(un)mount ramfs from C code

(un)mount ramfs from C code

[Brian McGrew]

Good morning all!

So I'm using a ramfs for temporary files, thank you whoever designed that,
it works great!

I can mkdir, mount, chmoud, readand write and then umount the thing from the
command line just fine.

What I need now is some method from within my C/C++ code to determine if the
ramfs is mounted, if not, then mount it so I can use it and unmount it when
I'm done, without making a system call.

Can this be done?  Is there any access to mount/unmount from C/C++?

-b

Re: (un)mount ramfs from C code

[Luciano Rocha]

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On Tue, Sep 22, 2009 at 09:04:15AM -0700, Brian McGrew wrote:
> Good morning all!
> 
> So I'm using a ramfs for temporary files, thank you whoever designed that,
> it works great!
> 
> I can mkdir, mount, chmoud, readand write and then umount the thing from the
> command line just fine.
> 
> What I need now is some method from within my C/C++ code to determine if the
> ramfs is mounted, if not, then mount it so I can use it and unmount it when
> I'm done, without making a system call.

You mean without using system(3), right? System call is how your program
interacts with the outside world.

The relevant system calls are:
  - mount(2): mount("none", "/mnt", "ramfs", 0, NULL);
  - umount(2): umount("/mnt");

The function(section) is the standard Unix way of specifying the manual
section of the function. Use it like this:
  $ man 2 mount

About checking whether it is already mounted, you could parse the file
/proc/mounts, or check the result of statfs(2).

Regards,
Luciano Rocha

-- 
Luciano Rocha <luciano@eurotux.com>
Eurotux Informática, S.A. <http://www.eurotux.com/>

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Re: (un)mount ramfs from C code

[Nikos Chantziaras]

On 09/22/2009 07:04 PM, Brian McGrew wrote:
> Good morning all!
>
> So I'm using a ramfs for temporary files, thank you whoever designed that,
> it works great!
>
> I can mkdir, mount, chmoud, readand write and then umount the thing from the
> command line just fine.
>
> What I need now is some method from within my C/C++ code to determine if the
> ramfs is mounted, if not, then mount it so I can use it and unmount it when
> I'm done, without making a system call.
>
> Can this be done?  Is there any access to mount/unmount from C/C++?

I suppose looking at the source code of the 'mount' utility could prove 
very enlightening.  You can find it at:

   http://www.kernel.org/pub/linux/utils/util-linux-ng

Re: (un)mount ramfs from C code

[Nikos Chantziaras]

On 09/22/2009 08:13 PM, Nikos Chantziaras wrote:
> On 09/22/2009 07:04 PM, Brian McGrew wrote:
>> Good morning all!
>>
>> So I'm using a ramfs for temporary files, thank you whoever designed
>> that,
>> it works great!
>>
>> I can mkdir, mount, chmoud, readand write and then umount the thing
>> from the
>> command line just fine.
>>
>> What I need now is some method from within my C/C++ code to determine
>> if the
>> ramfs is mounted, if not, then mount it so I can use it and unmount it
>> when
>> I'm done, without making a system call.
>>
>> Can this be done? Is there any access to mount/unmount from C/C++?
>
> I suppose looking at the source code of the 'mount' utility could prove
> very enlightening. You can find it at:
>
> http://www.kernel.org/pub/linux/utils/util-linux-ng

Also, "man 2 mount" will provide docs about the the mount() function in 
<sys/mount.h>.  If for some reason that man page isn't provided in your 
system, you can find a copy at:

   http://linux.die.net/man/2/mount

Re: (un)mount ramfs from C code

[Matthias Schniedermeyer]

On 22.09.2009 09:04, Brian McGrew wrote:
> Good morning all!
> 
> So I'm using a ramfs for temporary files, thank you whoever designed that,
> it works great!
> 
> I can mkdir, mount, chmoud, readand write and then umount the thing from the
> command line just fine.
> 
> What I need now is some method from within my C/C++ code to determine if the
> ramfs is mounted, if not, then mount it so I can use it and unmount it when
> I'm done, without making a system call.
> 
> Can this be done?  Is there any access to mount/unmount from C/C++?

The "/bin/mountpoint"-util appears to stat the mountpoint and it's 
parent directory and compares if they have the same device-no (st_dev). 
If they are different, then something is mounted.






Bis denn

-- 
Real Programmers consider "what you see is what you get" to be just as 
bad a concept in Text Editors as it is in women. No, the Real Programmer
wants a "you asked for it, you got it" text editor -- complicated, 
cryptic, powerful, unforgiving, dangerous.

Re: (un)mount ramfs from C code

[Brian McGrew]

> You mean without using system(3), right? System call is how your program
> interacts with the outside world.
> 
> The relevant system calls are:
>   - mount(2): mount("none", "/mnt", "ramfs", 0, NULL);
>   - umount(2): umount("/mnt");
> 

Thanks for the help!  I'm getting there.  Considering the following:

If (mount("ramfs", rd_path, "ramfs", MS_NOEXEC | MS_NOSUID, "size=2000m") <
0) {
    strerror(errno);
} else {
    std::cout << "mounted";
}

Works great!  Thank you.  However, even though the filesystem is mounted and
/bin/mountpoint confirms it's a mountpoint, I do not see the mountpoint
listed in /etc/mtab.

Also, according to what I've read about ramfs, the size is supposed to be
limited to size=, however, I can cat /dev/zero until the box it out of
memory (and swap).

I think I'm just missing something and google's not helping a whole lot
since ramfs isn't the most popular subject today.

Thanks,

-b

Re: (un)mount ramfs from C code

[Tim Walberg]

mount(2) (i.e. the system-call/library function) does not update /etc/mtab, IIRC...
mount(8) (the command-line utility) does...

In either case, I would expect the mount to show up in /proc/mounts (assuming
/proc is mounted).



On 09/22/2009 12:30 -0700, Brian McGrew wrote:
>>	> You mean without using system(3), right? System call is how your program
>>	> interacts with the outside world.
>>	> 
>>	> The relevant system calls are:
>>	>   - mount(2): mount("none", "/mnt", "ramfs", 0, NULL);
>>	>   - umount(2): umount("/mnt");
>>	> 
>>	
>>	Thanks for the help!  I'm getting there.  Considering the following:
>>	
>>	If (mount("ramfs", rd_path, "ramfs", MS_NOEXEC | MS_NOSUID, "size=2000m") <
>>	0) {
>>	    strerror(errno);
>>	} else {
>>	    std::cout << "mounted";
>>	}
>>	
>>	Works great!  Thank you.  However, even though the filesystem is mounted and
>>	/bin/mountpoint confirms it's a mountpoint, I do not see the mountpoint
>>	listed in /etc/mtab.
>>	
>>	Also, according to what I've read about ramfs, the size is supposed to be
>>	limited to size=, however, I can cat /dev/zero until the box it out of
>>	memory (and swap).
>>	
>>	I think I'm just missing something and google's not helping a whole lot
>>	since ramfs isn't the most popular subject today.
>>	
>>	Thanks,
>>	
>>	-b
>>	
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End of included message



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