allowed number of files in a fixed file set.
IORING_REGISTER_FILES was specified and adding nr_args file references would exceed the maximum allowed number of files the user is allowed to have according to the RLIMIT_NOFILE resource limit and the caller does not have CAP_SYS_RESOURCE capability. Note that this is a per user limit, not per process.
Insufficient kernel resources are available, or the caller had a non-zero RLIMIT_MEMLOCK soft resource limit, but tried to lock more memory than the limit permitted. This limit is not enforced if the process is privileged (CAP_IPC_LOCK).
IORING_UNREGISTER_BUFFERS or IORING_UNREGISTER_FILES was specified, but there were no buffers or files registered.
Attempt to register files or buffers on an io_uring instance that is already undergoing file or buffer registration, or is being torn down.
User buffers point to file-backed memory.
io_uring(7), io_uring_enter(2), io_uring_setup(2), syscalls(2).