# lttng_ust_tracef

## NAME



           lttng_ust_tracef, lttng_ust_vtracef - LTTng-UST printf(3)-like
           interface
    


## SYNOPSIS



           **#include <lttng/tracef.h>**
    
           #define **lttng_ust_tracef**(_fmt_ , ...)
           #define **lttng_ust_vtracef**(_fmt_ , _ap_)
    
           Link with:
    
           •   **-llttng-ust**
    
           •   If you define **_LGPL_SOURCE** before including **< lttng/tracef.h>**
               (directly or indirectly): **-llttng-ust-common**
    


## DESCRIPTION



           The LTTng-UST **lttng_ust_tracef()** and **lttng_ust_vtracef()** API
           allows you to trace your application with the help of simple
           printf(3)-like and vprintf(3)-like macros.
    
           The _fmt_ argument is passed directly as the _fmt_ parameter of
           vasprintf(3), as well as:
    
           For **lttng_ust_tracef()**
               The optional parameters following _fmt_.
    
           For **lttng_ust_vtracef()**
               The _ap_ parameter as the _ap_ parameter of vasprintf(3) (**va_list**
               type).
    
           To use **lttng_ust_tracef()** or **lttng_ust_vtracef()** , include
           **< lttng/tracef.h> **where you need it, and link your application with
           **liblttng-ust** and **liblttng-ust-common**. See the _EXAMPLE_ section
           below for a complete usage example.
    
           Once your application is instrumented with **lttng_ust_tracef()**
           and/or **lttng_ust_vtracef()** calls and ready to run, use
           lttng-enable-event(1) to enable the **lttng_ust_tracef:*** event.
    
           The **lttng_ust_tracef()** and **lttng_ust_vtracef()** events contain a
           single field, named **msg** , which is the formatted string output.
    
           If you need to attach a specific log level to a
           **lttng_ust_tracef()** /**lttng_ust_vtracef()** call, use
           lttng_ust_tracelog(3) and lttng_ust_vtracelog(3) instead.
    
           See also the _LIMITATIONS_ section below for important limitations
           to consider when using **lttng_ust_tracef()** or **lttng_ust_vtracef()**.
    


## EXAMPLE



           Here’s a usage example of **lttng_ust_tracef()** :
    
               #include <stdlib.h>
               #include <lttng/tracef.h>
    
               int main(void)
               {
                   int i;
    
                   for (i = 0; i < 25; i++) {
                       lttng_ust_tracef("my message: %s, this integer: %d",
                                        "a message", i);
                   }
    
                   return EXIT_SUCCESS;
               }
    
           This C source file, saved as **app.c** , can be compiled into a program
           like this:
    
               $ cc -o app app.c -llttng-ust -llttng-ust-common
    
           You can create an LTTng tracing session, enable the
           **lttng_ust_tracef()** events, and start the created tracing session
           like this:
    
               $ lttng create my-session
               $ lttng enable-event --userspace 'lttng_ust_tracef:*'
               $ lttng start
    
           Next, start the program to be traced:
    
               $ ./app
    
           Finally, stop the tracing session, and inspect the recorded
           events:
    
               $ lttng stop
               $ lttng view
    


## LIMITATIONS



           The **lttng_ust_tracef()** and **lttng_ust_vtracef()** utility macros were
           developed to make user space tracing super simple, albeit with
           notable disadvantages compared to custom, full-fledged tracepoint
           providers:
    
           •   All generated events have the same provider/event names.
    
           •   There’s no static type checking.
    
           •   The only event field with user data you actually get, named
               **msg** , is a string potentially containing the values you passed
               to the macro using your own format. This also means that you
               cannot use filtering using a custom expression at run time
               because there are no isolated fields.
    
           •   Since **lttng_ust_tracef()** and **lttng_ust_vtracef()** use C
               standard library’s vasprintf(3) function in the background to
               format the strings at run time, their expected performance is
               lower than using custom tracepoint providers with typed
               fields, which do not require a conversion to a string.
    
           •   Generally, a string containing the textual representation of
               the user data fields is not as compact as binary fields in the
               resulting trace.
    
           Thus, **lttng_ust_tracef()** /**lttng_ust_vtracef()** are useful for quick
           prototyping and debugging, but should not be considered for any
           permanent/serious application instrumentation.
    
           **lttng_ust_vtracef()** does not have a **STAP_PROBEV()** call, because
           **STAP_PROBEV()** does not support **va_list**. If you need it, you should
           emit this call yourself.
    
           See lttng-ust(3) to learn more about custom tracepoint providers.
    


## BUGS



           If you encounter any issue or usability problem, please report it
           on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-
           ust>.
    


## RESOURCES



           •   LTTng project website <http://lttng.org>
    
           •   LTTng documentation <http://lttng.org/docs>
    
           •   Git repositories <http://git.lttng.org>
    
           •   GitHub organization <http://github.com/lttng>
    
           •   Continuous integration <http://ci.lttng.org/>
    
           •   Mailing list <http://lists.lttng.org> for support and
               development: **lttng-dev@lists.lttng.org**
    
           •   IRC channel <irc://irc.oftc.net/lttng>: **#lttng** on **irc.oftc.net**
    


## THANKS



           Thanks to Ericsson for funding this work, providing real-life use
           cases, and testing.
    
           Special thanks to Michel Dagenais and the DORSAL laboratory
           <http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal
           for the LTTng journey.
    


* * *