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C-u C-SPC is used to

Move point to where the mark was, and restore the mark from the ring of former marks.

When trying to get the function bound to C-u C-SPC with describe-key it just stops processing the query, with reason, in C-u and returns the universal-argument description.

So the question can also extend to "How do you get the function bound to a keymap starting with C-u where it acts differently from an ordinary argument"?

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  • If you're trying to bind some key to something normally done with this prefix argument, you could curry the function.
    – Sean Allred
    Jun 12, 2015 at 17:18

3 Answers 3

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Leave the C-u off and check the binding for C-SPC (or whatever you're interested in). The universal argument (the C-u) is often used to make commands do different things. However, the docstring of the command will (or at least should) explain what the command does when preceded by universal arguments.

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"How do you get the function bound to a keymap starting with C-u where it acts differently from an ordinary argument"?

C-u C-SPC and C-SPC will run the same function, just with different arguments.

You'll need to read the documentation or the source code to figure out exactly what the difference are.

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C-u C-SPC is not bound to a single key sequence. C-u is bound to universal-argument, and C-SPC is bound to set-mark-command. If you consult the doc for each of those you will get the answer to your question.

C-u provides a prefix argument for the following command, in this case for command set-mark-command. Consulting the doc for set-mark-command tells you how it interprets a prefix argument:

With prefix argument (e.g., C-u C-SPC), jump to the mark, and set the mark from position popped off the local mark ring (this does not affect the global mark ring). Use C-x C-SPC to jump to a mark popped off the global mark ring (see pop-global-mark).

If set-mark-command-repeat-pop is non-nil, repeating the C-SPC command with no prefix argument pops the next position off the local (or global) mark ring and jumps there.

With C-u C-u as prefix argument, unconditionally set mark where point is, even if set-mark-command-repeat-pop is non-nil.

Consult the Emacs manual for more info about prefix arguments: C-h r i prefix arguments RET. That takes you to node Numeric Arguments of the manual.

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