Efficient way to pass socket data to structured data structs

Jerry Mcguire · 04-23-2018, 10:14 PM

In the old times when decoding some messages in the socket using `read`.

Code:

ssize_t read( int fd, void *buf, size_t count );

The senior programmers would get angry if you set count to 1. They would advocate to bulk read a chunk of data into a buffer and move the pointer along when decoding the buffer. This is what I have believed in up to this very date.

If the socket data is structured, I would simply define the struct and cast over the buffer.

Code:

typedef struct certain_data {
  int32_t a;
  int32_t b;
} certain_data_t;

Hooray! This is efficient enough.

However when the socket data is unstructured and breaks the byte-alignment, the old time method does not work?

For example, the protocol is something like this:

Code:

int32_t time;
uint8_t cType;
uint16_t len;
char name[len];

Things get nasty:
1) a single byte 'cType' is inserted which breaks the byte-alignment in my own data struct. I cannot simply cast the buffer.
2) the socket protocol adopts a variable length field. I cannot simply read the 'name' field without looking at 'len'.

Eventually I'll have to do a lot of expensive memcpy.

What would you suggest to tackle efficiently?

NevemTeve · 04-24-2018, 02:53 AM

There is a tool for this in libc: FILE. You can use fdopen, fread, fgetc and so on.

sundialsvcs · 04-24-2018, 07:00 AM

Compilers will by default insert alignment-bytes ("slack bytes") into structures for more-efficient access, but you can in various ways specify structure-by-structure that this should not be done. (In Pascal, for instance, the keyword is packed.)

- - -

Also, if the API is in any way public or might be used by different systems, you probably want to serialize the data e.g. with JSON or YAML. Something as basic as "little/big-endianness" or native word-size can torpedo such things, making the tiny "extra overhead" justifiable. These days, "efficiency" probably doesn't matter anymore, but robustness and transportability may well might.

camp0 · 04-25-2018, 04:34 AM

The key for send structs over network is to use packed on them so they will be align. Here is an example

Code:

struct dns_header {
        uint16_t	xid;           
	uint8_t 	rd :1; // recursion desired
    	uint8_t 	tc :1; // truncated message
    	uint8_t 	aa :1; // authoritive answer
    	uint8_t 	opcode :4; // purpose of message
    	uint8_t 	qr :1; // query/response flag
    	uint8_t 	rcode :4; // response code
    	uint8_t 	cd :1; // checking disabled
    	uint8_t 	ad :1; // authenticated data
    	uint8_t 	z :1; // its z! reserved
    	uint8_t 	ra :1; // recursion available
        uint16_t       	questions;       
        uint16_t       	answers;       
        uint16_t       	authorities;
	uint16_t	additionals;     
	uint8_t		data[0];
} __attribute__((packed));