Re: [Qemu-devel] How are the temporary files (-snapshot) created on Linux?

[Steve Fosdick]

On 12/09/08 11:18:50, Yann E. MORIN wrote:
> Hello!
> 
> On Friday 12 September 2008 08:04:02 EQX wrote:
> > The code says the -snapshot temporary files are created here:  
> > /tmp/vl.*, but they are never visible for users. Using lsof,
> > they have a state of 'deleted'. How does this work exactly?
> > What type of file is this?
> 
> It's done via some incantation of open(2) followed by unlink(2),
> something
> like:
> 
>   int fd;
>   fd = open( "/tmp/vl.xxx", O_CREAT|... );
>   unlink( "/tmp/vl.xxx" );
>   /* Use the file somehow */
>   close( fd );
> 
> Regards,
> Yann E. MORIN.

There is nothing special about the files concerned.  The key to this is 
that, unlike some other operating systems, Linux (like Unix) allows an 
open file to be deleted and has a well defined way to deal with that 
happenning.

When an open file is deleted only the file name is actually deleted.  
The data in the file (and any new data written to the file) are kept 
until the last process to have the file open closes the file whereupon 
the second half of the delete happens, i.e. deallocating the disk 
storage and returning it to the free space.

This mechanism is exploited by a process that opens a file and 
immediately deletes it like the example above for two reasons:

1. It provides a way to guarantee that the file not exist after the 
process concerned has finished even if it finishes abnormally or gets 
killed for some reason.

2. As a security measure.  Once the file name has been deleted there is 
no way for any other process to stumble upon the file and open it.

The second point is negated somewhat by the /proc filesystem.

Regards,
Steve.