io_uring_queue_init_mem(3) — Linux manual page


io_uring_queue_init(3)       liburing Manual      io_uring_queue_init(3)

NAME         top

       io_uring_queue_init - setup io_uring submission and completion

SYNOPSIS         top

       #include <liburing.h>

       int io_uring_queue_init(unsigned entries,
                               struct io_uring *ring,
                               unsigned flags);

       int io_uring_queue_init_params(unsigned entries,
                                      struct io_uring *ring,
                                      struct io_uring_params *params);

       int io_uring_queue_init_mem(unsigned entries,
                                   struct io_uring *ring,
                                   struct io_uring_params *params,
                                   void *buf, size_t buf_size);

DESCRIPTION         top

       The io_uring_queue_init(3) function executes the
       io_uring_setup(2) system call to initialize the submission and
       completion queues in the kernel with at least entries entries in
       the submission queue and then maps the resulting file descriptor
       to memory shared between the application and the kernel.

       By default, the CQ ring will have twice the number of entries as
       specified by entries for the SQ ring. This is adequate for
       regular file or storage workloads, but may be too small for
       networked workloads. The SQ ring entries do not impose a limit on
       the number of in-flight requests that the ring can support, it
       merely limits the number that can be submitted to the kernel in
       one go (batch). if the CQ ring overflows, e.g. more entries are
       generated than fits in the ring before the application can reap
       them, then if the kernel supports IORING_FEAT_NODROP the ring
       enters a CQ ring overflow state. Otherwise it drops the CQEs and
       increments cq.koverflow in struct io_uring with the number of
       CQEs dropped. The overflow state is indicated by
       IORING_SQ_CQ_OVERFLOW being set in the SQ ring flags. Unless the
       kernel runs out of available memory, entries are not dropped, but
       it is a much slower completion path and will slow down request
       processing. For that reason it should be avoided and the CQ ring
       sized appropriately for the workload. Setting cq_entries in
       struct io_uring_params will tell the kernel to allocate this many
       entries for the CQ ring, independent of the SQ ring size in given
       in entries.  If the value isn't a power of 2, it will be rounded
       up to the nearest power of 2.

       On success, io_uring_queue_init(3) returns 0 and ring will point
       to the shared memory containing the io_uring queues. On failure
       -errno is returned.

       flags will be passed through to the io_uring_setup syscall (see

       The io_uring_queue_init_params(3) and io_uring_queue_init_mem(3)
       variants will pass the parameters indicated by params straight
       through to the io_uring_setup(2) system call.

       The io_uring_queue_init_mem(3) variant uses the provided buf with
       associated size buf_size as the memory for the ring, using the
       IORING_SETUP_NO_MMAP flag to io_uring_setup(2).  The buffer
       passed to io_uring_queue_init_mem(3) must already be zeroed.
       Typically, the caller should allocate a huge page and pass that
       in to io_uring_queue_init_mem(3).  Pages allocated by mmap are
       already zeroed.  io_uring_queue_init_mem(3) returns the number of
       bytes used from the provided buffer, so that the app can reuse
       the buffer with the returned offset to put more rings in the same
       huge page.

       On success, the resources held by ring should be released via a
       corresponding call to io_uring_queue_exit(3).

RETURN VALUE         top

       io_uring_queue_init(3) and io_uring_queue_init_params(3) return 0
       on success and -errno on failure.

       io_uring_queue_init_mem(3) returns the number of bytes used from
       the provided buffer on success, and -errno on failure.

SEE ALSO         top

       io_uring_setup(2), io_uring_register_ring_fd(3), mmap(2),

COLOPHON         top

       This page is part of the liburing (A library for io_uring)
       project.  Information about the project can be found at 
       ⟨⟩.  If you have a bug report for
       this manual page, send it to  This page
       was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-19.)  If you discover any rendering
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       (which is not part of the original manual page), send a mail to

liburing-0.7                  July 10, 2020       io_uring_queue_init(3)

Pages that refer to this page: io_uring_queue_init(3)io_uring_queue_init_mem(3)io_uring_queue_init_params(3)