OpenBSD uses a different API:
https://man.openbsd.org/sosplice.9

int sosplice(struct socket *so, int fd, off_t max, struct timeval *tv);
int somove(struct socket *so, int wait);

"The function sosplice() is used to splice together a source and a drain socket."

"The function somove() transfers data from the source's receive buffer to the drain's send buffer."

"Socket splicing can be invoked from userland via the setsockopt(2) system-call at the SOL_SOCKET level with the socket option SO_SPLICE."