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

[Mathieu Desnoyers]

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.

Thanks,

Mathieu

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