| 1 | --- |
| 2 | id: tracef |
| 3 | since: 2.5 |
| 4 | --- |
| 5 | |
| 6 | `tracef()` is a small LTTng-UST API to avoid defining your own |
| 7 | tracepoints and tracepoint providers. The signature of `tracef()` is |
| 8 | the same as `printf()`'s. |
| 9 | |
| 10 | The `tracef()` utility function was developed to make user space tracing |
| 11 | super simple, albeit with notable disadvantages compared to custom, |
| 12 | full-fledged tracepoint providers: |
| 13 | |
| 14 | * All generated events have the same provider/event names, respectively |
| 15 | `lttng_ust_tracef` and `event`. |
| 16 | * There's no static type checking. |
| 17 | * The only event field you actually get, named `msg`, is a string |
| 18 | potentially containing the values you passed to the function |
| 19 | using your own format. This also means that you cannot use filtering |
| 20 | using a custom expression at runtime because there are no isolated |
| 21 | fields. |
| 22 | * Since `tracef()` uses C standard library's `vasprintf()` function |
| 23 | in the background to format the strings at runtime, its |
| 24 | expected performance is lower than using custom tracepoint providers |
| 25 | with typed fields, which do not require a conversion to a string. |
| 26 | |
| 27 | Thus, `tracef()` is useful for quick prototyping and debugging, but |
| 28 | should not be considered for any permanent/serious application |
| 29 | instrumentation. |
| 30 | |
| 31 | To use `tracef()`, first include `<lttng/tracef.h>` in the C source file |
| 32 | where you need to insert probes: |
| 33 | |
| 34 | ~~~ c |
| 35 | #include <lttng/tracef.h> |
| 36 | ~~~ |
| 37 | |
| 38 | Use `tracef()` like you would use `printf()` in your source code, for |
| 39 | example: |
| 40 | |
| 41 | ~~~ c |
| 42 | /* ... */ |
| 43 | |
| 44 | tracef("my message, my integer: %d", my_integer); |
| 45 | |
| 46 | /* ... */ |
| 47 | ~~~ |
| 48 | |
| 49 | Link your application with `liblttng-ust`: |
| 50 | |
| 51 | <pre class="term"> |
| 52 | gcc -o app app.c <strong>-llttng-ust</strong> |
| 53 | </pre> |
| 54 | |
| 55 | Execute the application as usual: |
| 56 | |
| 57 | <pre class="term"> |
| 58 | ./app |
| 59 | </pre> |
| 60 | |
| 61 | VoilĂ ! Use the `lttng` command line tool to |
| 62 | [control tracing](#doc-controlling-tracing). You can enable `tracef()` |
| 63 | events like this: |
| 64 | |
| 65 | <pre class="term"> |
| 66 | lttng enable-event --userspace 'lttng_ust_tracef:*' |
| 67 | </pre> |