io_uring_enter - Man Page

initiate and/or complete asynchronous I/O

Synopsis

#include <linux/io_uring.h>

int io_uring_enter(unsigned int fd, unsigned int to_submit,
                   unsigned int min_complete, unsigned int flags,
                   sigset_t *sig);

Description

io_uring_enter() is used to initiate and complete I/O using the shared submission and completion queues setup by a call to io_uring_setup(2). A single call can both submit new I/O and wait for completions of I/O initiated by this call or previous calls to io_uring_enter().

fd is the file descriptor returned by io_uring_setup(2). to_submit specifies the number of I/Os to submit from the submission queue.  If the IORING_ENTER_GETEVENTS bit is set in flags, then the system call will attempt to wait for min_complete event completions before returning.  If the io_uring instance was configured for polling, by specifying IORING_SETUP_IOPOLL in the call to io_uring_setup(2), then min_complete has a slightly different meaning.  Passing a value of 0 instructs the kernel to return any events which are already complete, without blocking.  If min_complete is a non-zero value, the kernel will still return immediately if any completion events are available.  If no event completions are available, then the call will poll either until one or more completions become available, or until the process has exceeded its scheduler time slice.

Note that, for interrupt driven I/O (where IORING_SETUP_IOPOLL was not specified in the call to io_uring_setup(2)), an application may check the completion queue for event completions without entering the kernel at all.

When the system call returns that a certain amount of SQEs have been consumed and submitted, it's safe to reuse SQE entries in the ring. This is true even if the actual IO submission had to be punted to async context, which means that the SQE may in fact not have been submitted yet. If the kernel requires later use of a particular SQE entry, it will have made a private copy of it.

sig is a pointer to a signal mask (see sigprocmask(2)); if sig is not NULL, io_uring_enter() first replaces the current signal mask by the one pointed to by sig, then waits for events to become available in the completion queue, and then restores the original signal mask.  The following io_uring_enter() call:

ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, &sig);

is equivalent to atomically executing the following calls:

pthread_sigmask(SIG_SETMASK, &sig, &orig);
ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, NULL);
pthread_sigmask(SIG_SETMASK, &orig, NULL);

See the description of pselect(2) for an explanation of why the sig parameter is necessary.

Submission queue entries are represented using the following data structure:

/*
 * IO submission data structure (Submission Queue Entry)
 */
struct io_uring_sqe {
    __u8    opcode;         /* type of operation for this sqe */
    __u8    flags;          /* IOSQE_ flags */
    __u16   ioprio;         /* ioprio for the request */
    __s32   fd;             /* file descriptor to do IO on */
    union {
        __u64   off;            /* offset into file */
        __u64   addr2;
    };
    union {
        __u64   addr;       /* pointer to buffer or iovecs */
        __u64   splice_off_in;
    }
    __u32   len;            /* buffer size or number of iovecs */
    union {
        __kernel_rwf_t  rw_flags;
        __u32    fsync_flags;
        __u16    poll_events;   /* compatibility */
        __u32    poll32_events; /* word-reversed for BE */
        __u32    sync_range_flags;
        __u32    msg_flags;
        __u32    timeout_flags;
        __u32    accept_flags;
        __u32    cancel_flags;
        __u32    open_flags;
        __u32    statx_flags;
        __u32    fadvise_advice;
        __u32    splice_flags;
    };
    __u64    user_data;     /* data to be passed back at completion time */
    union {
	struct {
	    /* index into fixed buffers, if used */
            union {
                /* index into fixed buffers, if used */
                __u16    buf_index;
                /* for grouped buffer selection */
                __u16    buf_group;
            }
	    /* personality to use, if used */
	    __u16    personality;
            __s32    splice_fd_in;
	};
        __u64    __pad2[3];
    };
};

The opcode describes the operation to be performed.  It can be one of:

IORING_OP_NOP

Do not perform any I/O.  This is useful for testing the performance of the io_uring implementation itself.

IORING_OP_READV
IORING_OP_WRITEV

Vectored read and write operations, similar to preadv2(2) and pwritev2(2).

IORING_OP_READ_FIXED
IORING_OP_WRITE_FIXED

Read from or write to pre-mapped buffers.  See io_uring_register(2) for details on how to setup a context for fixed reads and writes.

IORING_OP_FSYNC

File sync.  See also fsync(2). Note that, while I/O is initiated in the order in which it appears in the submission queue, completions are unordered.  For example, an application which places a write I/O followed by an fsync in the submission queue cannot expect the fsync to apply to the write.  The two operations execute in parallel, so the fsync may complete before the write is issued to the storage.  The same is also true for previously issued writes that have not completed prior to the fsync.

IORING_OP_POLL_ADD

Poll the fd specified in the submission queue entry for the events specified in the poll_events field.  Unlike poll or epoll without EPOLLONESHOT, this interface always works in one shot mode.  That is, once the poll operation is completed, it will have to be resubmitted.

IORING_OP_POLL_REMOVE

Remove an existing poll request.  If found, the res field of the struct io_uring_cqe will contain 0.  If not found, res will contain -ENOENT.

IORING_OP_EPOLL_CTL

Add, remove or modify entries in the interest list of epoll(7). See epoll_ctl(2) for details of the system call. fd holds the file descriptor that represents the epoll instance, addr holds the file descriptor to add, remove or modify, len holds the operation (EPOLL_CTL_ADD, EPOLL_CTL_DEL, EPOLL_CTL_MOD) to perform and, off holds a pointer to the epoll_events structure. Available since 5.6.

IORING_OP_SYNC_FILE_RANGE

Issue the equivalent of a sync_file_range (2) on the file descriptor. The fd field is the file descriptor to sync, the off field holds the offset in bytes, the len field holds the length in bytes, and the sync_range_flags field holds the flags for the command. See also sync_file_range(2) for the general description of the related system call. Available since 5.2.

IORING_OP_SENDMSG

Issue the equivalent of a sendmsg(2) system call. fd must be set to the socket file descriptor, addr must contain a pointer to the msghdr structure, and msg_flags holds the flags associated with the system call. See also sendmsg(2) for the general description of the related system call. Available since 5.3.

IORING_OP_RECVMSG

Works just like IORING_OP_SENDMSG, except for recvmsg(2) instead. See the description of IORING_OP_SENDMSG. Available since 5.3.

IORING_OP_SEND

Issue the equivalent of a send(2) system call. fd must be set to the socket file descriptor, addr must contain a pointer to the buffer, len denotes the length of the buffer to send, and msg_flags holds the flags associated with the system call. See also send(2) for the general description of the related system call. Available since 5.6.

IORING_OP_RECV

Works just like IORING_OP_SEND, except for recv(2) instead. See the description of IORING_OP_SEND. Available since 5.6.

IORING_OP_TIMEOUT

This command will register a timeout operation. The addr field must contain a pointer to a struct timespec64 structure, len must contain 1 to signify one timespec64 structure, timeout_flags may contain IORING_TIMEOUT_ABS for an absolute timeout value, or 0 for a relative timeout. off may contain a completion event count. A timeout will trigger a wakeup event on the completion ring for anyone waiting for events. A timeout condition is met when either the specified timeout expires, or the specified number of events have completed. Either condition will trigger the event. If set to 0, completed events are not counted, which effectively acts like a timer. io_uring timeouts use the CLOCK_MONOTONIC clock source. The request will complete with -ETIME if the timeout got completed through expiration of the timer, or 0 if the timeout got completed through requests completing on their own. If the timeout was cancelled before it expired, the request will complete with -ECANCELED. Available since 5.4.

IORING_OP_TIMEOUT_REMOVE

Attempt to remove an existing timeout operation. addr must contain the user_data field of the previously issued timeout operation. If the specified timeout request is found and cancelled successfully, this request will terminate with a result value of 0 If the timeout request was found but expiration was already in progress, this request will terminate with a result value of -EBUSY If the timeout request wasn't found, the request will terminate with a result value of -ENOENT Available since 5.5.

IORING_OP_ACCEPT

Issue the equivalent of an accept4(2) system call. fd must be set to the socket file descriptor, addr must contain the pointer to the sockaddr structure, and addr2 must contain a pointer to the socklen_t addrlen field. See also accept4(2) for the general description of the related system call. Available since 5.5.

IORING_OP_ASYNC_CANCEL

Attempt to cancel an already issued request. addr must contain the user_data field of the request that should be cancelled. The cancellation request will complete with one of the following results codes. If found, the res field of the cqe will contain 0. If not found, res will contain -ENOENT. If found and attempted cancelled, the res field will contain -EALREADY. In this case, the request may or may not terminate. In general, requests that are interruptible (like socket IO) will get cancelled, while disk IO requests cannot be cancelled if already started. Available since 5.5.

IORING_OP_LINK_TIMEOUT

This request must be linked with another request through IOSQE_IO_LINK which is described below. Unlike IORING_OP_TIMEOUT, IORING_OP_LINK_TIMEOUT acts on the linked request, not the completion queue. The format of the command is otherwise like IORING_OP_TIMEOUT, except there's no completion event count as it's tied to a specific request. If used, the timeout specified in the command will cancel the linked command, unless the linked command completes before the timeout. The timeout will complete with -ETIME if the timer expired and the linked request was attempted cancelled, or -ECANCELED if the timer got cancelled because of completion of the linked request. Like IORING_OP_TIMEOUT the clock source used is CLOCK_MONOTONIC Available since 5.5.

IORING_OP_CONNECT

Issue the equivalent of a connect(2) system call. fd must be set to the socket file descriptor, addr must contain the const pointer to the sockaddr structure, and off must contain the socklen_t addrlen field. See also connect(2) for the general description of the related system call. Available since 5.5.

IORING_OP_FALLOCATE

Issue the equivalent of a fallocate(2) system call. fd must be set to the file descriptor, off must contain the offset on which to operate, and len must contain the length. See also fallocate(2) for the general description of the related system call. Available since 5.6.

IORING_OP_FADVISE

Issue the equivalent of a posix_fadvise(2) system call. fd must be set to the file descriptor, off must contain the offset on which to operate, len must contain the length, and fadvise_advice must contain the advice associated with the operation. See also posix_fadvise(2) for the general description of the related system call. Available since 5.6.

IORING_OP_MADVISE

Issue the equivalent of a madvise(2) system call. addr must contain the address to operate on, len must contain the length on which to operate, and fadvise_advice must contain the advice associated with the operation. See also madvise(2) for the general description of the related system call. Available since 5.6.

IORING_OP_OPENAT

Issue the equivalent of a openat(2) system call. fd is the dirfd argument, addr must contain a pointer to the *pathname argument, open_flags should contain any flags passed in, and mode is access mode of the file. See also openat(2) for the general description of the related system call. Available since 5.6.

IORING_OP_OPENAT2

Issue the equivalent of a openat2(2) system call. fd is the dirfd argument, addr must contain a pointer to the *pathname argument, len should contain the size of the open_how structure, and off should be set to the address of the open_how structure. See also openat2(2) for the general description of the related system call. Available since 5.6.

IORING_OP_CLOSE

Issue the equivalent of a close(2) system call. fd is the file descriptor to be closed. See also close(2) for the general description of the related system call. Available since 5.6.

IORING_OP_STATX

Issue the equivalent of a statx(2) system call. fd is the dirfd argument, addr must contain a pointer to the *pathname string, statx_flags is the flags argument, len should be the mask argument, and off must contain a pointer to the statxbuf to be filled in. See also statx(2) for the general description of the related system call. Available since 5.6.

IORING_OP_READ
IORING_OP_WRITE

Issue the equivalent of a read(2) or write(2) system call. fd is the file descriptor to be operated on, addr contains the buffer in question, and len contains the length of the IO operation. These are non-vectored versions of the IORING_OP_READV and IORING_OP_WRITEV opcodes. See also read(2) and write(2) for the general description of the related system call. Available since 5.6.

IORING_OP_SPLICE

Issue the equivalent of a splice(2) system call. splice_fd_in is the file descriptor to read from, splice_off_in is an offset to read from, fd is the file descriptor to write to, off is an offset from which to start writing to. A sentinel value of -1 is used to pass the equivalent of a NULL for the offsets to splice(2). len contains the number of bytes to copy. splice_flags contains a bit mask for the flag field associated with the system call. Please note that one of the file descriptors must refer to a pipe. See also splice(2) for the general description of the related system call. Available since 5.7.

IORING_OP_TEE

Issue the equivalent of a tee(2) system call. splice_fd_in is the file descriptor to read from, fd is the file descriptor to write to, len contains the number of bytes to copy, and splice_flags contains a bit mask for the flag field associated with the system call. Please note that both of the file descriptors must refer to a pipe. See also tee(2) for the general description of the related system call. Available since 5.8.

IORING_OP_FILES_UPDATE

This command is an alternative to using IORING_REGISTER_FILES_UPDATE which then works in an async fashion, like the rest of the io_uring commands. The arguments passed in are the same. addr must contain a pointer to the array of file descriptors, len must contain the length of the array, and off must contain the offset at which to operate. Note that the array of file descriptors pointed to in addr must remain valid until this operation has completed. Available since 5.6.

The flags field is a bit mask. The supported flags are:

IOSQE_FIXED_FILE

When this flag is specified, fd is an index into the files array registered with the io_uring instance (see the IORING_REGISTER_FILES section of the io_uring_register(2) man page). Available since 5.1.

IOSQE_IO_DRAIN

When this flag is specified, the SQE will not be started before previously submitted SQEs have completed, and new SQEs will not be started before this one completes. Available since 5.2.

IOSQE_IO_LINK

When this flag is specified, it forms a link with the next SQE in the submission ring. That next SQE will not be started before this one completes. This, in effect, forms a chain of SQEs, which can be arbitrarily long. The tail of the chain is denoted by the first SQE that does not have this flag set. This flag has no effect on previous SQE submissions, nor does it impact SQEs that are outside of the chain tail. This means that multiple chains can be executing in parallel, or chains and individual SQEs. Only members inside the chain are serialized. A chain of SQEs will be broken, if any request in that chain ends in error. io_uring considers any unexpected result an error. This means that, eg, a short read will also terminate the remainder of the chain. If a chain of SQE links is broken, the remaining unstarted part of the chain will be terminated and completed with -ECANCELED as the error code. Available since 5.3.

IOSQE_IO_HARDLINK

Like IOSQE_IO_LINK, but it doesn't sever regardless of the completion result. Note that the link will still sever if we fail submitting the parent request, hard links are only resilient in the presence of completion results for requests that did submit correctly. IOSQE_IO_HARDLINK implies IOSQE_IO_LINK. Available since 5.5.

IOSQE_ASYNC

Normal operation for io_uring is to try and issue an sqe as non-blocking first, and if that fails, execute it in an async manner. To support more efficient overlapped operation of requests that the application knows/assumes will always (or most of the time) block, the application can ask for an sqe to be issued async from the start. Available since 5.6.

ioprio specifies the I/O priority.  See ioprio_get(2) for a description of Linux I/O priorities.

fd specifies the file descriptor against which the operation will be performed, with the exception noted above.

If the operation is one of IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED, addr and len must fall within the buffer located at buf_index in the fixed buffer array.  If the operation is either IORING_OP_READV or IORING_OP_WRITEV, then addr points to an iovec array of len entries.

rw_flags, specified for read and write operations, contains a bitwise OR of per-I/O flags, as described in the preadv2(2) man page.

The fsync_flags bit mask may contain either 0, for a normal file integrity sync, or IORING_FSYNC_DATASYNC to provide data sync only semantics.  See the descriptions of O_SYNC and O_DSYNC in the open(2) manual page for more information.

The bits that may be set in poll_events are defined in <poll.h>, and documented in poll(2).

user_data is an application-supplied value that will be copied into the completion queue entry (see below). buf_index is an index into an array of fixed buffers, and is only valid if fixed buffers were registered. personality is the credentials id to use for this operation. See io_uring_register(2) for how to register personalities with io_uring. If set to 0, the current personality of the submitting task is used.

Once the submission queue entry is initialized, I/O is submitted by placing the index of the submission queue entry into the tail of the submission queue.  After one or more indexes are added to the queue, and the queue tail is advanced, the io_uring_enter(2) system call can be invoked to initiate the I/O.

Completions use the following data structure:

/*
 * IO completion data structure (Completion Queue Entry)
 */
struct io_uring_cqe {
    __u64    user_data; /* sqe->data submission passed back */
    __s32    res;       /* result code for this event */
    __u32    flags;
};

user_data is copied from the field of the same name in the submission queue entry.  The primary use case is to store data that the application will need to access upon completion of this particular I/O.  The flags is reserved for future use. res is the operation-specific result.

For read and write opcodes, the return values match those documented in the preadv2(2) and pwritev2(2) man pages. Return codes for the io_uring-specific opcodes are documented in the description of the opcodes above.

Return Value

io_uring_enter() returns the number of I/Os successfully consumed.  This can be zero if to_submit was zero or if the submission queue was empty. The errors below that refer to an error in a submission queue entry will be returned though a completion queue entry, rather than through the system call itself.

Errors that occur not on behalf of a submission queue entry are returned via the system call directly. On such an error, -1 is returned and errno is set appropriately.

Errors

EAGAIN

The kernel was unable to allocate memory for the request, or otherwise ran out of resources to handle it. The application should wait for some completions and try again.

EBUSY

The application is attempting to overcommit the number of requests it can have pending. The application should wait for some completions and try again. May occur if the application tries to queue more requests than we have room for in the CQ ring.

EBADF

The fd field in the submission queue entry is invalid, or the IOSQE_FIXED_FILE flag was set in the submission queue entry, but no files were registered with the io_uring instance.

EFAULT

buffer is outside of the process' accessible address space

EFAULT

IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED was specified in the opcode field of the submission queue entry, but either buffers were not registered for this io_uring instance, or the address range described by addr and len does not fit within the buffer registered at buf_index.

EINVAL

The index member of the submission queue entry is invalid.

EINVAL

The flags field or opcode in a submission queue entry is invalid.

EINVAL

IORING_OP_NOP was specified in the submission queue entry, but the io_uring context was setup for polling (IORING_SETUP_IOPOLL was specified in the call to io_uring_setup).

EINVAL

IORING_OP_READV or IORING_OP_WRITEV was specified in the submission queue entry, but the io_uring instance has fixed buffers registered.

EINVAL

IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED was specified in the submission queue entry, and the buf_index is invalid.

EINVAL

IORING_OP_READV, IORING_OP_WRITEV, IORING_OP_READ_FIXED, IORING_OP_WRITE_FIXED or IORING_OP_FSYNC was specified in the submission queue entry, but the io_uring instance was configured for IOPOLLing, or any of addr, ioprio, off, len, or buf_index was set in the submission queue entry.

EINVAL

IORING_OP_POLL_ADD or IORING_OP_POLL_REMOVE was specified in the opcode field of the submission queue entry, but the io_uring instance was configured for busy-wait polling (IORING_SETUP_IOPOLL), or any of ioprio, off, len, or buf_index was non-zero in the submission queue entry.

EINVAL

IORING_OP_POLL_ADD was specified in the opcode field of the submission queue entry, and the addr field was non-zero.

ENXIO

The io_uring instance is in the process of being torn down.

EOPNOTSUPP

fd does not refer to an io_uring instance.

EOPNOTSUPP

opcode is valid, but not supported by this kernel.

EINTR

The operation was interrupted by a delivery of a signal before it could complete; see signal(7). Can happen while waiting for events with IORING_ENTER_GETEVENTS.

Referenced By

io_uring_register(2), io_uring_setup(2), syscalls(2).

2019-01-22 Linux Programmer's Manual