On 2011-06-11 10.08, T wrote:
> I'm writing a small program which changes working dir to a
> specific directory (using chdir()), and then opens, reads, and
> closes files in that directory, depending on user actions.
> Sometimes this directory is located on a mounted USB stick.
>
> I'm looking for a simple way to detect, from within my program,
> if a user have removed the USB stick without first unmounting it.
>
> The simplest way, I thought, was to check errno for EIO or ENXIO
> (depending on whether an error occurs in fread() or fopen()).
> However, that does not seem to work, because if I call fopen()
> after the USB stick has been removed, the return value is NULL,
> and the errno code is ENOENT instead of ENXIO, which sort of
> makes sense (the file certainly is not there any more), but the
> cause of the error cannot be differentiated from a regular
> mis-spelling of the file name.
>
> So is there some reliable way of detecting whether the underlying
> storage device has gone away when a library/system call fails,
> even if the OS still regards the filesystem as being mounted? Can
> I, upon detecting errno==ENOENT after fopen(), use some other
> calls (for example ioctl(), getfsstat(), or statfs()) to
> determine whether the cause is a mis-spelled filename or removed
> USB stick? What is the recommended/preferred way? Can/should I
> perhaps do something before attempting the fopen(), to find out
> whether the USB stick is still attached?
>
> I prefer simple solutions using standard library/system calls,
> and would like to avoid attaching to some notification mechanism,
> and tailing the kernel log looking for USB detach messages is a
> clumsy last chance solution.
Try to do an open(".", O_RDONLY) if your fopen()/fread() fails.
If you get -1 and ENOENT then the file system where you've parked
your cwd is no longer mounted (statfs() should also work equally
well, but then you'd need to declare a statfs struct you'd have
no further use for).
It's not wise to check before your fopen():s, because they may
still fail if the user pulls the USB stick in the right moment,
only use it as a verification of *what* actually went wrong.
Regards,
/Benny
--
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