mciSendString won't play an audio file if path is too long

问题

When the path+filename of a file is really long, I've noticed that

PlaySound(fName.c_str(), NULL, SND_ASYNC);

works, but not

mciSendString((L"open \"" + fName + L"\" type waveaudio alias sample").c_str(), NULL, 0, NULL);
mciSendString(L"play sample", NULL, 0, NULL);

Example of failing command:

open "C:\qisdjqldlkjsqdjqdqjslkdjqlksjlkdjqsldjlqjsdjqdksq\dajdjqjdlqjdlkjazejoizajoijoifjoifjdsfjsfszjfoijdsjfoijdsoifoidsjfojdsofjdsoijfoisjfoijoisdjfosjfqsd\Windows Critical Stop.wav" type waveaudio alias sample

But:

I really need mciSendString instead of PlaySound(), because PlaySound() doesn't play certain files (48 khz audio files, sometimes 24-bit files, etc.)
I need to be able to play audio files with potentially long paths because the end user of my app might have such files

How to make mciSendString accept long filenames?

Notes:

I've also tried with this MSDN example using mciSendCommand, but it's the same.
The max path+filename length is 127 (127: working, 128+: not working)
If really it's impossible to make mci* functions work with longer-than-127-char filenames, what could I use instead, just with winapi (without external libraries)? (PlaySound is not an option because doesn't work realiably with all the wav files, such as 48 khz: non-working, etc.)

回答1:

The 127 limit looks strange. I didn't find any information on MSDN about it.

There is an alternative syntax to open: open waveaudio!right.wav
An option You could try is to change the working directory to the directory of the file, then the limit only applies to filename. -> SetCurrentDiectory
To shorten the filename a Winapi function can be used GetShortPathName
But:

SMB 3.0 does not support short names on shares with continuous availability capability.

Resilient File System (ReFS) doesn't support short names. If you call GetShortPathName on a path that doesn't have any short names on-disk, the call will succeed, but will return the long-name path instead. This outcome is also possible with NTFS volumes because there's no guarantee that a short name will exist for a given long name.

Based on example from MSDN:

#include <string>
#include <Windows.h>

template<typename StringType>
std::pair<bool, StringType> shortPathName( const StringType& longPathName )
{
    // First obtain the size needed by passing NULL and 0.
    long length = GetShortPathName( longPathName.c_str(), NULL, 0 );
    if (length == 0) return std::make_pair( false, StringType() );

    // Dynamically allocate the correct size 
    // (terminating null char was included in length)
    StringType  shortName( length, ' ' );

    // Now simply call again using same long path.
    length = GetShortPathName( longPathName.c_str(), &shortName[ 0 ], length );
    if (length == 0) return std::make_pair( false, StringType() );

    return std::make_pair(true, shortName);
}


#include <locale>
#include <codecvt>

#include <iostream>
std::wstring_convert<std::codecvt_utf8_utf16<wchar_t>> converter;

//std::string narrow = converter.to_bytes( wide_utf16_source_string );
//std::wstring wide = converter.from_bytes( narrow_utf8_source_string );

int main( int argc, char** argv )
{
    std::wstring myPath = converter.from_bytes( argv[0] );

    auto result = shortPathName( myPath );
    if (result.first)
        std::wcout << result.second ;


    return 0;
}

回答2:

This is a limitation of the legacy MCI capabilities. There are two issues you face with using the MCI API:

The path name is too long, and this API cannot handle long filenames. The limitation is generally around 260 characters as noted on the page.
Not all files have a "short name". Starting with Windows 7, so called 8.3 (FILENAME.EXT) file creation could be disabled. This means that there may not be a path that GetShortPathName can return that will allow MCI to access the file.

Replacing the whole thing with a modern API is highly recommended. DirectDraw and Media Foundation, as mentioned by other commenters, would be suitable replacements.

回答3:

I have debugged it (on mciSendCommand example). The problem occurs when mwOpenDevice calls mmioOpen:

winmm.dll!_mciSendCommandW@16
winmm.dll!mciSendCommandInternal
winmm.dll!mciSendSingleCommand
winmm.dll!_mciOpenDevice@12
winmm.dll!mciLoadDevice
    winmm.dll!_mciSendCommandW@16
    winmm.dll!mciSendCommandInternal
    winmm.dll!mciSendSingleCommand
    winmmbase.dll!_DrvSendMessage@16
    winmmbase.dll!InternalBroadcastDriverMessage
        mciwave.dll!_DriverProc@20
        mciwave.dll!_mciDriverEntry@16
        mciwave.dll!_mwOpenDevice@12
        winmmbase.dll!_mmioOpenW@12

Here, mmioOpen is called with MMIO_PARSE flag to convert file path to fully qualified file path. According to MSDN, this has a limitation:

The buffer must be large enough to hold at least 128 characters.

That is, buffer is always assumed to be 128 bytes long. For long filenames, the buffer turns out to be insufficient and mmioOpen returns error, causing mciSendCommand to think that sound file is missing and return MCIERR_FILENAME_REQUIRED.

Unfortunately, since it's resolving fully qualified file path, SetCurrentDirectory will not help.

Since the problem is inside MCI driver (mciwave.dll) I doubt there is a way to force MCI subsystem to handle long path.

来源：https://stackoverflow.com/questions/45221390/mcisendstring-wont-play-an-audio-file-if-path-is-too-long

标签

c++

windows

winapi

audio

mci