[lttng-dev] Trying to understand use of lttng enable-event --kernel --userspace-probe=

[Brian Hutchinson]

On Thu, May 18, 2023 at 10:45 AM Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
>
> On 2023-05-18 10:10, Brian Hutchinson wrote:
> [...]
> > I updated my hello world to have a function I'd like to use the
> > --userspace-probe method on with the very original name of
> > 'probe_function':
> >
> > #include <stdio.h>
> > #include <lttng/tracef.h>
> >
> > void probe_function(int i);
> >
> > int main(int argc, char *argv[])
> > {
> >     unsigned int i;
> >     puts("Hello, World!\nPress Enter to continue...");
> >     /*
> >      * The following getchar() call only exists for the purpose of this
> >      * demonstration, to pause the application in order for you to have
> >      * time to list its tracepoints. You don't need it otherwise.
> >      */
> >     getchar();
> >
> >     lttng_ust_tracef("Number %d, string %s", 23, "hi there!");
> >     printf("Number %d, string %s", 23, "hi there!");
> >
> >     for (i = 0; i < argc; i++) {
> >         lttng_ust_tracef("Number %d, argv %s", i, argv[i]);
> >         printf("Number %d, argv %s", i, argv[i]);
> >     }
> >
> >     puts("Quitting now!");
> >
> >     probe_function(i);
> >
> >     return 0;
> > }
> >
> > void probe_function(int i) {
> >
> >     lttng_ust_tracef("Number %d, string %s", i * i, "i^2");
> >     printf("Number %d, string %s", i * i, "i^2");
> >
> > }
> >
> > ... and I get the same error as before when I try to enable the probe:
> > # lttng enable-event --kernel
> > --userspace-probe=/usr/local/bin/hello:probe_function
> > Error: Missing event name(s).
>
> As the error states, you are missing the event name. See
>
> man 1 lttng-enable-event
>
>         lttng [GENERAL OPTIONS] enable-event --kernel
>               [--probe=SOURCE | --function=SOURCE | --syscall |
>                --userspace-probe=SOURCE]
>               [--filter=EXPR] [--session=SESSION]
>               [--channel=CHANNEL] EVENT[,EVENT]...
>
>
> You will want something like:
>
> lttng enable-event --kernel --userspace-probe=/usr/local/bin/hello:probe_function my_probe_function
>
> Where "my_probe_function" is the event name that will appear in the collected traces.

Wow!  I must not have woken up this morning ha, ha.  Thanks for that!
The event is enabled now.  Hope to actually get tracing data now.

Regards,

Brian

>
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>