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#1 (permalink)

09-23-2009

emitrax

Registered User

Join Date: Apr 2009

Posts: 38

Function pointer to inline function ?

Hi.

Problem: I have to parse the payload of a packet. The payload could be in Big Endian Format (network byte order) or little. That depends on a flag present in the header of the packet.

Solution: A horrible solution could be to check for that flag everytime I have to read a field in the payload, that is

Code:

if (header->most_significan_byte)
    var = ntohl(payload->field32);
else
    var = payload->field32;

and that would be done in all the function body that is responsible for parsing the payload. Quite awful indeed.

I was thinking to use a function pointer, actually two, and init them to ntohl and ntohs if the payload was in network order, or init them to an empty function otherwise. Something like

Code:

uint32_t (*_ntohl)(uint32_t hostlong);
uint16_t (*_ntohs)(uint16_t hostshort);
...

uint32_t empty_ntohl(uint32_t hostlong)
{
     return hostlong;
}

uint16_t empty_ntohs(uint16_t hostshort)
{
     return hostshort;
}

...
if (header->most_sifnigican_byte) {
   _ntohl = ntohl;
   _ntohs =  ntohs;
} else {
   _ntohl = empty_ntohl;
   _ntohs = empty_ntohs;
}
..

var = _ntohl(payload->field32);

Now. Before going ahead and mess up with my already-working-code-without-byte-ordering-support, I'd like to know if this solution is
1 - correct
2 - efficient

or if any of you guys have already faced this problem and have a better and more elegant solution.

In the subject, I wrote function pointer to inline function, because I was wondering if the empty function could be inline to improve performances and avoid the function call
overhead.

Would the pointer function call be replaced with actual inline function in case it points to the inline one?
The answer if of course not, because the last one is done at
compile time. Yeah.. I'm basically answering myself while writing. :-)

Anyway, any comment and/or suggestion?

Thanks in advance.
S.

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