Device browsing problem

Question

I’m writing file browsing software and I want it to work correctly with all portable devices, such as cameras, smart phones and so on. My program shows thumbnails, so I need to read the content of each file.

Now I’m facing some problems:

1. With both my photo cameras I can open only one ISteam from device. For every additional stream I get ERROR_BUSY error. This is inconvenient as I get thumbnails in several background threads.

2. I can open multiple streams from my smart phone, but I cannot seek that streams! As workaround I have to copy the entire stream to temp file system location and process it there.

I wonder what it depends on. Device file system? Driver implementation? Or anything else?

Answer 1

Those seem like very reasonable restrictions on file access to a peripheral with very limited memory (limited fast volatile memory and code EEPROM are more of a concern than size of the flash card).

It's not the file system (which is almost universally FAT or FAT32 for these kinds of devices) or even limitations in the Windows driver (although the limits are probably enforced there to avoid confusing the device) but limited number of file descriptors in the device's embedded file access code.

As a result, you'll probably have to have workarounds for these and other unsupported driver features.

On a related note, multiple threads usually aren't the right way to do background I/O operations. If your devices support OVERLAPPED operation then you can use that along with events and MsgWaitForMultipleObjects (which replaces PeekMessage or GetMessage in the classic GetMessage/TranslateMessage/DispatchMessage main event loop). By keeping everything on one thread you avoid synchronization issues, most race conditions, and prevent the following problem:

• Your customer wants to select and use one of the files on her device, but oh no, the only IStream is being used on a thread reading thumbnails. Too bad, have to wait for that thread to finish its current file.