(foreign-declare "#include<sys/sendfile.h>")
(define %sendfile-implementation
  (foreign-lambda* double ((integer src) (integer dst) (double offset) (unsigned-integer to_send))
    "sendfilevec_t fvec;"
    "size_t written;"
    "fvec.sfv_fd = src;"
    "fvec.sfv_flag = 0;"
    "fvec.sfv_len = to_send;"
    "fvec.sfv_off = (off_t)offset;"
    "if(sendfilev(dst,&fvec,1,&written) < 0){"
    "  if(errno == EAGAIN){"
    "    C_return(written == 0 ? -2 : (double)(offset + written));"
    "  }else{"
    "    C_return(-1);"
    "  }"
    "}"
    "C_return((double)(offset + written));"

    ))