c-backslash-region is an interactive compiled Lisp function in `cc-cmds.el'.

(c-backslash-region FROM TO DELETE-FLAG &optional LINE-MODE)

Insert, align, or delete end-of-line backslashes on the lines in the region.
With no argument, inserts backslashes and aligns existing backslashes.
With an argument, deletes the backslashes. The backslash alignment is
done according to the settings in `c-backslash-column',
`c-backslash-max-column' and `c-auto-align-backslashes'.

This function does not modify blank lines at the start of the region.
If the region ends at the start of a line and the macro doesn't
continue below it, the backslash (if any) at the end of the previous
line is deleted.

You can put the region around an entire macro definition and use this
command to conveniently insert and align the necessary backslashes.