C++: Reading data from a memory mapped file

Question

I have used MapVirtualFile to map a file under Window using C++ VS2010. The void is

  void*                   m_pVoiceData;

I would now like to fill a structure with the data.

I have been using

void clsMapping::FeedPitchmarksFromMap(udtPitchmarks &uAudioInfo,unsigned long int uBytePos)
{

    unsigned long int iCountPM;
    memcpy(&iCountPM, &((char*)(m_pVoiceData))[uBytePos],sizeof(int));

    uAudioInfo.Pitchmarks.resize(iCountPM);

    for (unsigned long int i=0;i<iCountPM;i++)
    {
        iBytePos+=sizeof(int);
        unsigned long int iByteStart;
        memcpy(&iByteStart, &((char*)(m_pVoiceData))[iBytePos],4);

        iBytePos+=sizeof(int);
        unsigned long int iByteCount;
        memcpy(&iByteCount, &((char*)(m_pVoiceData))[iBytePos],4);

        iBytePos+=sizeof(int);
        unsigned int iF0;
        memcpy(&iF0, &((char*)(m_pVoiceData))[iBytePos],4);

        uAudioInfo.Pitchmarks[i].ByteStart =iByteStart;
        uAudioInfo.Pitchmarks[i].ByteCount =iByteCount;
        uAudioInfo.Pitchmarks[i].F0 =iF0;
    }
}

As one can see, I am currently using memcpy to get the data. Is there a faster way to do "get my data over"?

You declare a `long` variable and copy `sizeof(int)` to it. Remember that `sizeof(long)` doesn't have to equal to `sizeof(int)`. — Some programmer dude, Sep 14 '13 at 15:59
Also, is it really `iByteStart` as argument? Or is it `iBytePos` you mean? — Some programmer dude, Sep 14 '13 at 16:06
Thank you. I mistyped it and edited my post now. And yep, I made a mistake assuming the size of int and long is the same. However, in my case it is, so no error (for now). I will change that. — tmighty, Sep 14 '13 at 16:21

score 2 · Answer 1 · answered Sep 14 '13 at 16:04

2

Assuming that your memory-mapped file has exactly the same alignment and padding as the type of your uAudioInfo.Pitchmarks[0] (presumably a struct of some kind), you can just use one single memcpy to copy the whole lot, rather than memcpying individual fields like you're currently doing.

answered Sep 14 '13 at 16:04

C. K. Young

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You have seen my code. Can you tell me how I could do that in one rush? – tmighty Sep 19 '13 at 17:16

score 2 · Accepted Answer · answered Sep 14 '13 at 16:04

2

The "fastest" would probably be simple assignment:

unsigned long iCountPM = 
    *reinterpret_cast<unsigned long*>(reinterpret_cast<char*>(m_pVoiceData) + uBytePos);

Can be used instead for all those memcpy calls in the function.

answered Sep 14 '13 at 16:04

Some programmer dude

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And that would be faster or just more elegant? – tmighty Sep 14 '13 at 16:14
@tmighty Well, both I would guess, as it's one function call and (possible) loop less. – Some programmer dude Sep 14 '13 at 20:12
1

I'm suggesting even further, that if the memory-mapped file is laid out exactly the same as an array of the structures in memory, _all_ elements of the array can be copied with a single `memcpy`. That's where it's even faster than "simple assignment". – C. K. Young Sep 14 '13 at 20:35

C++: Reading data from a memory mapped file

2 Answers2