'\" t
.\"     Title: lttng_ust_tracelog
.\"    Author: [see the "AUTHORS" section]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 09/24/2024
.\"    Manual: LTTng Manual
.\"    Source: LTTng 2.13.8
.\"  Language: English
.\"
.TH "LTTNG_UST_TRACELOG" "3" "09/24/2024" "LTTng 2\&.13\&.8" "LTTng Manual"
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.nh
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.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
lttng_ust_tracelog, lttng_ust_vtracelog \- LTTng\-UST printf(3)\-like interface with a log level
.SH "SYNOPSIS"
.sp
.nf
\fB#include <lttng/tracelog\&.h>\fR
.fi
.sp
.nf
#define \fBlttng_ust_tracelog\fR(\fIlevel\fR, \fIfmt\fR, \&...)
#define \fBlttng_ust_vtracelog\fR(\fIlevel\fR, \fIfmt\fR, \fIap\fR)
.fi
.sp
Link with:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
\fB-llttng-ust\fR
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
If you define
\fB_LGPL_SOURCE\fR
before including
\fB<lttng/tracelog.h>\fR
(directly or indirectly):
\fB-llttng-ust-common\fR
.RE
.SH "DESCRIPTION"
.sp
The LTTng\-UST \fBlttng_ust_tracelog()\fR and \fBlttng_ust_vtracelog()\fR API allows you to trace your application with the help of simple \fBprintf\fR(3)\-like and \fBvprintf\fR(3)\-like macros, with an additional parameter for the desired log level\&.
.sp
The \fIfmt\fR argument is passed directly as the \fIfmt\fR parameter of \fBvasprintf\fR(3), as well as:
.PP
For \fBlttng_ust_tracelog()\fR
.RS 4
The optional parameters following
\fIfmt\fR\&.
.RE
.PP
For \fBlttng_ust_vtracelog()\fR
.RS 4
The
\fIap\fR
parameter as the
\fIap\fR
parameter of
\fBvasprintf\fR(3)
(\fBva_list\fR
type)\&.
.RE
.sp
The purpose of \fBlttng_ust_tracelog()\fR and \fBlttng_ust_vtracelog()\fR is to ease the migration from logging to tracing\&.
.sp
The available values for the \fIlevel\fR parameter are:
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_EMERG\fR
.RS 4
System is unusable\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_ALERT\fR
.RS 4
Action must be taken immediately\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT\fR
.RS 4
Critical conditions\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_ERR\fR
.RS 4
Error conditions\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING\fR
.RS 4
Warning conditions\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_NOTICE\fR
.RS 4
Normal, but significant, condition\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_INFO\fR
.RS 4
Informational message\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_SYSTEM\fR
.RS 4
Debug information with system\-level scope (set of programs)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROGRAM\fR
.RS 4
Debug information with program\-level scope (set of processes)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROCESS\fR
.RS 4
Debug information with process\-level scope (set of modules)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_MODULE\fR
.RS 4
Debug information with module (executable/library) scope (set of units)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT\fR
.RS 4
Debug information with compilation unit scope (set of functions)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION\fR
.RS 4
Debug information with function\-level scope\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_LINE\fR
.RS 4
Debug information with line\-level scope (default log level)\&.
.RE
.PP
\fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG\fR
.RS 4
Debug\-level message\&.
.RE
.sp
To use \fBlttng_ust_tracelog()\fR or \fBlttng_ust_vtracelog()\fR, include \fB<lttng/tracelog.h>\fR where you need it, and link your application with \fBliblttng-ust\fR and \fBliblttng-ust-common\fR\&. See the \fIEXAMPLE\fR section below for a complete usage example\&.
.sp
Once your application is instrumented with \fBlttng_ust_tracelog()\fR and/or \fBlttng_ust_vtracelog()\fR calls and ready to run, use \fBlttng-enable-event\fR(1) to enable the \fBlttng_ust_tracelog:*\fR event\&. You can isolate specific log levels with the \fB--loglevel\fR and \fB--loglevel-only\fR options of this command\&.
.sp
The \fBlttng_ust_tracelog()\fR and \fBlttng_ust_vtracelog()\fR events contain the following fields:
.TS
allbox tab(:);
ltB ltB.
T{
Field name
T}:T{
Description
T}
.T&
lt lt
lt lt
lt lt
lt lt.
T{
.sp
\fBline\fR
T}:T{
.sp
Line in source file where \fBlttng_ust_tracelog()\fR was called\&.
T}
T{
.sp
\fBfile\fR
T}:T{
.sp
Source file from which \fBlttng_ust_tracelog()\fR was called\&.
T}
T{
.sp
\fBfunc\fR
T}:T{
.sp
Function name from which \fBlttng_ust_tracelog()\fR was called\&.
T}
T{
.sp
\fBmsg\fR
T}:T{
.sp
Formatted string output\&.
T}
.TE
.sp 1
.sp
If you do not need to attach a specific log level to a \fBlttng_ust_tracelog()\fR/\fBlttng_ust_vtracelog()\fR call, use \fBlttng_ust_tracef\fR(3) instead\&.
.sp
See also the \fILIMITATIONS\fR section below for important limitations to consider when using \fBlttng_ust_tracelog()\fR or \fBlttng_ust_vtracelog()\fR\&.
.SH "EXAMPLE"
.sp
Here\(cqs a usage example of \fBlttng_ust_tracelog()\fR:
.sp
.if n \{\
.RS 4
.\}
.nf
#include <stdlib\&.h>
#include <lttng/tracelog\&.h>

int main(int argc, char *argv[])
{
    int i;

    if (argc < 2) {
        lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT,
                           "Not enough arguments: %d", argc);
        return EXIT_FAILURE;
    }

    lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO,
                       "Starting app with %d arguments", argc);

    for (i = 0; i < argc; i++) {
        lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG,
                           "Argument %d: %s", i, argv[i]);
    }

    lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO,
                       "Exiting app");
    return EXIT_SUCCESS;
}
.fi
.if n \{\
.RE
.\}
.sp
This C source file, saved as \fBapp.c\fR, can be compiled into a program like this:
.sp
.if n \{\
.RS 4
.\}
.nf
$ cc \-o app app\&.c \-llttng\-ust \-llttng\-ust\-common
.fi
.if n \{\
.RE
.\}
.sp
You can create an LTTng tracing session, enable all the \fBlttng_ust_tracelog()\fR events, and start the created tracing session like this:
.sp
.if n \{\
.RS 4
.\}
.nf
$ lttng create my\-session
$ lttng enable\-event \-\-userspace \*(Aqlttng_ust_tracelog:*\*(Aq
$ lttng start
.fi
.if n \{\
.RE
.\}
.sp
Or you can enable \fBlttng_ust_tracelog()\fR events matching a log level at least as severe as a given log level:
.sp
.if n \{\
.RS 4
.\}
.nf
$ lttng enable\-event \-\-userspace \*(Aqlttng_ust_tracelog:*\*(Aq \e
                     \-\-loglevel=INFO
.fi
.if n \{\
.RE
.\}
.sp
Next, start the program to be traced:
.sp
.if n \{\
.RS 4
.\}
.nf
$ \&./app a few arguments passed to this application
.fi
.if n \{\
.RE
.\}
.sp
Finally, stop the tracing session, and inspect the recorded events:
.sp
.if n \{\
.RS 4
.\}
.nf
$ lttng stop
$ lttng view
.fi
.if n \{\
.RE
.\}
.SH "LIMITATIONS"
.sp
The \fBlttng_ust_tracelog()\fR and \fBlttng_ust_vtracelog()\fR utility macros were developed to make user space tracing super simple, albeit with notable disadvantages compared to custom, full\-fledged tracepoint providers:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
All generated events have the same provider/event names\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
There\(cqs no static type checking\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The only event field with user data you actually get, named
\fBmsg\fR, 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\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Since
\fBlttng_ust_tracelog()\fR
and
\fBlttng_ust_vtracelog()\fR
use C standard library\(cqs
\fBvasprintf\fR(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\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Generally, a string containing the textual representation of the user data fields is not as compact as binary fields in the resulting trace\&.
.RE
.sp
Thus, \fBlttng_ust_tracelog()\fR/\fBlttng_ust_vtracelog()\fR are useful for quick prototyping and debugging, but should not be considered for any permanent/serious application instrumentation\&.
.sp
\fBlttng_ust_vtracelog()\fR does not have a \fBSTAP_PROBEV()\fR call, because \fBSTAP_PROBEV()\fR does not support \fBva_list\fR\&. If you need it, you should emit this call yourself\&.
.sp
See \fBlttng-ust\fR(3) to learn more about custom tracepoint providers\&.
.SH "BUGS"
.sp
If you encounter any issue or usability problem, please report it on the LTTng bug tracker <https://bugs.lttng.org/projects/lttng-ust>\&.
.SH "RESOURCES"
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng project website <http://lttng.org>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
LTTng documentation <http://lttng.org/docs>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Git repositories <http://git.lttng.org>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
GitHub organization <http://github.com/lttng>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Continuous integration <http://ci.lttng.org/>
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Mailing list <http://lists.lttng.org>
for support and development:
\fBlttng-dev@lists.lttng.org\fR
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
IRC channel <irc://irc.oftc.net/lttng>:
\fB#lttng\fR
on
\fBirc.oftc.net\fR
.RE
.SH "COPYRIGHTS"
.sp
This macro is part of the LTTng\-UST project\&.
.sp
This macro is distributed under the GNU Lesser General Public License, version 2\&.1 <http://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>\&. See the for more details\&.
.SH "THANKS"
.sp
Thanks to Ericsson for funding this work, providing real\-life use cases, and testing\&.
.sp
Special thanks to Michel Dagenais and the DORSAL laboratory <http://www.dorsal.polymtl.ca/> at \('Ecole Polytechnique de Montr\('eal for the LTTng journey\&.
.SH "AUTHORS"
.sp
LTTng\-UST was originally written by Mathieu Desnoyers, with additional contributions from various other people\&. It is currently maintained by Mathieu Desnoyers <mailto:mathieu.desnoyers@efficios.com>\&.
.SH "SEE ALSO"
.sp
\fBlttng_ust_tracef\fR(3), \fBlttng_ust_vtracef\fR(3), \fBlttng-ust\fR(3), \fBlttng\fR(1), \fBprintf\fR(3)