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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CQE ERRORS | COLOPHON |
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io_uring_enter(2) Linux Programmer's Manual io_uring_enter(2)
io_uring_enter - initiate and/or complete asynchronous I/O
#include <liburing.h>
int io_uring_enter(unsigned int fd, unsigned int to_submit,
unsigned int min_complete, unsigned int flags,
sigset_t *sig);
int io_uring_enter2(unsigned int fd, unsigned int to_submit,
unsigned int min_complete, unsigned int flags,
void *arg, size_t sz);
io_uring_enter(2) 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(2).
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. 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(2) or io_uring_enable_rings(3).
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:
struct io_uring_getevents_arg {
__u64 sigmask;
__u32 sigmask_sz;
__u32 pad;
__u64 ts;
};
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
structio_uring_getevents_arg, but merely an offset into an
area of wait regions previously registered with
io_uring_register(2) 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(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(2) 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(2) 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;
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(2)
and pwritev2(2). If the file is not seekable, off must be
set to zero or -1.
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). 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(7). See epoll_ctl(2) 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(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.
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.
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(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. Flags can be passed
using the accept_flags field. See also accept4(2) 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(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, 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(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 len is access mode of the file. See also
openat(2) 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(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.
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(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. 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.
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 pread(2) or pwrite(2) 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(2)
and write(2) system calls. 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.
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.
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.
IORING_OP_SHUTDOWN
Issue the equivalent of a shutdown(2) 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.
IORING_OP_RENAMEAT
Issue the equivalent of a renameat2(2) 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(2). Available since 5.11.
IORING_OP_UNLINKAT
Issue the equivalent of a unlinkat(2) 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(2). Available since 5.11.
IORING_OP_MKDIRAT
Issue the equivalent of a mkdirat(2) 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(2). Available since 5.15.
IORING_OP_SYMLINKAT
Issue the equivalent of a symlinkat(2) 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(2). Available since 5.15.
IORING_OP_LINKAT
Issue the equivalent of a linkat(2) 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(2). Available since 5.15.
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.
IORING_OP_SOCKET
Issue the equivalent of a socket(2) 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(2) 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.
IORING_OP_URING_CMD
Issues an asynchronous, per-file private operation, similar
to ioctl(2). Further information may be found in the
dedicated man page of IORING_OP_URING_CMD.
Available since 5.19.
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(2)
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(2) for details on how to setup a
context for fixed buffer I/O.
IORING_OP_SENDMSG_ZC
Issue the zerocopy equivalent of a sendmsg(2) 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
IORING_OP_WAITID
Issue the equivalent of a waitid(2) 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(2) for the general description
of the related system call. Available since 6.5.
IORING_OP_SETXATTR
IORING_OP_GETXATTR
IORING_OP_FSETXATTR
IORING_OP_FGETXATTR
Issue the equivalent of a setxattr(2) or getxattr(2) or
fsetxattr(2) or fgetxattr(2) 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(2) or getxattr(2) addr3 must contain a pointer to
the path of the file. For fsetxattr(2) or fgetxattr(2) fd
must contain the file descriptor of the file.
Available since 5.19.
IORING_OP_BIND
Issues the equivalent of the bind(2) 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.
IORING_OP_LISTEN
Issues the equivalent of the listen(2) system call. fd
must contain the file descriptor of the socket and addr
must contain the backlog parameter, i.e. the maximum amount
of pending queued connections.
Available since 6.11.
IORING_OP_FTRUNCATE
Issues the equivalent of the ftruncate(2) 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.
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(2) 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(2) can be woken by a call to futex_wake(2),
addr3 must hold the bitmask of this futex_wait(2) caller.
For a caller of futex_wake(2) 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.
IORING_OP_FUTEX_WAKE
Issues the equivalent of the futex_wake(2) 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(2) 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.
IORING_OP_FUTEX_WAITV
Issues the equivalent of the futex_waitv(2) 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.
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.
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). 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(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 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(2) and pwritev2(2) 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.
io_uring_enter(2) 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.
These are the errors returned by io_uring_enter(2) 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(2) 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(2) 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(7). 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.
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(2) 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_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.
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.
This page is part of the liburing (A library for io_uring)
project. Information about the project can be found at
⟨https://github.com/axboe/liburing⟩. If you have a bug report for
this manual page, send it to io-uring@vger.kernel.org. This page
was obtained from the project's upstream Git repository
⟨https://github.com/axboe/liburing⟩ on 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-02.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
Linux 2019-01-22 io_uring_enter(2)
Pages that refer to this page: io_uring_enter2(2), io_uring_enter(2), io_uring_register(2), io_uring_setup(2), syscalls(2), io_uring_prep_send_zc(3), io_uring_prep_send_zc_fixed(3), io_uring_register_ring_fd(3), io_uring_set_iowait(3), io_uring(7)
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