uring-api

uring-api is a small Python wrapper around Linux io_uring.

The goal is deliberately modest: expose enough of the native ring lifecycle, socket send/recv submission, completion waiting, and callback delivery to build higher-level completion abstractions in Python. It does not implement an event loop, scheduler, or asyncio compatibility layer.

Future work is tracked in ROADMAP.md, including queue resizing and specialised kernel tuning. Caller-owned provided-buffer receive with leased BufView delivery is already part of the Python surface.

Quick Check

import uring_api

print(uring_api.probe())

with uring_api.Ring() as ring:
    print(ring.fd)

Socket I/O

Need to drive socket work through a ring without building a full event loop? Ring exposes direct submit wrappers for the common Python-oriented cases:

• stream I/O: submit_recv(), and provided-buffer submit_recv_buf() / submit_recv_multishot() via create_buf_group();
• message I/O: submit_recvmsg(), submit_sendto(), submit_sendmsg(), and zero-copy submit_sendmsg_zc();
• listeners and setup: submit_accept(), submit_accept_multishot(), submit_connect(), and submit_socket();
• lifecycle: submit_shutdown(), submit_close(), send helpers submit_send() / submit_send_zc(), and wait() for completion reaping.

Each submitted operation carries a Python user_data object which comes back with its completion. Inspect the semantic operation with completion.kind (CompletionKind enum, or the matching COMPLETION_KIND_* constants) when callbacks need to branch on completion type rather than inferring from result alone.

import socket
import uring_api

reader, writer = socket.socketpair()
try:
    reader.setblocking(False)
    writer.setblocking(False)

    with uring_api.Ring() as ring:
        token = {"operation": "greeting"}
        buf = bytearray(5)
        ring.submit_recv(reader.fileno(), buf, token)
        writer.send(b"hello")

        batch = ring.wait(1.0)

    assert len(batch) == 1
    completion = batch[0]
    assert completion.user_data is token
    assert bytes(buf) == b"hello"
    print(completion.res, completion.result)
finally:
    reader.close()
    writer.close()

For sends, uring-api keeps the exported buffer alive until the kernel reports the completion. That avoids copying the outgoing payload into an internal bytes object just to keep memory valid. submit_send_zc() uses IORING_OP_SEND_ZC, while submit_sendmsg_zc() uses IORING_OP_SENDMSG_ZC for the sendmsg shape. Their ordinary operation CQE is delivered as the submitted Completion; the later IORING_CQE_F_NOTIF buffer-lifetime CQE is consumed internally and releases the retained buffer.

submit_shutdown() is a socket operation and mirrors shutdown(fd, how). submit_accept() and submit_accept_multishot() accept optional accept flags; pass socket.SOCK_NONBLOCK | socket.SOCK_CLOEXEC when accepted sockets should be ready for proactor ownership without a follow-up fcntl() call. submit_accept() and submit_accept_multishot() deliver the accepted fd in completion.res and completion.result. Call getpeername() on the fd when you need the peer address. submit_close() is lower-level: pass only a raw fd whose ownership has already been transferred away from Python objects such as socket.socket, for example with detach(). Otherwise, Python and the kernel may both believe they own the same descriptor.

File metadata and positioned I/O

Ring also exposes positioned file helpers for caller-owned fds:

• submit_openat(path, flags, mode=0, *, dfd=AT_FDCWD) opens a path and returns the new fd in the completion result;
• submit_read(fd, buf, offset) and submit_write(fd, data, offset) perform explicit-offset I/O into caller buffers;
• submit_statx_fdsize(fd) is the common fast path for open-file metadata: it runs fd-only statx internally and puts the byte length in completion.result on success (completion.kind == CompletionKind.STATX_FDSIZE);
• submit_statx(dfd, path, flags, mask, buf) fills a caller-provided 256-byte statx buffer asynchronously when you need a custom mask or path lookup.

The usual positioned-file case (append EOF, SEEK_END, sendfile bounds) is an open fd whose size you already own:

handle = ring.submit_statx_fdsize(fd)
[completion] = ring.wait()
if completion.res == 0:
    size = completion.result

No caller buffer is required for submit_statx_fdsize(). Use submit_statx() when you need path-based metadata or fields beyond stx_size.

Successful submit_statx() completions always leave completion.result as None; read fields from the caller-owned submit buffer (for example via statx_st_size(buf) when you requested STATX_SIZE). Only submit_statx_fdsize() puts the byte length in completion.result. If the internal buffer lacks size fields, completion.result is None and the completion is still delivered.

Behaviour change (since PR #34): successful submit_statx() no longer sets completion.result to 0; it stays None.

Provided-buffer receive uses a caller-owned ring created with create_buf_group(). Submit one-shot receives with submit_recv_buf() or stream receives with submit_recv_multishot(fd, buf_group, ...). Both paths return read-only BufView objects rather than copying into bytes. Export the payload with memoryview(view) and drop the export (or call memoryview.release()) before the kernel buffer is recycled:

buf_group = ring.create_buf_group(buffer_size=16384, buffer_count=256)
pending = ring.submit_recv_buf(reader.fileno(), buf_group, token)
[completion] = ring.wait(1.0)

view = memoryview(completion.result)
try:
    process(view)
finally:
    del view

Multishot receive reuses the same BufGroup contract. Each CQE delivers a leased BufView, sets completion.sequence for out-of-order callback reconstruction, and uses IORING_CQE_F_MORE until EOF, cancellation, or -ENOBUFS when the buffer ring is empty. After -ENOBUFS, return buffers to the ring and submit a fresh submit_recv_multishot(); stream consumers should continue ordinal indexing from the terminal completion's sequence.

handle = ring.submit_recv_multishot(reader.fileno(), buf_group, token)
[completion] = ring.wait(1.0)
view = memoryview(completion.result)
try:
    process(view)
finally:
    del view

BufView tracks active exported memoryviews and recycles the selected buffer back to the ring when the last export is released. Provided-buffer completions always return BufView, including EOF (completion.res == 0), where the view has length == 0 and is falsy. Detect stream end from completion.res, not from the result type. BufGroup and BufView cannot be constructed directly; use Ring.create_buf_group() and let receive completions create the views.

completion.kind uses RECV_MULTISHOT (13) for multishot provided-buffer receive and RECV_BUF (16) for one-shot submit_recv_buf().

CompletionKind values are stable across releases and mirror the C API constants in uring_api_completion_kinds.h. Prefer the enum in Python code; native clients include the same header and call completion_kind() on the completion object.

The local liburing headers expose more socket-adjacent operations than this wrapper publishes, but those are intentionally outside the core Python-oriented surface. Readiness polling is optional for a completion proactor, fixed-buffer send variants still need a different ownership contract than leased BufView receive, and socket command or NAPI controls are specialised tuning hooks. Those items are tracked in ROADMAP.md rather than implied by probe(), which remains a compact runtime availability check.

If the submission queue cannot provide another entry after flushing already prepared work to the kernel, submit methods raise SubmissionQueueFull. Treat that as backpressure rather than as a permanent ring failure: wait for completions, then retry or let a higher-level proactor defer the submission.

Checking Availability

io_uring availability depends on more than the Python package importing successfully. The kernel, container sandbox, seccomp profile, and process limits can all affect whether a ring can actually be created.

Use probe() when you want a compact availability and capability dictionary:

import uring_api

probe = uring_api.probe()

if probe:
    print("io_uring is available")
    print("capabilities:", probe)
else:
    print("io_uring is not available")

Use is_available() when you only need a boolean:

import uring_api

if not uring_api.is_available():
    raise RuntimeError("io_uring is not available in this environment")

probe() creates a tiny temporary ring and closes it right away to test ring creation with the requested entries and flags. Targeted capability probes run once per process and are cached in static variables; later probe() calls reuse those results. If ring creation fails, it returns an empty dictionary. If it succeeds, the dictionary contains "available": True plus named optional capabilities such as "IORING_ACCEPT_MULTISHOT", "IORING_POLL_MULTISHOT", "IORING_RECV_MULTISHOT", and "IORING_OP_SEND_ZC" and "IORING_OP_SENDMSG_ZC" (version-gated at kernel 6.0 per io_uring_enter(2)), and "IORING_OP_STATX" (version-gated at 5.6). Production code should still handle OSError when it creates the real ring because limits or sandbox policy may differ for larger settings.

Pass setup flags to probe(flags=...) to check whether this build and kernel combination accepts a ring mode before using it for the real ring:

import uring_api

flags = uring_api.IORING_SETUP_SINGLE_ISSUER
probe = uring_api.probe(flags=flags)

if probe:
    print("setup flags accepted")
else:
    print("setup flags rejected")

Some flags also impose application-level contracts. For example, IORING_SETUP_SINGLE_ISSUER means callers must submit SQEs from a single owning thread even on kernels that accept the flag. IORING_SETUP_DEFER_TASKRUN requires that same owning thread to reap completions too: wait() and serve_completions() must run there, not on a worker pool. Kernels expect IORING_SETUP_DEFER_TASKRUN together with IORING_SETUP_SINGLE_ISSUER.

The compiled liburing version fields report the header version used to build the binary extension. This is useful in CI because Linux distribution images can compile the same Python package against different liburing development packages while still running on the hosted runner's kernel.

submit_send_zc() and submit_sendmsg_zc() are best gated with probe()["IORING_OP_SEND_ZC"] and probe()["IORING_OP_SENDMSG_ZC"]. Both entries use the documented kernel 6.0 floor via uname(2). Zerocopy can still complete with ENOTSUP or EOPNOTSUPP for some protocols (for example AF_UNIX on WSL); higher layers should route those sockets through copying send paths. If your CI image is expected to support zerocopy on inet sockets, make that expectation explicit:

uv run --active python - <<'PY'
import uring_api

probe = uring_api.probe()
print(probe)
raise SystemExit(0 if probe.get("IORING_OP_SEND_ZC") and probe.get("IORING_OP_SENDMSG_ZC") else 1)
PY

If the native extension cannot be imported after installation, importing uring_api still succeeds and probe() returns {}. Source builds with unsupported native dependencies warn and install the pure Python wrapper without _uring_api.

The IORING_ACCEPT_MULTISHOT capability uses a runtime operation probe rather than a kernel version check. It creates a private temporary ring and loopback listener, submits one multishot accept request, connects a local client, and checks whether the first accept completion keeps the request armed. If the build headers do not expose the helper flag, the capability simply reports False.

The IORING_POLL_MULTISHOT capability uses a runtime operation probe. It creates a private socket pair, submits one multishot poll for POLLIN, writes one byte to the peer, and reports True only if the first completion reports readiness and keeps the request armed with IORING_CQE_F_MORE. Gate submit_poll_multishot() on this entry; one-shot submit_poll() and submit_poll_remove() are treated as baseline poll surface.

The IORING_RECV_MULTISHOT capability is also checked with a runtime operation probe because it requires newer kernel support than multishot accept. It creates a private socket pair and provided-buffer ring, submits one multishot receive, sends one byte, and reports True only if the first completion selects a buffer and keeps the request armed with IORING_CQE_F_MORE.

Initialising a Ring

The current wrapper exposes the native ring lifecycle. A ring is a file descriptor plus shared submission/completion queues owned by the process.

import uring_api

with uring_api.Ring(entries=8) as ring:
    print("fd:", ring.fd)
    print("kernel features:", ring.features)
    print("submission entries:", ring.sq_entries)
    print("completion entries:", ring.cq_entries)

entries is the requested submission queue depth. The kernel may round or size the actual submission and completion queues, so inspect sq_entries and cq_entries after initialisation if the exact capacity matters.

Pass flags= to request setup modes that were accepted by probe(flags=...):

import uring_api

flags = uring_api.IORING_SETUP_SINGLE_ISSUER

if uring_api.probe(flags=flags):
    with uring_api.Ring(entries=8, flags=flags) as ring:
        ...

The constructor passes these flags to io_uring_queue_init_params() for the real ring. The application is still responsible for the contracts implied by each flag; for example, IORING_SETUP_SINGLE_ISSUER requires all submissions to come from the owning thread.

If initialisation fails, the constructor raises OSError:

import errno
import uring_api

try:
    ring = uring_api.Ring(entries=256)
except OSError as exc:
    if exc.errno == errno.EPERM:
        raise RuntimeError("io_uring is blocked by seccomp or policy") from exc
    if exc.errno == errno.ENOMEM:
        raise RuntimeError("io_uring could not allocate or pin the requested resources") from exc
    raise
else:
    try:
        print(ring.fd)
    finally:
        ring.close()

Threading Model

Ring deliberately stays close to liburing's shared-ring model, but the Python object adds native locking around the parts that matter for normal use.

The intended baseline is simple:

• one thread may reap completions with wait();
• other threads may call submit-side methods such as submit_recv(), create_buf_group(), submit_recv_buf(), submit_recv_multishot(), submit_send(), submit_send_zc(), submit_recvmsg(), submit_sendto(), submit_sendmsg_zc(), submit_accept(), submit_accept_multishot(), submit_connect(), and break_wait();
• break_wait() is safe to call while another thread is blocked in wait();
• multiple concurrent wait() calls are serialised by the Ring object;
• alternatively, callers may start their own Python threads and have each one call serve_completions() to wait for completions and call the callback directly.

Rings created with IORING_SETUP_DEFER_TASKRUN do not follow that worker-pool model. Submit, wait(), serve_completions(), and break_wait() must all run on the owning thread established by the first gated call.

break_wait() prepares and submits an internal NOP. When the reaper consumes that completion, wait() returns an empty list rather than a user completion.

Serving workers use the same receive side as wait(), so public wait() calls raise RuntimeError while they are running. Each worker calls serve_completions(), then loops until stop_serving() asks the service to exit. Workers compete for an internal wait lock, so only one worker is inside io_uring_wait_cqe() at a time, while another worker can dispatch a completion callback.

stop_serving() asks workers to exit and wakes the active waiter with break_wait(). The caller owns the threads, so the caller must join them before closing the ring; close() and __exit__() raise while completion service is still active. reset_serving() clears the stop flag so a fresh set of workers can enter serve_completions() again. If a callback raises, the exception is reported as unraisable and the worker group exits.

Native C clients can register a worker-thread callback through the C API. When a C callback is present, the serving worker calls it instead of Ring.callback; otherwise it falls back to the Python callback property.

import uring_api
import threading


def delivered(batch):
    for completion in batch:
        print(completion.user_data, completion.res, completion.result)


with uring_api.Ring() as ring:
    ring.callback = delivered
    threads = [threading.Thread(target=ring.serve_completions) for _ in range(2)]
    for thread in threads:
        thread.start()
    try:
        ring.submit_recv(fd, bytearray(4096), 200)
    finally:
        ring.stop_serving()
        for thread in threads:
            thread.join()

close() is still an owner-coordinated shutdown operation for submissions. Do not close a ring while another thread may submit new user operations.

C API

Native clients can include uring_api_capi.h and import _uring_api._C_API with PyCapsule_Import(). Use uring_api.get_include() to find the installed header directory when compiling an extension module.

The capsule currently exposes:

• abi_version, struct_size, and feature_flags for compatibility checks. While the package remains pre-release, abi_version stays at 1 but the function table may be reordered or extended; clients should compare struct_size and null-check pointers they rely on;
• compiled_liburing_major and compiled_liburing_minor for build-time header visibility;
• probe(entries, flags), which returns a new reference to the same flat availability and capability dictionary as _uring_api.probe();
• ring_new(), lifecycle helpers, metadata helpers, ring_submit_recv(), ring_submit_recv_multishot(), ring_submit_send(), ring_submit_send_zc(), ring_submit_recvmsg(), ring_submit_sendto(), ring_submit_sendmsg(), ring_submit_sendmsg_zc(), ring_submit_accept(), ring_submit_accept_multishot(), ring_submit_connect(), ring_submit_shutdown(), ring_submit_close(), ring_submit_read(), ring_submit_write(), ring_submit_openat(), ring_submit_statx(), ring_submit_statx_fdsize(), statx_st_size(), ring_submit_socket(), ring_submit_poll(), ring_submit_poll_multishot(), ring_submit_poll_remove(), ring_break_wait(), and ring_wait();
• ring_set_callback(), ring_set_c_callback(), ring_serve_completions(), ring_stop_serving(), and ring_reset_serving() for completion-service control;
• completion_check(), completion_user_data(), completion_res(), completion_flags(), completion_sequence(), completion_result(), and completion_kind() for native completion inspection. Kind values match URING_API_COMPLETION_KIND_* in uring_api_completion_kinds.h and CompletionKind in Python.

Check URING_API_CAPI_FEATURE_CORE before calling the function table. The flag describes the capsule API surface, not runtime kernel support for individual operations. Use probe() to check whether this process can create a ring and to read runtime support for optional operation helpers from the returned flat dictionary. A C completion callback receives the ring object, a list of completions for one kernel drain batch, and the supplied user_data. Return 0 for success; return a negative value with a Python exception set to report an unraisable error and stop the serving worker group. Callback pointers must not be changed while serve_completions() workers are active.

Choosing Ring Sizes

Ring sizing is about queue depth, not payload buffer size. A modest application can start with a small number of in-flight operations; a server usually wants enough entries to cover its expected concurrent I/O without constantly draining and refilling the ring.

Typical starting points:

Ring entries and provided-buffer pools should be configured separately:

• ring entries control how many operations can be submitted or completed at once;
• create_buf_group() registers a provided-buffer ring whose storage stays pinned for receive operations that select buffers from that group;
• large provided-buffer pools can exceed RLIMIT_MEMLOCK even when ring creation itself succeeds.

uring-api does not yet expose fixed-buffer registration for send-side fixed zero-copy variants. When that is added, treat it as a separate pool from caller-owned BufGroup rings.

That distinction matters. During probing, a 64 MiB fixed-buffer pool exceeded a default 64 MiB memlock limit because the limit must cover the pinned payload memory plus kernel/accounting overhead.

You can inspect the process limit before choosing BufGroup sizes:

import resource

soft, hard = resource.getrlimit(resource.RLIMIT_MEMLOCK)

print("memlock soft limit:", soft)
print("memlock hard limit:", hard)

Size provided-buffer pools explicitly rather than assuming the largest useful value is safe:

buffer_size = 16 * 1024
buffer_count = 256
pool_bytes = buffer_size * buffer_count

print("planned pinned buffer pool:", pool_bytes)

Good default create_buf_group() profiles would look something like:

The server profile is intentionally near the common default memlock limit on some systems. In practice, leave headroom or raise the limit before registering that much memory.

Containers and Limits

Containers may block io_uring_setup() even when the host kernel supports it. For example, Docker's default seccomp profile commonly rejects ring creation with EPERM. A less restricted profile may be required for development.

Large BufGroup pools may also require raising RLIMIT_MEMLOCK. Prefer smaller buffers while developing the operation model, then make server profiles opt-in and explicit.

Build Requirements

uring-api links against system liburing:

sudo apt install liburing-dev

The native extension requires liburing >= 2.4. Older headers do not expose the version macros we use for build-time validation, and they also predate the data and ring entry helpers used by the extension. On Ubuntu, that means ubuntu-23.10 or newer from distro packages; ubuntu-22.04 needs a newer liburing installed from another source to build _uring_api.

The extension uses multi-phase module initialisation and declares itself safe to import without enabling the GIL on free-threaded CPython builds.