Initiate and/or complete asynchronous I/O
# 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]. 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].
*to_submit* specifies the number of I/Os to submit from the submission
queue. *flags* is a bitmask of the following values:
**IORING_ENTER_GETEVENTS**\
If this flag is set, then the system call will wait for the specified
number of events in *min_complete* before returning. This flag can be
set along with *to_submit* to both submit and complete events in a
single system call. If this flag is set either the flag
**IORING_SETUP_DEFER_TASKRUN** must not be set or the thread issuing the
syscall must be the thread that created the io_uring associated with
*fd,* or be the thread that enabled the ring originally created with
**IORING_SETUP_R_DISABLED** via [io_uring_register] or
[io_uring_enable_rings].
**IORING_ENTER_SQ_WAKEUP**\
If the ring has been created with **IORING_SETUP_SQPOLL,** then this
flag asks the kernel to wakeup the SQ kernel thread to submit IO.
**IORING_ENTER_SQ_WAIT**\
If the ring has been created with **IORING_SETUP_SQPOLL,** then the
application has no real insight into when the SQ kernel thread has
consumed entries from the SQ ring. This can lead to a situation where
the application can no longer get a free SQE entry to submit, without
knowing when one will become available as the SQ kernel thread consumes
them. If the system call is used with this flag set, then it will wait
until at least one entry is free in the SQ ring.
**IORING_ENTER_EXT_ARG**\
By default, *arg* is a *sigset_t* pointer. If **IORING_ENTER_EXT_ARG**
is set (supported since kernel 5.11), then *arg* is instead a pointer to
a *struct io_uring_getevents_arg* and *argsz* must be set to the size of
this structure. The definition is as follows:
``` c
"structio_uring_getevents_arg{
"__u64sigmask;
"__u32sigmask_sz;
"__u32pad;
"__u64ts;
"};
```
which allows passing in both a signal mask as well as pointer to a
*struct \_\_kernel_timespec* timeout value. If *ts* is set to a valid
pointer, then this time value indicates the timeout for waiting on
events. If an application is waiting on events and wishes to stop
waiting after a specified amount of time, then this can be accomplished
directly in version 5.11 and newer by using this feature.
**IORING_ENTER_REGISTERED_RING**\
If the ring file descriptor has been registered through use of
**IORING_REGISTER_RING_FDS**, then setting this flag will tell the
kernel that the *ring_fd* passed in is the registered ring offset rather
than a normal file descriptor.
**IORING_ENTER_ABS_TIMER**\
When this flag is set, the timeout argument passed in *struct
io_uring_getevents_arg* will be interpreted as an absolute time of the
registered clock (see **IORING_REGISTER_CLOCK)** until which the waiting
should end.
Available since 6.12
**IORING_ENTER_EXT_ARG_REG**\
When this flag is set, *arg* is not a pointer to a
*struct*io_uring_getevents_arg*,* but merely an offset into an area of
wait regions previously registered with [io_uring_register] using
the **IORING_REGISTER_MEM_REGION** operation.
Available since 6.13
**IORING_ENTER_NO_IOWAIT**\
When this flag is set, the system call will not mark the waiting task as
being in iowait if it is sleeping waiting on events and there are
pending requests. This is useful if iowait isn't expected when waiting
for events. It can also prevent extra power usage by allowing the CPU to
enter lower sleep states. This flag is only available if the kernel
supports the **IORING_FEAT_NO_IOWAIT** feature.
Available since 6.15.
If the io_uring instance was configured for polling, by specifying
**IORING_SETUP_IOPOLL** in the call to [io_uring_setup], 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]), 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](https://man7.org/linux/man-pages/man2/sigprocmask.2.html)); 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:
``` c
ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, &sig);
```
is equivalent to *atomically* executing the following calls:
``` c
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](https://man7.org/linux/man-pages/man2/pselect.2.html) for an explanation of why the
*sig* parameter is necessary.
Submission queue entries are represented using the following data
structure:
``` c
/*
* 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;
struct {
__u32 cmd_op;
__u32 __pad1;
};
};
union {
__u64 addr; /* pointer to buffer or iovecs */
__u64 splice_off_in;
struct {
__u32 level;
__u32 optname;
};
};
__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;
__u32 rename_flags;
__u32 unlink_flags;
__u32 hardlink_flags;
__u32 xattr_flags;
__u32 msg_ring_flags;
__u32 uring_cmd_flags;
__u32 waitid_flags;
__u32 futex_flags;
__u32 install_fd_flags;
__u32 nop_flags;
};
__u64 user_data; /* data to be passed back at completion time */
/* pack this to avoid bogus arm OABI complaints */
union {
/* index into fixed buffers, if used */
__u16 buf_index;
/* for grouped buffer selection */
__u16 buf_group;
} __attribute__((packed));
/* personality to use, if used */
__u16 personality;
union {
__s32 splice_fd_in;
__u32 file_index;
__u32 optlen;
struct {
__u16 addr_len;
__u16 __pad3[1];
};
};
union {
struct {
__u64 addr3;
__u64 __pad2[1];
};
__u64 optval;
/*
* If the ring is initialized with IORING_SETUP_SQE128, then
* this field is used for 80 bytes of arbitrary command data
*/
__u8 cmd[0];
};
};
```
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](https://man7.org/linux/man-pages/man2/preadv2.2.html) and
[pwritev2](https://man7.org/linux/man-pages/man2/pwritev2.2.html). If the file is not seekable, *off* must be set to zero
or -1.
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**IORING_OP_READ_FIXED**\
**IORING_OP_WRITE_FIXED**\
Read from or write to pre-mapped buffers. See [io_uring_register]
for details on how to setup a context for fixed reads and writes.
<!-- -->
**IORING_OP_FSYNC**\
File sync. See also [fsync](https://man7.org/linux/man-pages/man2/fsync.2.html). Optionally *off* and *len* can be used
to specify a range within the file to be synced rather than syncing the
entire file, which is the default behavior. 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. To enforce ordering one may utilize linked SQEs,
**IOSQE_IO_DRAIN** or wait for the arrival of CQEs of requests which
have to be ordered before a given request before submitting its SQE.
<!-- -->
**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**, by default this interface always works in one shot
mode. That is, once the poll operation is completed, it will have to be
resubmitted.
If **IORING_POLL_ADD_MULTI** is set in the SQE *len* field, then the
poll will work in multi shot mode instead. That means it'll repatedly
trigger when the requested event becomes true, and hence multiple CQEs
can be generated from this single SQE. The CQE *flags* field will have
**IORING_CQE_F_MORE** set on completion if the application should expect
further CQE entries from the original request. If this flag isn't set on
completion, then the poll request has been terminated and no further
events will be generated. This mode is available since 5.13.
This command works like an async **poll(2)** and the completion event
result is the returned mask of events.
Without **IORING_POLL_ADD_MULTI** and the initial poll operation with
**IORING_POLL_ADD_MULTI** the operation is level triggered, i.e. if
there is data ready or events pending etc. at the time of submission a
corresponding CQE will be posted. Potential further completions beyond
the first caused by a **IORING_POLL_ADD_MULTI** are edge triggered.
<!-- -->
**IORING_OP_POLL_REMOVE**\
Remove or update 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,** or **-EALREADY** if the poll request was in the
process of completing already.
If **IORING_POLL_UPDATE_EVENTS** is set in the SQE *len* field, then the
request will update an existing poll request with the mask of events
passed in with this request. The lookup is based on the *user_data*
field of the original SQE submitted, and this values is passed in the
*addr* field of the SQE. If **IORING_POLL_UPDATE_USER_DATA** is set in
the SQE *len* field, then the request will update the *user_data* of an
existing poll request based on the value passed in the *off* field.
Updating an existing poll is available since 5.13.
<!-- -->
**IORING_OP_EPOLL_CTL**\
Add, remove or modify entries in the interest list of [epoll](https://man7.org/linux/man-pages/man2/epoll.2.html). See
[epoll_ctl](https://man7.org/linux/man-pages/man2/epoll_ctl.2.html) for details of the system call. *fd* holds the file
descriptor that represents the epoll instance, *off* 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,
*addr* holds a pointer to the *epoll_event* 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](https://man7.org/linux/man-pages/man2/sync_file_range.2.html) 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](https://man7.org/linux/man-pages/man2/sendmsg.2.html) for the general description of the
related system call. Available since 5.3.
This command also supports the following modifiers in *ioprio:*
**IORING_RECVSEND_POLL_FIRST** If set, io_uring will assume the socket
is currently full and attempting to send data will be unsuccessful. For
this case, io_uring will arm internal poll and trigger a send of the
data when there is enough space available. This initial send attempt can
be wasteful for the case where the socket is expected to be full,
setting this flag will bypass the initial send attempt and go straight
to arming poll. If poll does indicate that data can be sent, the
operation will proceed.
**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.
This command also supports the following modifiers in *ioprio:*
**IORING_RECVSEND_POLL_FIRST** If set, io_uring will assume the socket
is currently empty and attempting to receive data will be unsuccessful.
For this case, io_uring will arm internal poll and trigger a receive of
the data when the socket has data to be read. This initial receive
attempt can be wasteful for the case where the socket is expected to be
empty, setting this flag will bypass the initial receive attempt and go
straight to arming poll. If poll does indicate that data is ready to be
received, the operation will proceed.
**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.
This command also supports the following modifiers in *ioprio:*
**IORING_RECVSEND_POLL_FIRST** If set, io_uring will assume the socket
is currently full and attempting to send data will be unsuccessful. For
this case, io_uring will arm internal poll and trigger a send of the
data when there is enough space available. This initial send attempt can
be wasteful for the case where the socket is expected to be full,
setting this flag will bypass the initial send attempt and go straight
to arming poll. If poll does indicate that data can be sent, the
operation will proceed.
**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.
This command also supports the following modifiers in *ioprio:*
**IORING_RECVSEND_POLL_FIRST** If set, io_uring will assume the socket
is currently empty and attempting to receive data will be unsuccessful.
For this case, io_uring will arm internal poll and trigger a receive of
the data when the socket has data to be read. This initial receive
attempt can be wasteful for the case where the socket is expected to be
empty, setting this flag will bypass the initial receive attempt and go
straight to arming poll. If poll does indicate that data is ready to be
received, the operation will proceed.
**IORING_OP_TIMEOUT**\
This command will register a timeout operation. The *addr* field must
contain a pointer to a struct \_\_kernel_timespec structure, *len* must
contain 1 to signify one \_\_kernel_timespec 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** as the default 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 canceled before it
expired, the request will complete with **-ECANCELED.** Available since
5.4.
Since 5.15, this command also supports the following modifiers in
*timeout_flags:*
**IORING_TIMEOUT_BOOTTIME** If set, then the clocksource used is
**CLOCK_BOOTTIME** instead of **CLOCK_MONOTONIC**. This clocksource
differs in that it includes time elapsed if the system was suspend while
having a timeout request in-flight.
**IORING_TIMEOUT_REALTIME** If set, then the clocksource used is
**CLOCK_REALTIME** instead of **CLOCK_MONOTONIC**.
Since 5.16, **IORING_TIMEOUT_ETIME_SUCCESS** can be set in
*timeout_flags*, which will result in the expiration of the timer and
subsequent completion with **-ETIME** not being interpreted as an error.
This is mostly relevant for linked SQEs, as subsequent requests in the
chain would not get canceled by the timeout, if this flag is set. See
**IOSQE_IO_LINK** for more details on linked SQEs.
Since 6.4, **IORING_TIMEOUT_MULTISHOT** can be set in *timeout_flags*,
which will result in the timer producing multiple consecutive
completions like other multi shot operations e.g.
**IORING_OP_READ_MULTISHOT** or **IORING_POLL_ADD_MULTI**. *off* must be
set to the amount of desired completions. **IORING_TIMEOUT_MULTISHOT**
must not be used with **IORING_TIMEOUT_ABS**.
Since kernel 7.1, **IORING_TIMEOUT_IMMEDIATE_ARG** can be set in
*timeout_flags*, which causes the *addr* field to be interpreted as a
timeout value in nanoseconds rather than a pointer to a **struct
\_\_kernel_timespec.** This avoids the need to keep a timespec structure
valid in user memory until the request is submitted.
**IORING_OP_TIMEOUT_REMOVE**\
If *timeout_flags* are zero, then it attempts 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 canceled 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.
If *timeout_flags* contain **IORING_TIMEOUT_UPDATE**, instead of
removing an existing operation, it updates it. *addr* and return values
are same as before. *addr2* field must contain a pointer to a struct
\_\_kernel_timespec structure. *timeout_flags* may also contain
IORING_TIMEOUT_ABS, in which case the value given is an absolute one,
not a relative one. Available since 5.11.
<!-- -->
**IORING_OP_ACCEPT**\
Issue the equivalent of an [accept4](https://man7.org/linux/man-pages/man2/accept4.2.html) 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. Flags can be passed using the *accept_flags* field. See
also [accept4](https://man7.org/linux/man-pages/man2/accept4.2.html) for the general description of the related system
call. Available since 5.5.
If the *file_index* field is set to a positive number, the file won't be
installed into the normal file table as usual but will be placed into
the fixed file table at index *file_index* - 1. In this case, instead of
returning a file descriptor, the result will contain either 0 on success
or an error. If the index points to a valid empty slot, the installation
is guaranteed to not fail. If there is already a file in the slot, it
will be replaced, similar to **IORING_OP_FILES_UPDATE.** Please note
that only io_uring has access to such files and no other syscall can use
them. See **IOSQE_FIXED_FILE** and **IORING_REGISTER_FILES**.
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 canceled. The
cancelation 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 canceled,
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 canceled, while disk IO requests cannot be
canceled 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 canceled, or
**-ECANCELED** if the timer got canceled 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](https://man7.org/linux/man-pages/man2/connect.2.html) 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](https://man7.org/linux/man-pages/man2/connect.2.html) for the general description of the
related system call. Available since 5.5.
<!-- -->
**IORING_OP_FALLOCATE**\
Issue the equivalent of a [fallocate](https://man7.org/linux/man-pages/man2/fallocate.2.html) system call. *fd* must be set
to the file descriptor, *len* must contain the mode associated with the
operation, *off* must contain the offset on which to operate, and *addr*
must contain the length. See also [fallocate](https://man7.org/linux/man-pages/man2/fallocate.2.html) for the general
description of the related system call. Available since 5.6.
<!-- -->
**IORING_OP_FADVISE**\
Issue the equivalent of a [posix_fadvise](https://man7.org/linux/man-pages/man2/posix_fadvise.2.html) 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](https://man7.org/linux/man-pages/man2/posix_fadvise.2.html) for the general description of the related system
call. Available since 5.6.
<!-- -->
**IORING_OP_MADVISE**\
Issue the equivalent of a [madvise](https://man7.org/linux/man-pages/man2/madvise.2.html) 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](https://man7.org/linux/man-pages/man2/madvise.2.html) for the general
description of the related system call. Available since 5.6.
<!-- -->
**IORING_OP_OPENAT**\
Issue the equivalent of a [openat](https://man7.org/linux/man-pages/man2/openat.2.html) 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 *len* is access
mode of the file. See also [openat](https://man7.org/linux/man-pages/man2/openat.2.html) for the general description of
the related system call. Available since 5.6.
If the *file_index* field is set to a positive number, the file won't be
installed into the normal file table as usual but will be placed into
the fixed file table at index *file_index - 1.* In this case, instead of
returning a file descriptor, the result will contain either 0 on success
or an error. If the index points to a valid empty slot, the installation
is guaranteed to not fail. If there is already a file in the slot, it
will be replaced, similar to **IORING_OP_FILES_UPDATE.** Please note
that only io_uring has access to such files and no other syscall can use
them. See **IOSQE_FIXED_FILE** and **IORING_REGISTER_FILES**.
Available since 5.15.
<!-- -->
**IORING_OP_OPENAT2**\
Issue the equivalent of a [openat2](https://man7.org/linux/man-pages/man2/openat2.2.html) 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](https://man7.org/linux/man-pages/man2/openat2.2.html) for the general description of the related system call.
Available since 5.6.
If the *file_index* field is set to a positive number, the file won't be
installed into the normal file table as usual but will be placed into
the fixed file table at index *file_index - 1.* In this case, instead of
returning a file descriptor, the result will contain either 0 on success
or an error. If the index points to a valid empty slot, the installation
is guaranteed to not fail. If there is already a file in the slot, it
will be replaced, similar to **IORING_OP_FILES_UPDATE**. Please note
that only io_uring has access to such files and no other syscall can use
them. See **IOSQE_FIXED_FILE** and **IORING_REGISTER_FILES**.
Available since 5.15.
<!-- -->
**IORING_OP_CLOSE**\
Issue the equivalent of a [close](https://man7.org/linux/man-pages/man2/close.2.html) system call. *fd* is the file
descriptor to be closed. See also [close](https://man7.org/linux/man-pages/man2/close.2.html) for the general
description of the related system call. Available since 5.6. If the
*file_index* field is set to a positive number, this command can be used
to close files that were direct opened through **IORING_OP_OPENAT**,
**IORING_OP_OPENAT2**, or **IORING_OP_ACCEPT** using the io_uring
specific direct descriptors. Note that only one of the descriptor fields
may be set. The direct close feature is available since the 5.15 kernel,
where direct descriptors were introduced.
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**IORING_OP_STATX**\
Issue the equivalent of a [statx](https://man7.org/linux/man-pages/man2/statx.2.html) 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](https://man7.org/linux/man-pages/man2/statx.2.html) for the general description of the
related system call. Available since 5.6.
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**IORING_OP_READ**\
**IORING_OP_WRITE**\
Issue the equivalent of a [pread](https://man7.org/linux/man-pages/man2/pread.2.html) or [pwrite](https://man7.org/linux/man-pages/man2/pwrite.2.html) system call.
*fd* is the file descriptor to be operated on, *addr* contains the
buffer in question, *len* contains the length of the IO operation, and
*offs* contains the read or write offset. If *fd* does not refer to a
seekable file, *off* must be set to zero or -1. If *offs* is set to
**-1** , the offset will use (and advance) the file position, like the
[read](https://man7.org/linux/man-pages/man2/read.2.html) and [write](https://man7.org/linux/man-pages/man2/write.2.html) system calls. These are non-vectored
versions of the **IORING_OP_READV** and **IORING_OP_WRITEV** opcodes.
See also [read](https://man7.org/linux/man-pages/man2/read.2.html) and [write](https://man7.org/linux/man-pages/man2/write.2.html) for the general description of the
related system call. Available since 5.6.
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**IORING_OP_SPLICE**\
Issue the equivalent of a [splice](https://man7.org/linux/man-pages/man2/splice.2.html) 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](https://man7.org/linux/man-pages/man2/splice.2.html). *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](https://man7.org/linux/man-pages/man2/splice.2.html) for
the general description of the related system call. Available since 5.7.
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**IORING_OP_TEE**\
Issue the equivalent of a [tee](https://man7.org/linux/man-pages/man2/tee.2.html) 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](https://man7.org/linux/man-pages/man2/tee.2.html) for the general description of the related system call.
Available since 5.8.
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**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.
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**IORING_OP_PROVIDE_BUFFERS**\
This command allows an application to register a group of buffers to be
used by commands that read/receive data. Using buffers in this manner
can eliminate the need to separate the poll + read, which provides a
convenient point in time to allocate a buffer for a given request. It's
often infeasible to have as many buffers available as pending reads or
receive. With this feature, the application can have its pool of buffers
ready in the kernel, and when the file or socket is ready to
read/receive data, a buffer can be selected for the operation. *fd* must
contain the number of buffers to provide, *addr* must contain the
starting address to add buffers from, *len* must contain the length of
each buffer to add from the range, *buf_group* must contain the group ID
of this range of buffers, and *off* must contain the starting buffer ID
of this range of buffers. With that set, the kernel adds buffers
starting with the memory address in *addr,* each with a length of *len.*
Hence the application should provide *len \* fd* worth of memory in
*addr.* Buffers are grouped by the group ID, and each buffer within this
group will be identical in size according to the above arguments. This
allows the application to provide different groups of buffers, and this
is often used to have differently sized buffers available depending on
what the expectations are of the individual request. When submitting a
request that should use a provided buffer, the **IOSQE_BUFFER_SELECT**
flag must be set, and *buf_group* must be set to the desired buffer
group ID where the buffer should be selected from. Available since 5.7.
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**IORING_OP_REMOVE_BUFFERS**\
Remove buffers previously registered with **IORING_OP_PROVIDE_BUFFERS**.
*fd* must contain the number of buffers to remove, and *buf_group* must
contain the buffer group ID from which to remove the buffers. Available
since 5.7.
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**IORING_OP_SHUTDOWN**\
Issue the equivalent of a [shutdown](https://man7.org/linux/man-pages/man2/shutdown.2.html) system call. *fd* is the file
descriptor to the socket being shutdown, and *len* must be set to the
*how* argument. No no other fields should be set. Available since 5.11.
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**IORING_OP_RENAMEAT**\
Issue the equivalent of a [renameat2](https://man7.org/linux/man-pages/man2/renameat2.2.html) system call. *fd* should be
set to the *olddirfd*, *addr* should be set to the *oldpath*, *len*
should be set to the *newdirfd*, *addr* should be set to the *oldpath*,
*addr2* should be set to the *newpath*, and finally *rename_flags*
should be set to the *flags* passed in to [renameat2](https://man7.org/linux/man-pages/man2/renameat2.2.html). Available
since 5.11.
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**IORING_OP_UNLINKAT**\
Issue the equivalent of a [unlinkat](https://man7.org/linux/man-pages/man2/unlinkat.2.html) system call. *fd* should be
set to the *dirfd*, *addr* should be set to the *pathname*, and
*unlink_flags* should be set to the *flags* being passed in to
[unlinkat](https://man7.org/linux/man-pages/man2/unlinkat.2.html). Available since 5.11.
<!-- -->
**IORING_OP_MKDIRAT**\
Issue the equivalent of a [mkdirat](https://man7.org/linux/man-pages/man2/mkdirat.2.html) system call. *fd* should be set
to the *dirfd*, *addr* should be set to the *pathname*, and *len* should
be set to the *mode* being passed in to [mkdirat](https://man7.org/linux/man-pages/man2/mkdirat.2.html). Available since
5.15.
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**IORING_OP_SYMLINKAT**\
Issue the equivalent of a [symlinkat](https://man7.org/linux/man-pages/man2/symlinkat.2.html) system call. *fd* should be
set to the *newdirfd*, *addr* should be set to the *target* and *addr2*
should be set to the *linkpath* being passed in to [symlinkat](https://man7.org/linux/man-pages/man2/symlinkat.2.html).
Available since 5.15.
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**IORING_OP_LINKAT**\
Issue the equivalent of a [linkat](https://man7.org/linux/man-pages/man2/linkat.2.html) system call. *fd* should be set
to the *olddirfd*, *addr* should be set to the *oldpath*, *len* should
be set to the *newdirfd*, *addr2* should be set to the *newpath*, and
*hardlink_flags* should be set to the *flags* being passed in to
[linkat](https://man7.org/linux/man-pages/man2/linkat.2.html). Available since 5.15.
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**IORING_OP_MSG_RING**\
Send a message to an io_uring. *fd* must be set to a file descriptor of
a ring that the application has access to, *len* can be set to any
32-bit value that the application wishes to pass on, and *off* should be
set any 64-bit value that the application wishes to send. On the target
ring, a CQE will be posted with the *res* field matching the *len* set,
and a *user_data* field matching the *off* value being passed in. This
request type can be used to either just wake or interrupt anyone waiting
for completions on the target ring, or it can be used to pass messages
via the two fields. Available since 5.18.
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**IORING_OP_SOCKET**\
Issue the equivalent of a [socket](https://man7.org/linux/man-pages/man2/socket.2.html) system call. *fd* must contain
the communication domain, *off* must contain the communication type,
*len* must contain the protocol, and *rw_flags* is currently unused and
must be set to zero. See also [socket](https://man7.org/linux/man-pages/man2/socket.2.html) for the general description
of the related system call. Available since 5.19.
If the *file_index* field is set to a positive number, the file won't be
installed into the normal file table as usual but will be placed into
the fixed file table at index *file_index* - 1. In this case, instead of
returning a file descriptor, the result will contain either 0 on success
or an error. If the index points to a valid empty slot, the installation
is guaranteed to not fail. If there is already a file in the slot, it
will be replaced, similar to **IORING_OP_FILES_UPDATE**. Please note
that only io_uring has access to such files and no other syscall can use
them. See **IOSQE_FIXED_FILE** and **IORING_REGISTER_FILES**.
Available since 5.19.
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**IORING_OP_URING_CMD**\
Issues an asynchronous, per-file private operation, similar to
[ioctl](https://man7.org/linux/man-pages/man2/ioctl.2.html). Further information may be found in the dedicated man page
of **IORING_OP_URING_CMD**.
Available since 5.19.
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**IORING_OP_SEND_ZC**\
Issue the zerocopy equivalent of a **send(2)** system call. Similar to
**IORING_OP_SEND**, but tries to avoid making intermediate copies of
data. Zerocopy execution is not guaranteed and may fall back to copying.
The request may also fail with **-EOPNOTSUPP**, when a protocol doesn't
support zerocopy, in which case users are recommended to use copying
sends instead.
The *flags* field of the first *struct io_uring_cqe* may likely contain
**IORING_CQE_F_MORE**, which means that there will be a second
completion event / notification for the request, with the *user_data*
field set to the same value. The user must not modify the data buffer
until the notification is posted. The first cqe follows the usual rules
and so its *res* field will contain the number of bytes sent or a
negative error code. The notification's *res* field will be set to zero
and the *flags* field will contain **IORING_CQE_F_NOTIF**. The two step
model is needed because the kernel may hold on to buffers for a long
time, e.g. waiting for a TCP ACK, and having a separate cqe for request
completions allows userspace to push more data without extra delays.
Note, notifications are only responsible for controlling the lifetime of
the buffers, and as such don't mean anything about whether the data has
atually been sent out or received by the other end. Even errored
requests may generate a notification, and the user must check for
**IORING_CQE_F_MORE** rather than relying on the result.
*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. When
*addr2* is non-zero it points to the address of the target with
*addr_len* specifying its size, turning the request into a [sendto](https://man7.org/linux/man-pages/man2/sendto.2.html)
system call equivalent.
Available since 6.0.
This command also supports the following modifiers in *ioprio:*
**IORING_RECVSEND_POLL_FIRST** If set, io_uring will assume the socket
is currently full and attempting to send data will be unsuccessful. For
this case, io_uring will arm internal poll and trigger a send of the
data when there is enough space available. This initial send attempt can
be wasteful for the case where the socket is expected to be full,
setting this flag will bypass the initial send attempt and go straight
to arming poll. If poll does indicate that data can be sent, the
operation will proceed.
**IORING_RECVSEND_FIXED_BUF** If set, instructs io_uring to use a
pre-mapped buffer. The *buf_index* field should contain an index into an
array of fixed buffers. See [io_uring_register] for details on how
to setup a context for fixed buffer I/O.
**IORING_OP_SENDMSG_ZC**\
Issue the zerocopy equivalent of a [sendmsg](https://man7.org/linux/man-pages/man2/sendmsg.2.html) system call. Works
just like **IORING_OP_SENDMSG**, but like **IORING_OP_SEND_ZC** supports
**IORING_RECVSEND_FIXED_BUF**. For additional notes regarding zero copy
see **IORING_OP_SEND_ZC**.
Available since 6.1
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**IORING_OP_WAITID**\
Issue the equivalent of a [waitid](https://man7.org/linux/man-pages/man2/waitid.2.html) system call. *len* must contain
the idtype being queried/waited for and *fd* must contain the 'pid' (or
id) being waited for. *file_index* is the 'options' being set (the child
state changes to wait for). *addr2* is a pointer to siginfo_t, if any,
being filled in. See also [waitid](https://man7.org/linux/man-pages/man2/waitid.2.html) for the general description of
the related system call. Available since 6.5.
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**IORING_OP_SETXATTR**\
**IORING_OP_GETXATTR**\
**IORING_OP_FSETXATTR**\
**IORING_OP_FGETXATTR**\
Issue the equivalent of a [setxattr](https://man7.org/linux/man-pages/man2/setxattr.2.html) or [getxattr](https://man7.org/linux/man-pages/man2/getxattr.2.html) or
[fsetxattr](https://man7.org/linux/man-pages/man2/fsetxattr.2.html) or [fgetxattr](https://man7.org/linux/man-pages/man2/fgetxattr.2.html) system call. *addr* must contain a
pointer to a buffer containing the name of the extended attribute.
*addr2* must contain a pointer to a buffer of maximum length *len*, in
which the value of the extended attribute is to be placed or is read
from. Additional flags maybe provided in *xattr_flags*. For
[setxattr](https://man7.org/linux/man-pages/man2/setxattr.2.html) or [getxattr](https://man7.org/linux/man-pages/man2/getxattr.2.html) *addr3* must contain a pointer to the
path of the file. For [fsetxattr](https://man7.org/linux/man-pages/man2/fsetxattr.2.html) or [fgetxattr](https://man7.org/linux/man-pages/man2/fgetxattr.2.html) *fd* must
contain the file descriptor of the file.
Available since 5.19.
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**IORING_OP_BIND**\
Issues the equivalent of the [bind](https://man7.org/linux/man-pages/man2/bind.2.html) system call. *fd* must contain
the file descriptor of the socket, *addr* must contain a pointer to the
sockaddr struct containing the address to assign and *addr2* must
contain the length of the address.
Available since 6.11.
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**IORING_OP_LISTEN**\
Issues the equivalent of the [listen](https://man7.org/linux/man-pages/man2/listen.2.html) system call. *fd* must
contain the file descriptor of the socket and *len* must contain the
backlog parameter, i.e. the maximum amount of pending queued
connections.
Available since 6.11.
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**IORING_OP_FTRUNCATE**\
Issues the equivalent of the [ftruncate](https://man7.org/linux/man-pages/man2/ftruncate.2.html) system call. *fd* must
contain the file descriptor of the file to truncate and *off* must
contain the length to which the file will be truncated.
Available since 6.9.
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**IORING_OP_READ_MULTISHOT**\
Like **IORING_OP_READ**, but similar to requests prepared with
*io_uring_prep_multishot_accept*(3) additional reads and thus CQEs will
be performed based on this single SQE once there is more data available.
Is restricted to pollable files and will fall back to single shot if the
file does not support **NOWAIT**. Like other multishot type requests,
the application should look at the CQE flags and see if
**IORING_CQE_F_MORE** is set on completion as an indication of whether
or not the read request will generate further CQEs. Available since 6.7.
<!-- -->
**IORING_OP_FUTEX_WAIT**\
Issues the equivalent of the [futex_wait](https://man7.org/linux/man-pages/man2/futex_wait.2.html) system call. *addr* must
hold a pointer to the futex, *addr2* must hold the value to which the
futex has to be changed so this caller to [futex_wait](https://man7.org/linux/man-pages/man2/futex_wait.2.html) can be woken
by a call to [futex_wake](https://man7.org/linux/man-pages/man2/futex_wake.2.html), *addr3* must hold the bitmask of this
[futex_wait](https://man7.org/linux/man-pages/man2/futex_wait.2.html) caller. For a caller of [futex_wake](https://man7.org/linux/man-pages/man2/futex_wake.2.html) to wake a
waiter additionally the bitmask of the waiter and waker must have at
least one set bit in common. *fd* must contain additional flags passed
in.
Available since 6.7.
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**IORING_OP_FUTEX_WAKE**\
Issues the equivalent of the [futex_wake](https://man7.org/linux/man-pages/man2/futex_wake.2.html) system call. *addr* must
hold a pointer to the futex, *addr2* must hold the maximum number of
waiters waiting on this futex to wake, *addr3* must hold the bitmask of
this [futex_wake](https://man7.org/linux/man-pages/man2/futex_wake.2.html) call. To wake a waiter additionally the bitmask
of the waiter and waker must have at least one set bit in common. *fd*
must contain additional flags passed in.
Available since 6.7.
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**IORING_OP_FUTEX_WAITV**\
Issues the equivalent of the [futex_waitv](https://man7.org/linux/man-pages/man2/futex_waitv.2.html) system call. *addr* must
hold a pointer to the futexv struct, *len* must hold the length of the
futexv struct, which may not be 0 and must be smaller than
**FUTEX_WAITV_MAX** (as of 6.11 == 128).
Available since 6.7.
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**IORING_OP_FIXED_FD_INSTALL**\
This operation is used to insert a registered file into the regular
process file table. Consequently *fd* must contain the file index and
**IOSQE_FIXED_FILE** must be set. The resulting regular fd is returned
via cqe-\>res. Additional flags may be passed in via *install_fd_flags*.
Currently supported flags are: **IORING_FIXED_FD_NO_CLOEXEC**, which
overrides a potentially set **O_CLOEXEC** flag set on the initial file.
Available since 6.8.
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**IORING_OP_PIPE**\
This operation is used to create a pipe, a set of file descriptors that
can be used for communication. The pipe may either be created as a set
of normal file descriptors, or it can be created as fixed/direct
descriptors. *addr* must contain a pointer to an array of two integers,
where upon successful completion of the request, index 0 will contain
the read side and index 1 the write side of the pipe. *pipe_flags* may
contain flags associated with pipe creation. Currently **O_CLOEXEC |
O_NONBLOCK | O_DIRECT | O_NOTIFICATION_PIPE** are supported.
*file_index* may contain the the desired starting point for a fixed
descriptor pipe creation. If this is set to **0,** then regular file
descriptors are used. If set to **IORING_FILE_INDEX_ALLOC,** then the
kernel will allocate descriptors from the previously registered direct
descriptor table. If set to any non-zero value, then it sets the exact
direct descriptor value for index 0 of the pipe, and index 1 will be the
following integer value.
If used with direct descriptors rather than normal file descriptors, a
direct descriptor table must have been previously registered with the
kernel.
Available since 6.16.
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**IORING_OP_RECV_ZC**\
Receive data from a socket using zero-copy techniques. Unlike
**IORING_OP_RECV**, this operation does not use a user-provided buffer.
Instead, data is delivered through a pre-registered zero-copy RX
interface queue. *fd* must be set to the socket file descriptor.
*zcrx_ifq_idx* specifies the index of the registered zero-copy RX
interface queue. *len* specifies the maximum amount of data to receive.
*ioprio* can contain flags such as **IORING_RECVSEND_POLL_FIRST** and
**IORING_RECV_MULTISHOT**. This operation requires multishot mode.
Before using this command, a zero-copy RX interface queue must be
registered via [io_uring_register] using
**IORING_REGISTER_ZCRX_IFQ**. Data completions are posted as auxiliary
CQEs.
Available since 6.15.
<!-- -->
**IORING_OP_EPOLL_WAIT**\
Wait for events on an epoll instance. This is an async version of
[epoll_wait](https://man7.org/linux/man-pages/man2/epoll_wait.2.html). *fd* must be set to the epoll file descriptor, *addr*
must point to an array of *struct epoll_event* to receive the events,
and *len* must contain the maximum number of events to return.
The primary use case is for legacy event loops that still use epoll for
some file descriptors. By using io_uring to wait on epoll events, the
application can unify its event handling through io_uring while
maintaining backwards compatibility with epoll-based components.
Available since 6.15.
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**IORING_OP_READV_FIXED**\
**IORING_OP_WRITEV_FIXED**\
Vectored read and write operations using pre-registered buffers,
combining the functionality of **IORING_OP_READV**/**IORING_OP_WRITEV**
with **IORING_OP_READ_FIXED**/**IORING_OP_WRITE_FIXED**. The *buf_index*
field specifies the index into the registered buffer table. Unlike the
non-fixed vectored operations, the iovec entries point into the
registered buffer region. This allows vectored I/O while still
benefiting from the reduced overhead of pre-registered buffers.
Available since 6.15.
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**IORING_OP_NOP128**\
No operation, similar to **IORING_OP_NOP**, but explicitly uses a
128-byte SQE. This can be useful for testing or alignment purposes when
using mixed 64/128-byte SQE rings (**IORING_SETUP_SQE_MIXED**).
Available since 6.19.
<!-- -->
**IORING_OP_URING_CMD128**\
Passthrough command to the underlying file, identical to
**IORING_OP_URING_CMD**, but explicitly uses a 128-byte SQE. The extra
64 bytes provide additional space for command-specific data. This is
useful with **IORING_SETUP_SQE_MIXED** rings where some commands need
the larger SQE size while others do not.
See **IORING_OP_URING_CMD** for general usage details.
Available since 6.19.
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] man page). Note that this isn't
always available for all commands. If used on a command that doesn't
support fixed files, the SQE will error with **-EBADF**. 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, the SQE forms a link with the next SQE in
the submission ring. That next SQE will not be started before the
previous request 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. Chains are not supported
across submission boundaries. Even if the last SQE in a submission has
this flag set, it will still terminate the current chain. 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.
**IOSQE_BUFFER_SELECT**\
Used in conjunction with the **IORING_OP_PROVIDE_BUFFERS** command,
which registers a pool of buffers to be used by commands that read or
receive data. When buffers are registered for this use case, and this
flag is set in the command, io_uring will grab a buffer from this pool
when the request is ready to receive or read data. If successful, the
resulting CQE will have **IORING_CQE_F_BUFFER** set in the flags part of
the struct, and the upper **IORING_CQE_BUFFER_SHIFT** bits will contain
the ID of the selected buffers. This allows the application to know
exactly which buffer was selected for the operation. If no buffers are
available and this flag is set, then the request will fail with
**-ENOBUFS** as the error code. Once a buffer has been used, it is no
longer available in the kernel pool. The application must re-register
the given buffer again when it is ready to recycle it (eg has completed
using it). Available since 5.7.
**IOSQE_CQE_SKIP_SUCCESS**\
Don't generate a CQE if the request completes successfully. If the
request fails, an appropriate CQE will be posted as usual and if there
is no **IOSQE_IO_HARDLINK,** CQEs for all linked requests will be
omitted. The notion of failure/success is opcode specific and is the
same as with breaking chains of **IOSQE_IO_LINK**. One special case is
when the request has a linked timeout, then the CQE generation for the
linked timeout is decided solely by whether it has
**IOSQE_CQE_SKIP_SUCCESS** set, regardless whether it timed out or was
canceled. In other words, if a linked timeout has the flag set, it's
guaranteed to not post a CQE.
The semantics are chosen to accommodate several use cases. First, when
all but the last request of a normal link without linked timeouts are
marked with the flag, only one CQE per link is posted. Additionally, it
enables suppression of CQEs in cases where the side effects of a
successfully executed operation is enough for userspace to know the
state of the system. One such example would be writing to a
synchronisation file.
This flag is incompatible with **IOSQE_IO_DRAIN**. Using both of them in
a single ring is undefined behavior, even when they are not used
together in a single request. Currently, after the first request with
**IOSQE_CQE_SKIP_SUCCESS**, all subsequent requests marked with drain
will be failed at submission time. Note that the error reporting is best
effort only, and restrictions may change in the future.
Available since 5.17.
*ioprio* specifies the I/O priority. See [ioprio_get](https://man7.org/linux/man-pages/man2/ioprio_get.2.html) 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](https://man7.org/linux/man-pages/man2/preadv2.2.html) 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](https://man7.org/linux/man-pages/man2/open.2.html) manual page for more information.
The bits that may be set in *poll_events* are defined in *\<poll.h\>*,
and documented in [poll](https://man7.org/linux/man-pages/man2/poll.2.html).
*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] 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] system call can be
invoked to initiate the I/O.
Completions use the following data structure:
``` c
/*
* 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 used for certain commands, like **IORING_OP_POLL_ADD** or in
conjunction with **IOSQE_BUFFER_SELECT** or **IORING_OP_MSG_RING**, see
those entries for details. *res* is the operation-specific result, but
io_uring-specific errors (e.g. flags or opcode invalid) are returned
through this field. They are described in section **CQE** **ERRORS**.
For read and write opcodes, the return values match *errno* values
documented in the [preadv2](https://man7.org/linux/man-pages/man2/preadv2.2.html) and [pwritev2](https://man7.org/linux/man-pages/man2/pwritev2.2.html) man pages, with
*res* holding the equivalent of *-errno* for error cases, or the
transferred number of bytes in case the operation is successful. Hence
both error and success return can be found in that field in the CQE. For
other request types, the return values are documented in the matching
man page for that type, or in the opcodes section above for
io_uring-specific opcodes.
# 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. Note that if the ring was created with **IORING_SETUP_SQPOLL**
specified, then the return value will generally be the same as
*to_submit* as submission happens outside the context of the system
call.
The errors related to a submission queue entry will be returned through
a completion queue entry (see section **CQE** **ERRORS**), 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, a negative error code is
returned. The caller should not rely on *errno* variable.
# ERRORS
These are the errors returned by [io_uring_enter] system call.
**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.
**EBADF**\
*fd* is not a valid file descriptor.
**EBADFD**\
*fd* is a valid file descriptor, but the io_uring ring is not in the
right state (enabled). See [io_uring_register] for details on how
to enable the ring.
**EBADR**\
At least one CQE was dropped even with the **IORING_FEAT_NODROP**
feature, and there are no otherwise available CQEs. This clears the
error state and so with no other changes the next call to
[io_uring_enter] will not have this error. This error should be
extremely rare and indicates the machine is running critically low on
memory. It may be reasonable for the application to terminate running
unless it is able to safely handle any CQE being lost.
**EBUSY**\
If the **IORING_FEAT_NODROP** feature flag is set, then **EBUSY** will
be returned if there were overflow entries, **IORING_ENTER_GETEVENTS**
flag is set and not all of the overflow entries were able to be flushed
to the CQ ring.
Without **IORING_FEAT_NODROP** 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, or if the application attempts to wait for more events without
having reaped the ones already present in the CQ ring.
**EEXIST**\
The thread submitting the work is invalid. This may occur if
**IORING_ENTER_GETEVENTS** and **IORING_SETUP_DEFER_TASKRUN** is set,
but the submitting thread is not the thread that initially created or
enabled the io_uring associated with *fd.*
**EINVAL**\
Some bits in the *flags* argument are invalid.
**EFAULT**\
An invalid user space address was specified for the *sig* argument.
**ENXIO**\
The io_uring instance is in the process of being torn down.
**EOPNOTSUPP**\
*fd* does not refer to an io_uring instance.
**EINTR**\
The operation was interrupted by a delivery of a signal before it could
complete; see [signal](https://man7.org/linux/man-pages/man2/signal.2.html). Can happen while waiting for events with
**IORING_ENTER_GETEVENTS.**
**EOWNERDEAD**\
The ring has been setup with **IORING_SETUP_SQPOLL** and the sq poll
kernel thread has been killed.
# CQE ERRORS
These io_uring-specific errors are returned as a negative value in the
*res* field of the completion queue entry.
**EACCES**\
The *flags* field or *opcode* in a submission queue entry is not allowed
due to registered restrictions. See [io_uring_register] for details
on how restrictions work.
**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 *flags* field or *opcode* in a submission queue entry is invalid.
**EINVAL**\
The *buf_index* member of the submission queue entry is invalid.
**EINVAL**\
The *personality* field in a submission queue entry is invalid.
**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.
**EOPNOTSUPP**\
*opcode* is valid, but not supported by this kernel.
**EOPNOTSUPP**\
**IOSQE_BUFFER_SELECT** was set in the *flags* field of the submission
queue entry, but the *opcode* doesn't support buffer selection.
**EINVAL**\
**IORING_OP_TIMEOUT** was specified, but *timeout_flags* specified more
than one clock source or **IORING_TIMEOUT_MULTISHOT** was set alongside
**IORING_TIMEOUT_ABS**.