Add io_uring_register_sync_cancel() man page

Link: https://github.com/axboe/liburing/issues/659
Signed-off-by: Jens Axboe <axboe@kernel.dk>

man/io_uring_register_sync_cancel.3

@@ -0,0 +1,71 @@
+.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
+.\"
+.\" SPDX-License-Identifier: LGPL-2.0-or-later
+.\"
+.TH io_uring_register_sync_cancel 3 "September 21, 2022" "liburing-2.3" "liburing Manual"
+.SH NAME
+io_uring_register_sync_cancel \- issue a synchronous cancelation request
+.SH SYNOPSIS
+.nf
+.B #include <liburing.h>
+.PP
+.BI "int io_uring_register_buffers(struct io_uring *" ring ",
+.BI "                              struct io_uring_sync_cancel_reg *" reg ");
+.PP
+.SH DESCRIPTION
+.PP
+The
+.BR io_uring_register_sync_cancel (3)
+function performs a synchronous cancelation request based on the parameters
+specified in
+.I reg .
+
+The
+.I reg
+argument must be filled in with the appropriate information for the
+cancelation request. It looks as follows:
+.PP
+.in +4n
+.EX
+struct io_uring_sync_cancel_reg {
+    __u64 addr;
+    __s32 fd;
+    __u32 flags;
+    struct __kernel_timespec timeout;
+    __u64 pad[4];
+};
+.EE
+.in
+.PP
+
+The arguments largely mirror what the async prep functions support, see
+.BR io_uring_prep_cancel (3)
+for details. Similarly, the return value is the same. The exception is the
+.I timeout
+argument, which can be used to limit the time that the kernel will wait for
+cancelations to be successful. If the
+.I tv_sec
+and
+.I tv_nsec
+values are set to anything but
+.B -1UL ,
+then they indicate a relative timeout upon which cancelations should be
+completed by.
+
+The
+.I pad
+values must be zero filled.
+
+.SH RETURN VALUE
+See
+.BR io_uring_prep_cancel (3)
+for details on the return value. If
+.I timeout
+is set to indicate a timeout, then
+.B -ETIME
+will be return if exceeded. If an unknown value is set in the request,
+or if the pad values are not cleared to zero, then
+.I -EINVAL
+is returned.
+.SH SEE ALSO
+.BR io_uring_prep_cancel (3)