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Full Discussion: Function pointer to inline function ?
Top Forums Programming Function pointer to inline function ? Post 302355575 by emitrax on Wednesday 23rd of September 2009 05:47:41 AM
Old 09-23-2009
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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LEARN ABOUT CENTOS

htonl

BYTEORDER(3)						     Linux Programmer's Manual						      BYTEORDER(3)

NAME

       htonl, htons, ntohl, ntohs - convert values between host and network byte order

SYNOPSIS

       #include <arpa/inet.h>

       uint32_t htonl(uint32_t hostlong);

       uint16_t htons(uint16_t hostshort);

       uint32_t ntohl(uint32_t netlong);

       uint16_t ntohs(uint16_t netshort);

DESCRIPTION

       The htonl() function converts the unsigned integer hostlong from host byte order to network byte order.

       The htons() function converts the unsigned short integer hostshort from host byte order to network byte order.

       The ntohl() function converts the unsigned integer netlong from network byte order to host byte order.

       The ntohs() function converts the unsigned short integer netshort from network byte order to host byte order.

       On  the i386 the host byte order is Least Significant Byte first, whereas the network byte order, as used on the Internet, is Most Signifi-
       cant Byte first.

CONFORMING TO

       POSIX.1-2001.

       Some systems require the inclusion of <netinet/in.h> instead of <arpa/inet.h>.

SEE ALSO

       endian(3), gethostbyname(3), getservent(3)

COLOPHON

       This page is part of release 3.53 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2009-01-15							      BYTEORDER(3)
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