Conference turris::digital_unix

8759.0. "XDR question" by HYDRA::DORHAMER () Fri Feb 07 1997 15:59

A software developer who is porting his application to Digital UNIX v4.0
would like to know if there is a function or method to find out the size
of data that he is transferring using XDR.  Currently he is estimating the
size using strlen and padding with a fixed amount, but he would like a more
exact method to do this.  Here are his questions:

"I use XDR to transfer structure through the socket. My structure is very
complicated and of variable length. Inside there are several linked list.
The list can be 1,000 nodes or 1 node. Currently I allocate a big enough
buffer to store the XDR string. This is very unefficient when the list has
only a few values.  Is there a strlen function for XDR so I can know the length 
of binary string."

"Currently what I did is, I estimate how many bytes I need, for string I
allocate (strlen + 20) but this is not sharp solution. What I need is a function

like that

int
xdrlen(xdrstring, ... structure definition)

I want to know if DEC has coded it. I have read the whole network
programming manual, but found nothing."
    

I don't know of libc routines that let you estimate how long XDR data will
be.  It would be fairly easy to write a new XDR module that merely counts
the length of data to be encoded, and using the public domain RPC code would
provide enough hints.  (That has a lot of 32/64 bit issues, so it doesn't
replace our libc code.)

Some sizes (I may not be exactly right):

xdr_bytes, xdr_string:	The number of bytes, rounded up to next multiple of 4,
			plus 4.
xdr_char, xdr_short, xdr_int, xdr_long, xdr_float xdr_bool, xdr_enum:	4 bytes.
xdr_longlong, xdr_hyper, xdr_double:	8 bytes

Ah - here's an idea - try calling xdr_getpos before and after encoding
something.  The difference is the number of bytes used.

Me?  I'd just look at the messages sent via tcpdump and figure the length and
structure.

[Posted by WWW Notes gateway]

    Thanks for your help.  I'll relay the info to the customer.
    
    Karen