'\" t
.\"     Title: lttng-ust-cyg-profile
.\"    Author: [see the "AUTHORS" section]
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 01/10/2024
.\"    Manual: LTTng Manual
.\"    Source: LTTng 2.13.7
.\"  Language: English
.\"
.TH "LTTNG\-UST\-CYG\-PRO" "3" "01/10/2024" "LTTng 2\&.13\&.7" "LTTng Manual"
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.SH "NAME"
lttng-ust-cyg-profile \- Function tracing (LTTng\-UST helper)
.SH "SYNOPSIS"
.sp
Compile your application with compiler option \fB-finstrument-functions\fR\&.
.sp
Launch your application by preloading \fBliblttng-ust-cyg-profile-fast.so\fR for fast function tracing:
.sp
.nf
$ \fBLD_PRELOAD=liblttng\-ust\-cyg\-profile\-fast\&.so\fR my\-app
.fi
.sp
Launch your application by preloading \fBliblttng-ust-cyg-profile.so\fR for slower, more verbose function tracing:
.sp
.nf
$ \fBLD_PRELOAD=liblttng\-ust\-cyg\-profile\&.so\fR my\-app
.fi
.SH "DESCRIPTION"
.sp
When the \fBliblttng-ust-cyg-profile.so\fR or the \fBliblttng-ust-cyg-profile-fast.so\fR library is preloaded before a given application starts, all function entry and return points are traced by LTTng\-UST (see \fBlttng-ust\fR(3)), provided said application was compiled with the \fB-finstrument-functions\fR compiler option\&.
.sp
See \fBlttng\fR(1) to learn more about how to control LTTng tracing sessions\&.
.sp
Function tracing with LTTng\-UST comes in two flavors, each one providing a different trade\-off between performance and robustness:
.PP
\fBliblttng-ust-cyg-profile-fast.so\fR
.RS 4
This is a lightweight variant that should only be used where it can be
\fIguaranteed\fR
that the complete event stream is recorded without any missing events\&. Any kind of duplicate information is left out\&.
.sp
At each function entry, the address of the called function is recorded in an LTTng\-UST event\&. Function exits are recorded as another, empty LTTng\-UST event\&.
.sp
See the
\fIFast function tracing\fR
section below for the complete list of emitted events and their fields\&.
.RE
.PP
\fBliblttng-ust-cyg-profile.so\fR
.RS 4
This is a more robust variant which also works for use cases where events might get discarded, or not recorded from application startup\&. In these cases, the trace analyzer needs extra information to be able to reconstruct the program flow\&.
.sp
At each function entry
\fIand\fR
exit, the address of the called function
\fIand\fR
the call site address are recorded in an LTTng\-UST event\&.
.sp
See the
\fIVerbose function tracing\fR
section below for the complete list of emitted events and their fields\&.
.RE
.SS "Usage"
.sp
To use LTTng\-UST function tracing, you need to make sure the sources of your application are compiled with the \fB-finstrument-functions\fR compiler option\&.
.sp
It might be necessary to limit the number of source files where this option is used to prevent excessive amount of trace data to be generated at run time\&. Usually, there are additional compiler flags that allow you to specify a more fine\-grained selection of function instrumentation\&.
.sp
For each instrumented function, the executable will contain calls to profiling function hooks (after function entry, named \fB__cyg_profile_func_enter()\fR, and just before function exit, named \fB__cyg_profile_func_exit()\fR)\&.
.sp
By preloading (using the \fBLD_PRELOAD\fR environment variable) one of the provided shared libraries, these profiling hooks get defined to emit LTTng events (as described below)\&.
.if n \{\
.sp
.\}
.RS 4
.it 1 an-trap
.nr an-no-space-flag 1
.nr an-break-flag 1
.br
.ps +1
\fBNote\fR
.ps -1
.br
.sp
Using this feature can result in a \fBmassive amount\fR of trace data to be generated by the instrumented application\&. Application run time is also considerably affected\&. Be careful on systems with limited resources\&.
.sp .5v
.RE
.SS "Fast function tracing"
.sp
The following LTTng\-UST events are available when using \fBliblttng-ust-cyg-profile-fast.so\fR\&. Their log level is set to \fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION\fR\&.
.PP
\fBlttng_ust_cyg_profile_fast:func_entry\fR
.RS 4
Emitted when an application function is entered, or more specifically, when
\fB__cyg_profile_func_enter()\fR
is called\&.
.sp
Fields:
.TS
allbox tab(:);
ltB ltB.
T{
Field name
T}:T{
Description
T}
.T&
lt lt.
T{
\fBfunc_addr\fR
T}:T{
Function address\&.
T}
.TE
.sp 1
.RE
.PP
\fBlttng_ust_cyg_profile_fast:func_exit\fR
.RS 4
Emitted when an application function returns, or more specifically, when
\fB__cyg_profile_func_exit()\fR
is called\&.
.sp
This event has no fields\&. Since the
\fBliblttng-ust-cyg-profile-fast.so\fR
library should only be used when it can be guaranteed that the complete event stream is recorded without any missing events, a per\-thread, stack\-based approach can be used on the trace analyzer side to match function entry and return events\&.
.RE
.SS "Verbose function tracing"
.sp
The following LTTng\-UST events are available when using \fBliblttng-ust-cyg-profile.so\fR\&. Their log level is set to \fBLTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION\fR\&.
.PP
\fBlttng_ust_cyg_profile:func_entry\fR
.RS 4
Emitted when an application function is entered, or more specifically, when
\fB__cyg_profile_func_enter()\fR
is called\&.
.sp
Fields:
.TS
allbox tab(:);
ltB ltB.
T{
Field name
T}:T{
Description
T}
.T&
lt lt
lt lt.
T{
\fBfunc_addr\fR
T}:T{
Function address\&.
T}
T{
\fBcall_site\fR
T}:T{
Address from which this function was called\&.
T}
.TE
.sp 1
.RE
.PP
\fBlttng_ust_cyg_profile:func_exit\fR
.RS 4
Emitted when an application function returns, or more specifically, when
\fB__cyg_profile_func_exit()\fR
is called\&.
.sp
Fields:
.TS
allbox tab(:);
ltB ltB.
T{
Field name
T}:T{
Description
T}
.T&
lt lt
lt lt.
T{
\fBfunc_addr\fR
T}:T{
Function address\&.
T}
T{
\fBcall_site\fR
T}:T{
Address from which this function was called\&.
T}
.TE
.sp 1
.RE
.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 library is part of the LTTng\-UST project\&.
.sp
This library 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\fR(3), \fBlttng\fR(1), \fBgcc\fR(1), \fBld.so\fR(8)