Re: Command line systemtap macro setting

[Craig Ringer <craig@2ndquadrant.com>]

Frank,

Any thoughts on this? I'd be happy to submit a patch but I'd like to know
if you think it's reasonable to do first, so I don't spend the time writing
something that won't be committable.

It'd be a big usability improvement in systemtap for me to be able to
define systemtap language macros on the command line.


On Fri, 8 Jan 2021 at 13:07, Craig Ringer <craig@2ndquadrant.com> wrote:

> Hi
>
> I keep on finding situations where I want to set systemtap language macros
> on the command line. AFAICS there is no such feature. Right? The closest
> seems to be to generate a .stpm file with the desired macro definition and
> put on the include path.
>
> Would you object to a patch that added this capability? If so, how do you
> think it should be spelled on the command line? "-D" is taken. So maybe
> "-M" ?
>
> The most important time I need command line macro definitions is to work
> around the issues with relative paths. I pretty much always require
> absolute paths for executables in my tapscripts. Otherwise @var("var@cu",
> "exec_path") doesn't work properly or didn't last I checked. And probes
> that fail to match seem to like to fail silently if the executable isn't
> found on the PATH, where they'll report an explicit error if the executable
> is a fully qualified path. So I try to run with absolute paths all the
> time. But I don't really want users to have to hack the script to supply
> them, so I'd like to be able to e.g.
>
>     -D POSTGRES_EXECUTABLE="/path/to/postgres"
>
> However, this won't work - it'd set a C preprocessor macro for the runtime
> compilation, not the systemtap language processing phase. And you can't
>
>     -G POSTGRES_EXECUTABLE="/path/to/postgres"
>
> because probes don't accept globals in their arguments; this isn't legal:
>
>     probe process(POSTGRES_EXECUTABLE).function("foo") { ...}
>
> That can be handled with a positional parameter instead, but doing so is
> less user friendly. It's much less obvious what the param is and does when
> reading the cmdline.
>
> Similarly, if I want to make it easy to customise script behaviour,
> something like this won't work:
>
>     global stats_report_interval_ms=5000
>     probe timer.ms(stats_report_interval_ms ) { do_something(); }
>
>
>
>
> --
>  Craig Ringer                   http://www.2ndQuadrant.com/
>  2ndQuadrant - PostgreSQL Solutions for the Enterprise
>


-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 2ndQuadrant - PostgreSQL Solutions for the Enterprise