.\" Copyright (C) 2026 Yitang Yang <yi1tang.yang@gmail.com>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_register_query 3 "March 26, 2026" "liburing-2.15" "liburing Manual"
.SH NAME
io_uring_register_query \- query io_uring capabilities and feature support
.SH SYNOPSIS
.nf
.B #include <liburing.h>
.PP
.BI "int io_uring_register_query(struct io_uring_query_hdr *" query ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_register_query (3)
function queries io_uring capabilities and feature support. It provides
information about supported opcodes, flags, and subsystem-specific
capabilities.

The
.I query
argument must point to a
.I struct io_uring_query_hdr
structure that describes the query to perform:

.PP
.in +4n
.EX
struct io_uring_query_hdr {
    __u64 next_entry;
    __u64 query_data;
    __u32 query_op;
    __u32 size;
    __s32 result;
    __u32 __resv[3];
};
.EE
.in
.PP

The
.I next_entry
field can be used to chain multiple queries together. It should point to the
next
.I struct io_uring_query_hdr
structure, or be set to 0 for the last entry in the chain.

The
.I query_data
field must point to a data structure appropriate for the query type specified in
.IR query_op .

The
.I query_op
field specifies the type of query to perform and can be one of:

.TP
.B IO_URING_QUERY_OPCODES
Returns information about supported opcodes and flags. The
.I query_data
field must point to a
.I struct io_uring_query_opcode
structure, which will be filled with information about supported request
opcodes, register opcodes, feature flags, setup flags, enter flags, and SQE
flags.

.TP
.B IO_URING_QUERY_ZCRX
Returns information about zero-copy receive support. The
.I query_data
field must point to a
.I struct io_uring_query_zcrx
structure, which will be filled with information about supported zero-copy
receive flags, features, and configuration details.

.TP
.B IO_URING_QUERY_SCQ
Returns information about the SQ/CQ ring layout. The
.I query_data
field must point to a
.I struct io_uring_query_scq
structure, which will be filled with information about ring header size and
alignment requirements.

.PP
The
.I size
field should be set to the size of the data structure pointed to by
.IR query_data .

Upon return, the
.I result
field will contain 0 on success, or a negative error code on failure.

The reserved
.I __resv
fields must be cleared to zero.

.SH RETURN VALUE
Returns 0 on success. On error, a negative errno value is returned.

.SH NOTES
This function is available since Linux kernel 6.15.

Multiple queries can be efficiently performed in a single system call by
chaining them together using the
.I next_entry
field.

.SH SEE ALSO
.BR io_uring_register (2),
.BR io_uring_setup (2)