io_uring_prep_provide_buffers(3) liburing Manual io_uring_prep_provide_buffers(3)

io_uring_prep_provide_buffers - prepare a provide buffers request

#include <liburing.h>
void io_uring_prep_provide_buffers(struct io_uring_sqe *sqe,
                                   void *addr,
                                   int len,
                                   int nr,
                                   int bgid,
                                   int bid);

The io_uring_prep_provide_buffers(3) function prepares a request for providing the kernel with buffers. The submission queue entry sqe is setup to consume nr number of len sized buffers starting at addr and identified by the buffer group ID of bgid and numbered sequentially starting at bid.

This function sets up a request to provide buffers to the io_uring context that can be used by read or receive operations. This is done by filling in the SQE buf_group field and setting IOSQE_BUFFER_SELECT in the SQE flags member. If buffer selection is used for a request, no buffer should be provided in the address field. Instead, the group ID is set to match one that was previously provided to the kernel. The kernel will then select a buffer from this group for the IO operation. On successful completion of the IO request, the CQE flags field will have IORING_CQE_F_BUFFER set and the selected buffer ID will be indicated by the upper 16-bits of the flags field.

Different buffer group IDs can be used by the application to have different sizes or types of buffers available. Once a buffer has been consumed for an operation, it is no longer known to io_uring. It must be re-provided if so desired or freed by the application if no longer needed.

The buffer IDs are internally tracked from bid and sequentially ascending from that value. If 16 buffers are provided and start with an initial bid of 0, then the buffer IDs will range from 0..15. The application must be aware of this to make sense of the buffer ID passed back in the CQE.

Buffer IDs always range from 0 to 65535 , as there are only 16-bits available in the CQE to pass them back. This range is independent of how the buffer group initially got created. Attempting to add buffer IDs larger than that, or buffer IDs that will wrap when cast to a 16-bit value, will cause the request to fail with -E2BIG or -EINVAL .

Not all requests support buffer selection, as it only really makes sense for requests that receive data from the kernel rather than write or provide data. Currently, this mode of operation is supported for any file read or socket receive request. Attempting to use IOSQE_BUFFER_SELECT with a command that doesn't support it will result in a CQE res error of -EINVAL. Buffer selection will work with operations that take a struct iovec as its data destination, but only if 1 iovec is provided.


These are the errors that are reported in the CQE res field. On success, res will contain 0 or the number of successfully provided buffers.

The kernel was unable to allocate memory for the request.
One of the fields set in the SQE was invalid.
The number of buffers provided was too big, or the bid was too big. A max value of USHRT_MAX buffers can be specified.
Some of the user memory given was invalid for the application.
The product of len and nr exceed the valid amount or overflowed, or the sum of addr and the length of buffers overflowed.
Attempt to update a slot that is already used.

io_uring_get_sqe(3), io_uring_submit(3), io_uring_register(2), io_uring_prep_remove_buffers(3)

March 13, 2022 liburing-2.2