Understanding second argument in the function "xdrmem_create"

Question

I was debugging ganglia and came through sending data using XDR through UDP channel. I found the second argument of the function xdrmem_create ( xdrs, addr, size, op) strange. Where the syntax of the arguments are given as:

XDR *xdrs;
char *addr;
u_int size;
enum xdr_op op;

The reference of this function is here.

As you can see, the second argument (xdrs) of this function is a character array. And this is similarly declared in one of the ganglia's function as char msgbuf[GANGLIA_MAX_MESSAGE_LEN];.

After calling above function as xdrmem_create(&x, msgbuf, GANGLIA_MAX_MESSAGE_LEN, XDR_ENCODE); in ganglia , appropriate data in the ganglia's specific structure (cb->msg) is encoded to XDR format by calling the function xdr_Ganglia_value_msg(&x, &(cb->msg)) where x is an XDR type of variable.

Later, to send the encoded data through UDP channel, the function Ganglia_udp_send_message( udp_send_channels, msgbuf, len); is called. To understand how this XDR data is sent, I tried to print the output of the content of msgbuf using fprintf but it always print nothing in spite of the fact that it is a character array . And it is also evident that the encoded data is sent successfully.

So, my question is, How data encoded into XDR format is being sent through UDP channel here?

I have pasted a part of code from ganglia here. You can see from the line 131 to 136 for your reference.

I had tried to print the length of `msgbuf` using `strlen()` and it always returned zero. while `sizeof()` returns 8. — Rohit, Sep 22 '14 at 11:33

score 1 · Accepted Answer · answered Sep 22 '14 at 11:32

The messages are encoded with XDR, a binary format. The XDR libraries used are old, and a modern version of the API probably would have used uint8_t instead of char. Despite it using char, it is binary data - i.e. you cannot print the data as a string.

If you want to print this data, use a loop where you print each byte as hex e.g. by doing printf("%02X ", msgbuf[i]);

Read RFC4506 to learn about the XDR encoding. The actual messages are described in an XDR language specification, and run through a code generation tool (e.g. rpcgen for C code) to generate code for encoding/decoding. See https://github.com/fastly/ganglia/blob/master/lib/gm_protocol.x for the message definitions that Ganglia defines.

Understanding second argument in the function "xdrmem_create"

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