How to convert little-endian 64 to host byte order in C++

Question

I need to convert little-endian 64 to host byte order. In winapi i can't find such functions, so i need to write my own, can anyone help me? Thanks!

Considering that you mention "winapi" I guess you're on a Windows system, which means that unless you're on a **very** old system (we're talking 1990's when Windows NT was ported to some big-endian machines) chances are that you're on a x86/x86_64 system, which already is little-endian. So converting little-endian to little-endian is a no-op. — Some programmer dude, Jan 23 '14 at 08:29
A quick search on google returns http://stackoverflow.com/questions/2182002/convert-big-endian-to-little-endian-in-c-without-using-provided-func as the top result - it has some answers you might find useful. — Nick, Jan 23 '14 at 08:35

score 3 · Answer 1 · answered Jan 23 '14 at 08:51

3

Use htonll. It converts an unsigned __int64 to network byte order.

answered Jan 23 '14 at 08:51

Sean

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It should be noted that this is not a portable function, it's Windows only. – Some programmer dude Jan 23 '14 at 09:15
1

@JoachimPileborg - yes, but the questioner mentions the Windows API in his question, so it seems reasonable. – Sean Jan 23 '14 at 09:42
This converts host order to network byte order which is big endian. The OP asked for converting little endian to host order. – Andre Holzner Sep 10 '21 at 13:22

score 2 · Answer 2 · answered Jan 23 '14 at 09:53

I think you need to get the host's endiannes first and you can decide after that if you need to convert anything:

#define BIG_ENDIAN 1
#define LITTLE_ENDIAN 0

int getEndiannes()
{
   int n = 1;
   char *p = &n;
   if(*p) 
       return LITTLE_ENDIAN; 
   else
       return BIG_ENDIAN ;  

}

score 2 · Answer 3 · edited Jan 23 '14 at 11:34

2

In Linux you can use uint64_t htobe64(uint64_t host_64bits);

Check the man page for more details.

edited Jan 23 '14 at 11:34

Angew is no longer proud of SO

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answered Jan 23 '14 at 11:14

Rahul_cs12

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score 1 · Answer 4 · answered Jan 23 '14 at 09:28

If you're reading external data, the usual solution is to build up the individual values as specified:

unsigned long long      //  The only type guaranteed long enough for 64 bits
readData( std::istream& source )
{
    unsigned long long results = source.get();
    results |= source.get() <<  8;
    results |= source.get() << 16;
    results |= source.get() << 24;
    results |= source.get() << 32;
    results |= source.get() << 40;
    results |= source.get() << 48;
    results |= source.get() << 56;
    return results;
}

Of course, you really need some sort of error checking, in case the file ends in the middle of the 8 bytes. (But it is sufficient to check once, after all of the bytes have been read.)

If the data is already in a buffer, then just substitute static_cast<unsigned char>(*p++) for source.get() (where p points to the position in the buffer). In this case, you also have to ensure that there are 8 bytes between the initial p and the end of the buffer before doing the conversion.

How to convert little-endian 64 to host byte order in C++

4 Answers4