io_uring_register(2)

NAME
io_uring_register - register files or user buffers for asynchronous I/O

SYNOPSIS
#include <liburing.h>

int io_uring_register(unsigned int fd, unsigned int opcode,
                      void *arg, unsigned int nr_args);

DESCRIPTION

The io_uring_register(2) 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). If opcode has the flag IORING_REGISTER_USE_REGISTERED_RING ored into it, fd is instead the index of a registered ring fd. opcode can be one of:

IORING_REGISTER_BUFFERS
    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(2) with the new buffers. Note that before 5.13 registering buffers would 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. Note, however, that shutdown processing may run asynchronously within the kernel. As a result, it is not guaranteed that pages are immediately unpinned in this case. Available since 5.1.

IORING_REGISTER_BUFFERS2
    Register buffers for I/O. Similar to IORING_REGISTER_BUFFERS but aims to have a more extensible ABI. arg points to a struct io_uring_rsrc_register, and nr_args should be set to the number of bytes in the structure.

        struct io_uring_rsrc_register {
            __u32 nr;
            __u32 flags;
            __u64 resv2;
            __aligned_u64 data;
            __aligned_u64 tags;
        };

    The data field contains a pointer to a struct iovec array of nr entries. The tags field should either be 0, then tagging is disabled, or point to an array of nr "tags" (unsigned 64 bit integers). If a tag is zero, then tagging for this particular resource (a buffer in this case) is disabled. Otherwise, after the resource had been unregistered and it's not used anymore, a CQE will be posted with user_data set to the specified tag and all other fields zeroed. The flags field supports the following flags:

    IORING_RSRC_REGISTER_SPARSE If set, io_uring will register nr empty buffers, which need to be updated before use. When this flag is set, data and tags must be NULL. Available since 5.19.

    Note that resource updates, e.g. IORING_REGISTER_BUFFERS_UPDATE, don't necessarily deallocate resources by the time it returns, but they might be held alive until all requests using it complete. Available since 5.13.

IORING_REGISTER_BUFFERS_UPDATE
    Updates registered buffers with new ones, either turning a sparse entry into a real one, or replacing an existing entry. arg must contain a pointer to a struct io_uring_rsrc_update2, which contains an offset on which to start the update, and an array of struct iovec. tags points to an array of tags. nr must contain the number of descriptors in the passed in arrays. See IORING_REGISTER_BUFFERS2 for the resource tagging description.

    struct io_uring_rsrc_update2 {
        __u32 offset;
        __u32 resv;
        __aligned_u64 data;
        __aligned_u64 tags;
        __u32 nr;
        __u32 resv2;
    };

Available since 5.13.

IORING_UNREGISTER_BUFFERS
    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 synchronously. Available since 5.1.

IORING_REGISTER_FILES
    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 before 5.13 registering files would 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 needs only unregister if it wishes to register a new set of fds. Available since 5.1.

IORING_REGISTER_FILES2
    Register files for I/O. Similar to IORING_REGISTER_FILES. arg points to a struct io_uring_rsrc_register, and nr_args should be set to the number of bytes in the structure. The data field contains a pointer to an array of nr file descriptors (signed 32 bit integers). tags field should either be 0 or or point to an array of nr "tags" (unsigned 64 bit integers). See IORING_REGISTER_BUFFERS2 for more info on resource tagging. Note that resource updates, e.g. IORING_REGISTER_FILES_UPDATE, don't necessarily deallocate resources, they might be held until all requests using that resource complete. Available since 5.13.

IORING_REGISTER_FILES_UPDATE
    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.

IORING_REGISTER_FILES_UPDATE2
    Similar to IORING_REGISTER_FILES_UPDATE, 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_rsrc_update2, which contains an offset on which to start the update, and an array of file descriptors to use for the update stored in data. tags points to an array of tags. nr must contain the number of descriptors in the passed in arrays. See IORING_REGISTER_BUFFERS2 for the resource tagging description. Available since 5.13.

IORING_UNREGISTER_FILES
    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.

IORING_REGISTER_EVENTFD
    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. Note that while io_uring generally takes care to avoid spurious events, they can occur. Similarly, batched completions of CQEs may only trigger a single eventfd notification even if multiple CQEs are posted. The application should make no assumptions on number of events being available having a direct correlation to eventfd notifications posted. An eventfd notification must thus only be treated as a hint to check the CQ ring for completions. 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.

IORING_REGISTER_EVENTFD_ASYNC
    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.

IORING_UNREGISTER_EVENTFD
    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.

IORING_REGISTER_PROBE
    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.

IORING_REGISTER_PERSONALITY
    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.

IORING_UNREGISTER_PERSONALITY
    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.

IORING_REGISTER_ENABLE_RINGS
    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.

IORING_REGISTER_RESTRICTIONS
    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(2) 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(2) 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.

IORING_REGISTER_IOWQ_AFF
    By default, async workers created by io_uring will inherit the CPU mask of its parent. This is usually all the CPUs in the system, unless the parent is being run with a limited set. If this isn't the desired outcome, the application may explicitly tell io_uring what CPUs the async workers may run on. arg must point to a cpu_set_t mask, and nr_args the byte size of that mask. Available since 5.14.

IORING_UNREGISTER_IOWQ_AFF
    Undoes a CPU mask previously set with IORING_REGISTER_IOWQ_AFF. Must not have arg or nr_args set. Available since 5.14.

IORING_REGISTER_IOWQ_MAX_WORKERS
    By default, io_uring limits the unbounded workers created to the maximum processor count set by RLIMIT_NPROC and the bounded workers is a function of the SQ ring size and the number of CPUs in the system. Sometimes this can be excessive (or too little, for bounded), and this command provides a way to change the count per ring (per NUMA node) instead. arg must be set to an unsigned int pointer to an array of two values, with the values in the array being set to the maximum count of workers per NUMA node. Index 0 holds the bounded worker count, and index 1 holds the unbounded worker count. On successful return, the passed in array will contain the previous maximum values for each type. If the count being passed in is 0, then this command returns the current maximum values and doesn't modify the current setting. nr_args must be set to 2, as the command takes two values. Available since 5.15.

IORING_REGISTER_RING_FDS
    Whenever io_uring_enter(2) is called to submit request or wait for completions, the kernel must grab a reference to the file descriptor. If the application using io_uring is threaded, the file table is marked as shared, and the reference grab and put of the file descriptor count is more expensive than it is for a non-threaded application. Similarly to how io_uring allows registration of files, this allow registration of the ring file descriptor itself. This reduces the overhead of the io_uring_enter(2) system call. arg must be set to a pointer to an array of type struct io_uring_rsrc_update of nr_args number of entries. The data field of this struct must contain an io_uring file descriptor, and the offset field can be either -1 or an explicit offset desired for the registered file descriptor value. If -1 is used, then upon successful return of this system call, the field will contain the value of the registered file descriptor to be used for future io_uring_enter(2) system calls. On successful completion of this request, the returned descriptors may be used instead of the real file descriptor for io_uring_enter(2), provided that IORING_ENTER_REGISTERED_RING is set in the flags for the system call. This flag tells the kernel that a registered descriptor is used rather than a real file descriptor. Each thread or process using a ring must register the file descriptor directly by issuing this request. The maximum number of supported registered ring descriptors is currently limited to 16. Available since 5.18.

IORING_UNREGISTER_RING_FDS
    Unregister descriptors previously registered with IORING_REGISTER_RING_FDS. arg must be set to a pointer to an array of type struct io_uring_rsrc_update of nr_args number of entries. Only the offset field should be set in the structure, containing the registered file descriptor offset previously returned from IORING_REGISTER_RING_FDS that the application wishes to unregister. Note that this isn't done automatically on ring exit, if the thread or task that previously registered a ring file descriptor isn't exiting. It is recommended to manually unregister any previously registered ring descriptors if the ring is closed and the task persists. This will free up a registration slot, making it available for future use. Available since 5.18.

IORING_REGISTER_PBUF_RING
    Registers a shared buffer ring to be used with provided buffers. This is a newer alternative to using IORING_OP_PROVIDE_BUFFERS which is more efficient, to be used with request types that support the IOSQE_BUFFER_SELECT flag. The arg argument must be filled in with the appropriate information. It looks as follows:

    struct io_uring_buf_reg {
        __u64 ring_addr;
        __u32 ring_entries;
        __u16 bgid;
        __u16 pad;
        __u64 resv[3];
    };

The ring_addr field must contain the address to the memory allocated to fit this ring. The memory must be page aligned and hence allocated appropriately using eg posix_memalign(3) or similar. The size of the ring is the product of ring_entries and the size of struct io_uring_buf. ring_entries is the desired size of the ring, and must be a power-of-2 in size. The maximum size allowed is 2^15 (32768). bgid is the buffer group ID associated with this ring. SQEs that select a buffer have a buffer group associated with them in their buf_group field, and the associated CQEs will have IORING_CQE_F_BUFFER set in their flags member, which will also contain the specific ID of the buffer selected. The rest of the fields are reserved and must be cleared to zero. nr_args must be set to 1. Also see io_uring_register_buf_ring(3) for more details. Available since 5.19.

IORING_UNREGISTER_PBUF_RING
    Unregister a previously registered provided buffer ring. arg must be set to the address of a struct io_uring_buf_reg, with just the bgid field set to the buffer group ID of the previously registered provided buffer group. nr_args must be set to 1. Also see IORING_REGISTER_PBUF_RING. Available since 5.19.

IORING_REGISTER_SYNC_CANCEL
    Performs a synchronous cancelation request, which works in a similar fashion to IORING_OP_ASYNC_CANCEL except it completes inline. This can be useful for scenarios where cancelations should happen synchronously, rather than needing to issue an SQE and wait for completion of that specific CQE. arg must be set to a pointer to a struct io_uring_sync_cancel_reg structure, with the details filled in for what request(s) to target for cancelation. See io_uring_register_sync_cancel(3) for details on that. The return values are the same, except they are passed back synchronously rather than through the CQE res field. nr_args must be set to 1. Available since 6.0.

IORING_REGISTER_FILE_ALLOC_RANGE
    sets the allowable range for fixed file index allocations within the kernel. When requests that can instantiate a new fixed file are used with IORING_FILE_INDEX_ALLOC, the application is asking the kernel to allocate a new fixed file descriptor rather than pass in a specific value for one. By default, the kernel will pick any available fixed file descriptor within the range available. This effectively allows the application to set aside a range just for dynamic allocations, with the remainder being used for specific values. nr_args must be set to 1 and arg must be set to a pointer to a struct io_uring_file_index_range:

    struct io_uring_file_index_range {
        __u32 off;
        __u32 len;
        __u64 resv;
    };

with off being set to the starting value for the range, and len being set to the number of descriptors. The reserved resv field must be cleared to zero. The application must have registered a file table first. Available since 6.0.

IORING_REGISTER_PBUF_STATUS
    Can be used to retrieve the current head of a ringbuffer provided earlier via IORING_REGISTER_PBUF_RING. arg must point to a

    struct io_uring_buf_status {
    	__u32	buf_group;	/* input */
    	__u32	head;		/* output */
    	__u32	resv[8];
    };

of which arg->buf_group should contain the buffer group ID for the buffer ring in question, nr_args should be set to 1 and arg->resv should be zeroed out. The current head of the ringbuffer will be returned in arg->head. Available since 6.8.

IORING_REGISTER_NAPI
    Registers a napi instance with the io_uring instance of fd. arg should point to a

    struct io_uring_napi {
    	__u32	busy_poll_to;
    	__u8	prefer_busy_poll;
    	__u8	pad[3];
    	__u64	resv;
    };

in which arg->busy_poll_to should contain the busy poll timeout in micro seconds and arg->prefer_busy_poll should specify whether busy polling should be used rather than IRQs. nr_args should be set to 1 and arg->pad and arg->resv should be zeroed out. On successful return the io_uring_napi struct pointed to by arg will contain the previously used settings. Must be used with a ring initially created with IORING_SETUP_IOPOLL. Available since 6.9.

IORING_UNREGISTER_NAPI
    Unregisters a napi instance previously registered via IORING_REGISTER_NAPI to the io_uring instance of fd. arg should point to a struct io_uring_napi. On successful return the io_uring_napi struct pointed to by arg will contain the previously used settings. Available since 6.9.

IORING_REGISTER_CLOCK
    Specifies which clock id io_uring will use for timers while waiting for completion events with IORING_ENTER_GETEVENTS. It's only effective if the timeout argument in struct io_uring_getevents_arg is passed, ignored otherwise. When used in conjunction with IORING_ENTER_ABS_TIMER, interprets the timeout argument as absolute time of the specified clock. The default clock is CLOCK_MONOTONIC. Available since 6.12 and supports CLOCK_MONOTONIC and CLOCK_BOOTTIME.

IORING_REGISTER_CLONE_BUFFERS
    Supports cloning buffers from a source ring to a destination ring, duplicating previously registered buffers from source to destination. arg must be set to a pointer to a struct io_uring_clone_buffers and nr_args must be set to 1 . struct io_uring_buf_reg looks as follows:

    struct io_uring_clone_buffers {
        __u32 src_fd;
        __u32 flags;
        __u32 src_off;
        __u32 dst_off;
        __u32 nr;
        __u32 pad[3];
    };

where
    src_fd indicates the fd of the source ring, flags are modifier flags for the operation, src_off indicates the offset from where to start the cloning from the source ring, dst_off indicates the offset from where to start the cloning into the destination ring, and nr indicates the number of buffers to clone at the given offsets. pad must be zero filled. Kernel 6.12 added support for full range cloning, where src_off, dst_off, and nr must all be set to 0, indicating cloning of the entire table in source to destination. Kernel 6.13 added support for specifying the offsets and how many buffers to clone. Additionally, it added support for cloning into a previously registered table in the destination as well, 6.12 would fail that operation with -EBUSY if attempted. To replace existing nodes, or clone into an existing table, IORING_REGISTER_DST_REPLACE must be set in the flags member.

IORING_REGISTER_SEND_MSG_RING
    Supports sending of the equivalent of a IORING_OP_MSG_RING request, but without having a source ring available. Takes a pointer to a structio_uring_sqe which must be prepared with io_uring_prep_msg_ring(3) before being submitted. Only supports IORING_MSG_DATA type of requests. Available since kernel 6.12.

IORING_REGISTER_RESIZE_RINGS
    Supports resizing the SQ and CQ rings. Takes a pointer to a structio_uring_params as the argument, where sq_entries and cq_entries may be set to the desired values. Only supports a limited set of flags set in the structio_uring_params argument, notably IORING_SETUP_CQSIZE and IORING_SETUP_CLAMP to modify the CQ ring sizing. See io_uring_resize_rings(3) for details. Note that while liburing takes care of the ring unmap and mapping for a resize operation, manual users of this register syscall must perform those operations, similarly to when a new ring is created. The structio_uring_params structure will get the necessary offsets copied back upon successful completion of this system call, which can be used to memory map the ring just like how a new ring would've been mapped. Available since kernel 6.13.

IORING_REGISTER_CQWAIT_REG
    Supports registering fixed wait regions, avoiding unnecessary copying in of structio_uring_getevents_arg for wait operations that specify a timeout or minimum timeout. Takes a pointer to a structio_uring_cqwait_reg_arg structure, which looks as follows:

    struct io_uring_cqwait_reg_arg {
        __u32 flags;
        __u32 struct_size;
        __u32 nr_entries;
        __u32 pad;
        __u64 user_addr;
        __u64 pad2[2];
    };

where
    flags specifies modifier flags (must currently be 0 ), struct_size must be set to the size of the struct, and user_addr must be set to the region being registered as wait regions. The pad fields must all be cleared to 0 . Each wait regions looks as follows:

    struct io_uring_reg_wait {
        struct __kernel_timespec ts;
        __u32                    min_wait_usec;
        __u32                    flags;
        __u64                    sigmask;
        __u32                    sigmask_sz;
        __u32                    pad[3];
        __u64                    pad2[2];
    };

where
    ts holds the timeout information for this region flags holds information about the timeout region, sigmask is a pointer to a signal mask, if used, and sigmask_sz is the size of that signal mask. The pad fields must all be cleared to 0 . Currently the only valid flag is IORING_REG_WAIT_TS , which, if set, says that the values in ts are valid and should be used for a timeout operation. The user_addr field of structio_uring_cqwait_reg_arg must be set to an address of structio_uring_cqwait_reg members, an up to a page size can be mapped. At the size of 64 bytes per region, that allows at least 64 individual regions on a 4k page size system. The offsets of these regions are used for an io_uring_enter(2) system call, with the first one being 0, second one 1, and so forth. After registration of the wait regions, io_uring_enter(2) may be used with the enter flag of IORING_ENTER_EXT_ARG_REG and an argp set to the wait region offset, rather than a pointer to a structio_uring_getevent_arg structure. If used with IORING_ENTER_GETEVENTS , then the wait operation will use the information in the registered wait region rather than needing a io_uring_getevent_arg structure copied for each operation. For high frequency waits, this can save considerable CPU cycles. Note: once a region has been registered, it cannot get unregistered. It lives for the life of the ring. Individual wait region offset may be modified before any io_uring_enter(2) system call. Available since kernel 6.13.

RETURN VALUE
On success, io_uring_register(2) returns either 0 or a positive value, depending on the opcode used. On error, a negative error value is returned. The caller should not rely on the errno variable.

ERRORS

EACCES
    The opcode field is not allowed due to registered restrictions.

EBADF
    One or more fds in the fd array are invalid.

EBADFD
    IORING_REGISTER_ENABLE_RINGS or IORING_REGISTER_RESTRICTIONS was specified, but the io_uring ring is not disabled.

EBUSY
    IORING_REGISTER_BUFFERS or IORING_REGISTER_FILES or IORING_REGISTER_RESTRICTIONS was specified, but there were already buffers, files, or restrictions registered.

EEXIST
    The thread performing the registration is invalid.

EFAULT
    buffer is outside of the process' accessible address space, or iov_len is greater than 1GiB.

EINVAL
    IORING_REGISTER_BUFFERS or IORING_REGISTER_FILES was specified, but nr_args is 0.

EINVAL
    IORING_REGISTER_BUFFERS was specified, but nr_args exceeds UIO_MAXIOV

EINVAL
    IORING_UNREGISTER_BUFFERS or IORING_UNREGISTER_FILES was specified, and nr_args is non-zero or arg is non-NULL.

EINVAL
    IORING_REGISTER_RESTRICTIONS was specified, but nr_args exceeds the maximum allowed number of restrictions or restriction opcode is invalid.

EMFILE
    IORING_REGISTER_FILES was specified and nr_args exceeds the maximum allowed number of files in a fixed file set.

EMFILE
    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.

ENOMEM
    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).

ENXIO
    IORING_UNREGISTER_BUFFERS or IORING_UNREGISTER_FILES was specified, but there were no buffers or files registered.

ENXIO
    Attempt to register files or buffers on an io_uring instance that is already undergoing file or buffer registration, or is being torn down.

EOPNOTSUPP
    User buffers point to file-backed memory.

EFAULT
    User buffers point to file-backed memory (newer kernels).

ENOENT
    IORING_REGISTER_PBUF_STATUS was specified, but buf_group did not refer to a currently valid buffer group.

EINVAL
    IORING_REGISTER_PBUF_STATUS was specified, but the valid buffer group specified by buf_group did not refer to a buffer group registered via IORING_REGISTER_PBUF_RING.

EINVAL
    IORING_REGISTER_NAPI was specified, but the ring associated with fd has not been created with IORING_SETUP_IOPOLL.