io_uring_prep_splice(3)

NAME
io_uring_prep_splice - prepare an splice request

SYNOPSIS
#include <fcntl.h>
#include <liburing.h>

void io_uring_prep_splice(struct io_uring_sqe *sqe,
                          int fd_in,
                          int64_t off_in,
                          int fd_out,
                          int64_t off_out,
                          unsigned int nbytes,
                          unsigned int splice_flags);

DESCRIPTION

The io_uring_prep_splice(3) function prepares a splice request. The submission queue entry sqe is setup to use as input the file descriptor fd_in at offset off_in, splicing data to the file descriptor at fd_out and at offset off_out. nbytes bytes of data should be spliced between the two descriptors. splice_flags are modifier flags for the operation. See splice(2) for the generic splice flags.

If fd_out is a direct descriptor, IOSQE_FIXED_FILE can be set in the SQE to indicate that. For the input file, the io_uring specific SPLICE_F_FD_IN_FIXED can be set in splice_flags and fd_in given as a registered file descriptor offset.

If fd_in refers to a pipe, off_in is ignored and must be set to -1.

If fd_in does not refer to a pipe and off_in is -1, then nbytes are read from fd_in starting from the file offset, which is incremented by the number of bytes read.

If fd_in does not refer to a pipe and off_in is not -1, then the starting offset of fd_in will be off_in.

The same rules apply to fd_out and off_out.

This function prepares an async splice(2) request. See that man page for details.

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.

SEE ALSO
io_uring_get_sqe(3), io_uring_submit(3), io_uring_register(2), splice(2)

NOTES
Note that even if fd_in or fd_out refers to a pipe, the splice operation can still fail with EINVAL if one of the fd doesn't explicitly support splice operation, e.g. reading from terminal is unsupported from kernel 5.7 to 5.11.

Despite accepting an unsigned number of bytes, this function can transfer at most INT_MAX bytes per call (the maximum for the underlying syscall interface). In practice, limits as low as 65536 have been observed (just like with splice(2) itself).