Global constants

[Stone, Joshua I]

Martin Hunt wrote:
> Also it would be nice to have a way to declare these constants in
> embedded C, so we can use the C header files to get the values, rather
> than look them up.
> 
> something like
> %{ const_O_CREAT = O_CREAT; %}
>   instead of
> O_CREAT = 64

Along these same lines, it would be nice to have a more direct way to
access macros, instead of resorting to embedded-C.  Perhaps something
like:
	import O_CREAT, O_TRUNC, O_APPEND

The translator could make references to "imported" values as direct
macro accesses, instead of needing a global systemtap variable.

There are of course safety issues here - the imported name should be a
macro, and cannot have side-effects.  An #ifndef,#error check fixes the
first, but the second is tricky.  There are also type-safety issues to
consider.

Direct access to macros would make script parameters easy:

	# openwatch.stp
	import FILE_TO_WATCH
	probe syscall.open {
	    if (FILE_TO_WATCH == filename)
	        log(<relevant info>)
	}


	$ stap openwatch.stp -D FILE_TO_WATCH="/etc/passwd"

This can be imitated with an embedded-C function and a begin probe to
read it, but guru-mode shouldn't be required for simple parameter
access.

Josh