io_uring_prep_send_zc(3)

NAME
io_uring_prep_send_zc - prepare a zerocopy send request

SYNOPSIS
#include <liburing.h>

void io_uring_prep_send_zc(struct io_uring_sqe *sqe,
                           int sockfd,
                           const void *buf,
                           size_t len,
                           int flags,
                           unsigned zc_flags);

void io_uring_prep_send_zc_fixed(struct io_uring_sqe *sqe,
                                 int sockfd,
                                 const void *buf,
                                 size_t len,
                                 int flags,
                                 unsigned zc_flags);
                                 unsigned buf_index);

DESCRIPTION

The io_uring_prep_send_zc(3) function prepares a zerocopy send request. The submission queue entry sqe is setup to use the file descriptor sockfd to start sending the data from buf of size len bytes with send modifier flags flags and zerocopy modifier flags zc_flags.

The io_uring_prep_send_zc_fixed(3) works just like io_uring_prep_send_zc(3) except it requires the use of buffers that have been registered with io_uring_register_buffers(3). The buf and len arguments must fall within a region specified by buf_index in the previously registered buffer. The buffer need not be aligned with the start of the registered buffer.

See io_uring_prep_send(3) for a description of flags that can be set in the SQE ioprio field. In addition to those, the zero-copy send also supports setting IORING_SEND_ZC_REPORT_USAGE . If set, the notification CQE res field will report the number of bytes that were copied rather than sent with zero copy. A value of 0 indicates success. If the value is IORING_NOTIF_USAGE_ZC_COPIED , then data was copied.

As opposed to non-zerocopy send requests, a zerocopy send will usually generate two CQEs. The first CQE holds the result of the send operation itself, and if that CQE has IORING_CQE_F_MORE set in the CQE flags field, then a second notification CQE will be posted for the operation. This second notification tells the application that the memory associated with the send is safe to get reused. The second CQE will have IORING_CQE_F_NOTIF set in the CQE flags field. Also see the io_uring_enter(2) man page for a fuller description of the notification CQE.

Note that using IOSQE_IO_LINK with this request type requires the setting of MSG_WAITALL in the flags argument, as a short send isn't considered an error condition without that being set.

These functions prepare an async zerocopy send(2) request. See that man page for details. For details on the zerocopy nature of it, see io_uring_enter(2).

RETURN VALUE
None

ERRORS
The CQE res field will contain the result of the operation. See the related man page for details on possible values. Note that where synchronous system calls will return -1 on failure and set errno to the actual error value, io_uring never uses errno. Instead it returns the negated errno directly in the CQE res field. Some common error cases are:

-ENOMEM
    The ulimit(1) -l setting is too low to support the size of the attempted zero copy send. Increasing the limit may help

-ENOMEM
    The kernel ran out of memory.

SEE ALSO
io_uring_get_sqe(3), io_uring_submit(3), io_uring_prep_send(3), io_uring_enter(2), send(2)