C-u C-space

C-u C-space

[A Soare]

Today I learned about this.

Now I am trying to find out what command is associated with C-u C-space. I do

C-h k C-u .

It appears <<C-u runs the command universal-argument>> . I did not arrive to press C-space.

Is it a bug? Or I used wrong describe-key ?

Re: C-u C-space

[Maciej Katafiasz]

Den Thu, 10 May 2007 00:45:36 +0200 skrev A Soare:

> Now I am trying to find out what command is associated with C-u C-space.
> I do
> 
> C-h k C-u .
> 
> It appears <<C-u runs the command universal-argument>> . I did not
> arrive to press C-space.
> 
> Is it a bug? Or I used wrong describe-key ?

No, C-u modifies the command that follows. Strictly speaking, it supplies
the so called universal argument, which commands are free to read and
modify their behaviour accordingly. So what you really want is C-h k
C-SPC, which should (and does) explain what happens if you supply the
universal argument.

Cheers,
Maciej

RE: C-u C-space

[Drew Adams]

> Now I am trying to find out what command is associated with C-u 
> C-space. I do C-h k C-u .
> 
> It appears <<C-u runs the command universal-argument>> . I did 
> not arrive to press C-space.
> 
> Is it a bug? Or I used wrong describe-key ?

It is not a bug. It is the way Emacs works. You need to read the Emacs manual section on C-u (prefix argument). 

Once you understand that, you'll understand that you need only do C-h C-SPC. That will give you an explanation of the command bound to C-SPC, `set-mark-command', and that explanation will tell you what `C-u' means in that context:

 C-SPC runs the command set-mark-command
 ...

 With argument, e.g. C-u C-@, jump to mark, and pop a
 new position for mark off the local mark ring...

 With a double C-u prefix argument, e.g. C-u C-u C-@,
 unconditionally set mark where point is.

IOW, C-u has different meanings and behaviors, depending on what key sequence follows it. Use C-h on the base key sequence to find out about its behavior when it is modified by C-u.