Faster way to get File Size information C++

问题

I have a function to get a FileSize of a file. I am running this on WinCE. Here is my current code which seems particularily slow

int Directory::GetFileSize(const std::string &filepath)
{
    int filesize = -1;

#ifdef linux
    struct stat fileStats;
    if(stat(filepath.c_str(), &fileStats) != -1)
      filesize = fileStats.st_size;
#else
    std::wstring widePath;
    Unicode::AnsiToUnicode(widePath, filepath);

    HANDLE hFile = CreateFile(widePath.c_str(), 0, FILE_SHARE_READ | FILE_SHARE_WRITE, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
    if (hFile > 0)
    {
      filesize = ::GetFileSize( hFile, NULL); 
    }

    CloseHandle(hFile);
#endif

    return filesize;
}

回答1:

At least for Windows, I think I'd use something like this:

__int64 Directory::GetFileSize(std::wstring const &path) { 

    WIN32_FIND_DATAW data;
    HANDLE h = FindFirstFileW(path.c_str(), &data);
    if (h == INVALID_HANDLE_VALUE)
        return -1;

    FindClose(h);

    return data.nFileSizeLow | (__int64)data.nFileSizeHigh << 32;
}

If the compiler you're using supports it, you might want to use long long instead of __int64. You probably do not want to use int though, as that will only work correctly for files up to 2 gigabytes, and files larger than that are now pretty common (though perhaps not so common on a WinCE device).

I'd expect this to be faster than most other methods though. It doesn't require opening the file itself at all, just finding the file's directory entry (or, in the case of something like NTFS, its master file table entry).

回答2:

Your solution is already rather fast to query the size of a file.

Under Windows, at least for NTFS and FAT, the file system driver will keep the file size in the cache, so it is rather fast to query it. The most time-consuming work involved is switching from user-mode to kernel-mode, rather than the file system driver's processing.

If you want to make it even faster, you have to use your own cache policy in user-mode, e.g. a special hash table, to avoid switching from user-mode to kernel-mode. But I don't recommend you to do that, because you will gain little performance.

PS: You'd better avoid the statement Unicode::AnsiToUnicode(widePath, filepath); in your function body. This function is rather time-consuming.

回答3:

Just an idea (I haven't tested it), but I would expect GetFileAttributesEx to be fastest at the system level. It avoids having to open the file, and logically, I would expect it to be faster than FindFirstFile, since it doesn't have to maintain any information for continuing the search.

回答4:

You could roll your own but I don't see why your approach is slow:

int Get_Size( string path )
{
// #include <fstream>
FILE *pFile = NULL;

// get the file stream
fopen_s( &pFile, path.c_str(), "rb" );

// set the file pointer to end of file
fseek( pFile, 0, SEEK_END );

// get the file size
int Size = ftell( pFile );

// return the file pointer to begin of file if you want to read it
// rewind( pFile );

// close stream and release buffer
fclose( pFile );

return Size;
}

来源：https://stackoverflow.com/questions/14628141/faster-way-to-get-file-size-information-c

标签

c++

optimization

windows-ce