473 lines
13 KiB
TeX
473 lines
13 KiB
TeX
\documentclass[slidestop,compress,blue]{beamer}
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%\usetheme{default}
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%\usetheme{Malmoe}
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%\usetheme{Boadilla}
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%\usetheme{umbc2}
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%\usetheme{Antibes}
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%\usetheme{Berlin}
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%\usetheme{Madrid}
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% Pacotes úteis
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\usepackage[utf8]{inputenc}
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% Fonte bonita!
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\usepackage{ae,aecompl}
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\usepackage[english]{babel}
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\usepackage[T1]{fontenc}
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\usepackage{listings}
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\title{GDB, Your New Best Friend}
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\author{Sergio Durigan Junior \\
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\texttt{sergiodj@\{redhat.com,sergiodj.net\}}}
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\date{\today}
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\usetheme[secheader]{Madrid}
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\setbeamertemplate{navigation symbols}{}
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\defbeamertemplate*{footline}{infolines theme without institution}
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{
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\leavevmode%
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\hbox{%
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\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{author in head/foot}%
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\usebeamerfont{author in head/foot}\insertshortauthor
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\end{beamercolorbox}%
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\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,center]{title in head/foot}%
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\usebeamerfont{title in head/foot}\insertshorttitle
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\end{beamercolorbox}%
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\begin{beamercolorbox}[wd=.333333\paperwidth,ht=2.25ex,dp=1ex,right]{date in head/foot}%
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\usebeamerfont{date in head/foot}\insertshortdate{}\hspace*{2em}
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\insertframenumber{} / \inserttotalframenumber\hspace*{2ex}
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\end{beamercolorbox}}%
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\vskip0pt%
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}
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\newcommand{\aspas}[1]{``#1''}
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\newcommand{\gnu}{\texttt{GNU}}
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\newcommand{\gdb}{\texttt{GDB}}
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\newcommand{\gcc}{\texttt{GCC}}
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\newcommand{\stap}{\texttt{SystemTap}}
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\newcommand{\python}{\texttt{Python}}
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\newcommand{\glibc}{\texttt{GLIBC}}
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\newcommand{\kernel}{\textit{kernel}}
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\newcommand{\probe}{\textit{probe}}
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\newtheorem{sourcecode}{Source Code}
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\begin{document}
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\lstset{escapeinside={(*}{*)}}
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\begin{frame}
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\titlepage
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\end{frame}
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\begin{frame}
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\frametitle{License}
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\begin{itemize}
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\item{License: \textbf{Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)}}
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\item{\url{http://creativecommons.org/licenses/by-sa/3.0/}}
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\end{itemize}
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\end{frame}
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\section{Agenda}
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\begin{frame}
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\frametitle{Agenda}
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\begin{itemize}
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\item{Introduction}
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% corefile
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\item{The core of the file}
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% Falar de multiprocess
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\item{Multiprocess}
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% Falar de probes, stap
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\item{\stap{} \texttt{SDT} probes}
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% Falar de catchpoints? catch syscall
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% \item{Análise de uma \probe{}}
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% Falar de depuração reversa
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\item{gniggubeD esreveR}
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% Falar de Python
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\item{\python{}}
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\item{Conclusion}
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\end{itemize}
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\end{frame}
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\section{Introduction}
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\begin{frame}[fragile]
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\frametitle{Introduction}
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\begin{itemize}
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\item{\gdb{}: \gnu{} project's Debugger \pause{} (it is \textbf{not} a \textit{database}...). Supports several programming languages.}
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\begin{itemize}
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\item{\verb|#> gcc -g program.c -o program|}
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\item{\verb|#> gdb ./program|}
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\end{itemize}
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\pause
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\item{Before anything else! \verb|C-x a|}
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\end{itemize}
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\end{frame}
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\section{The core of the file}
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\begin{frame}[fragile]
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\frametitle{The core of the file}
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\begin{itemize}
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\item{\texttt{corefile}s are ``frozen images'' of a certain point of execution inside a program.}
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\pause
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\item{To generate one on \gdb{}, at any time:}
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\begin{itemize}
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\item{\verb|(gdb) generate-core-file|}
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\end{itemize}
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\pause
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\item{If you are seeing a segmentation fault, enable the \texttt{corefile} generation in your \texttt{shell}:}
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\begin{itemize}
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\item{\verb|#> ulimit -c unlimited|}
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\end{itemize}
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\pause
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\item{And then open the \texttt{corefile} on \gdb{}:}
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\begin{itemize}
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\item{\verb|#> gdb program core.12345|}
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\end{itemize}
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\pause
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\item{Inside \gdb{}, almost everything works. You can print variables, request a \texttt{backtrace}, change the current \texttt{frame}. You just cannot ``resurrect'' the program (i.e., execute it from that point) \verb|:-)|.}
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\end{itemize}
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\end{frame}
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\section{Multiprocess}
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\begin{frame}[fragile]
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\frametitle{Multiprocess}
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\begin{itemize}
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\item{Given:}
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\begin{block}{Makefile}
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\tiny
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\begin{lstlisting}[language=sh,showstringspaces=false]
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all: run
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run: segv
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./segv
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segv: segv.c
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gcc -g -o segv segv.c
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\end{lstlisting}
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\end{block}
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\begin{block}{segv.c}
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\tiny
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\begin{lstlisting}[language=c,showstringspaces=false]
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int
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main (int argc, char *argv[])
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{
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int *x = 0;
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*x = 52;
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return 0;
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}
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\end{lstlisting}
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\end{block}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{Multiprocess$^2$}
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\begin{itemize}
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\item{How do you debug the program from the example, which has a segmentation fault?}
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\pause
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\item{Multiprocess!}
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\pause
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\item{On \gdb{}:}
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\begin{itemize}
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\item{\verb|(gdb) set detach-on-fork off|}
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\item{\verb|(gdb) set target-async on|}
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\item{\verb|(gdb) set non-stop on|}
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\item{\verb|(gdb) run|}
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\item{(...)}
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\item{\verb|(gdb) info inferiors|}
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\end{itemize}
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\item{(Yes, it is boring to type. Yes, we will improve it.)}
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\pause
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\item{Time for a hands-on!}
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\end{itemize}
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\end{frame}
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\section{SystemTap SDT probes}
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\begin{frame}[fragile]
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\frametitle{\stap{} \texttt{SDT} probes}
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\begin{itemize}
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\item{Pre-req: \probe{} v3, \verb|systemtap-sdt-devel >= 1.4|.}
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\begin{block}{Example code -- \texttt{example-stap.c}}
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\tiny
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\begin{lstlisting}[language=c,showstringspaces=false]
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(*\textcolor{red}{\#\textbf{include}} \textcolor{red}{\textless{}sys/sdt.h\textgreater{}}*)
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void foo (int a)
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{
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(*\textcolor{red}{STAP\_PROBE1 (}\textcolor{red}{example-stap, my\_probe, a}\textcolor{red}{);}*)
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}
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int main (int argc, char *argv[])
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{
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foo (10);
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return 0;
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}
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\end{lstlisting}
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\end{block}
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\pause
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\begin{block}{Compiling and verifying the \probe{}}
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\tiny
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\begin{lstlisting}[language=sh,showstringspaces=false]
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$> gcc exemplo-stap.c -o exemplo-stap
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$> stap -L '(*\textcolor{red}{process("./example-stap").mark("*")}*)'
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process("./example-stap").mark("my_probe") $arg1:long
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\end{lstlisting}
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\end{block}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\stap{} \texttt{SDT} probes$^2$}
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\begin{itemize}
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\item{Available since \gdb{} 7.5, release in August, 2012.}
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\pause
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\item{Integrated with the \textit{breakpoint} system. Support for printing a \probe{}'s argument value.}
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\pause
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\item{\textcolor{red}{Not necessary to include debug information in your binary (\texttt{-g} flag); arguments are encoded directly in the asm.}}
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\pause
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\item{Also possible to use optmization flags (\texttt{-O}).}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\stap{} \texttt{SDT} probes$^3$}
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\begin{itemize}
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\item{Command \texttt{info probes}, list information about available \textit{probes}.}
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\pause
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\item{Command \texttt{break -probe-stap <probe\_name>}.}
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\pause
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\begin{itemize}
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\item{Also possible to use \texttt{-probe} or \texttt{-p}.}
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\end{itemize}
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\pause
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\item{Printing arguments for each \probe{}: internal \gdb{} variables.}
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\begin{itemize}
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\item{\texttt{\$\_probe\_argc}: number of arguments of the \probe{} (max: 12).}
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\item{\texttt{\$\_probe\_arg$\{$0..11$\}$}: print argument \{0..11\}.}
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\end{itemize}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\stap{} \texttt{SDT} probes$^4$}
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\begin{itemize}
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\item{After compiling your program (as shown previously):}
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\begin{block}{Debugging \stap{} \texttt{SDT} probes}
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\tiny
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\begin{lstlisting}[language=sh,showstringspaces=false]
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\$> gdb /tmp/example-stap
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...
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(gdb) info probes
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Provider Name Where Semaphore Object
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example-stap my_probe 0x000000000040047b /tmp/example-stap
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(gdb) break -probe-stap my_probe
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Breakpoint 1 at 0x40047b
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(gdb) run
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Starting program: /tmp/example-stap
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Breakpoint 1, 0x000000000040047b in foo ()
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(gdb) print $_probe_argc
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$1 = 1
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(gdb) print $_probe_arg0
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$2 = 10
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\end{lstlisting}
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\end{block}
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\end{itemize}
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\end{frame}
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\section{gniggubeD esreveR}
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\begin{frame}[fragile]
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\frametitle{gniggubeD esreveR}
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\begin{itemize}
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\item{Available since \gdb{} 7.0, released on September, 2009!}
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\pause
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\item{Implementation is kind of messy, but works in most cases...}
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\pause
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\item{Just works on x86/x86\_64 (patch for PowerPC is being made).}
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\pause
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\item{The hacker who did almost everything passed away. \verb|:-(|}
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\begin{itemize}
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\item{Old story: ``... if I die, who is going to maintain this code?''}
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\end{itemize}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{gniggubeD esreveR$^2$}
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\begin{itemize}
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\item{\verb|(gdb) start|}
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\item{\verb|(gdb) target record-full|}
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\item{Or, with a recent \gdb{}: \verb|(gdb) record|}
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\item{(...)}
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\item{Use the commands with the prefix \texttt{reverse}. \verb|reverse-next, reverse-step, reverse-nexti, reverse-stepi, set exec-direction reverse|...}
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\end{itemize}
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\end{frame}
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\section{Python}
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\begin{frame}[fragile]
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\frametitle{\python{}}
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\begin{itemize}
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\item{Available since \gdb{} 7.0, too.}
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\pause
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\item{Motivation: make it easy to extend \gdb{} without having to mess with its internals.}
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\pause
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\item{Very well maintained, several extensions implemented.}
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\pause
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\item{Several projects already make use of it. Yours can be the next! \verb|:-)|}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\python{}$^2$}
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\begin{itemize}
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\item{Problem: \gdb{} knows about the \texttt{call stack}. \gdb{} also has conditional \texttt{breakpoints}.}
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\pause
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\item{\emph{However, you cannot use information from the \texttt{stack} in the condition of a \texttt{breakpoint}.}}
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\pause
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\item{Solution? Python!}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\python{}$^3$}
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\begin{block}{caller\_is.py}
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\tiny
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\begin{lstlisting}[language=python,showstringspaces=false]
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import gdb
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class CallerIs (gdb.Function):
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"""Return True if the calling function's name is equal to a string.
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This function takes one or two arguments.
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The first argument is the name of a function; if the calling function's
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name is equal to this argument, this function returns True.
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The optional second argument tells this function how many stack frames
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to traverse to find the calling function. The default is 1."""
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def __init__ (self):
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super (CallerIs, self).__init__ ("caller_is")
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def invoke (self, name, nframes = 1):
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frame = gdb.selected_frame ()
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while nframes > 0:
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frame = frame.older ()
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nframes = nframes - 1
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return frame.name () == name.string ()
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CallerIs()
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\end{lstlisting}
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\end{block}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\python{}$^4$}
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\begin{itemize}
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\item{To use it:}
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\begin{block}{Using caller\_is.py}
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\tiny
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\begin{lstlisting}[language=sh,showstringspaces=false]
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(gdb) source caller_is.py
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(gdb) break foo if $caller_is ("bar")
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\end{lstlisting}
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\end{block}
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\item{Internal variables and functions on \gdb{} start with \verb|$|.}
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\item{\texttt{caller\_is.py} is part of upstream \gdb{}.}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\python{}$^5$}
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\begin{itemize}
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\item{Problem: \texttt{std::string s = "Hello, World";}}
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\begin{block}{Printing ``s'' inside \gdb{}}
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\tiny
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\begin{lstlisting}[language=sh,showstringspaces=false]
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(gdb) print s
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$1 = {static npos = <optimized out>,
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_M_dataplus = {<std::allocator<char>> =
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{<__gnu_cxx::new_allocator<char>> = {<No data fields>},
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<No data fields>}, _M_p = 0x602028 ``Hello, World''}}
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\end{lstlisting}
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\end{block}
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\pause
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\item{\gdb{} knows about \verb|_M_dataplus -> _M_p|. How to tell that this is the only field you are interested in?}
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\pause
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\item{Python!}
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\end{itemize}
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\end{frame}
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\begin{frame}[fragile]
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\frametitle{\python{}$^6$}
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\begin{block}{print-std-string.py}
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\tiny
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\begin{lstlisting}[language=python,showstringspaces=false]
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class StdStringPrinter:
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"Print a std::basic_string of some kind"
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def __init__(self, typename, val):
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self.val = val
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def to_string(self):
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# ... snip ...
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# Calculate the length of the string so that to_string returns
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# the string according to length, not according to first null
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# encountered.
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(*\textcolor{red}{ptr = self.val ['\_M\_dataplus']['\_M\_p']}*)
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realtype = type.unqualified ().strip_typedefs ()
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reptype = gdb.lookup_type (str (realtype) + '::_Rep').pointer ()
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header = ptr.cast(reptype) - 1
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len = header.dereference ()['_M_length']
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if hasattr(ptr, "lazy_string"):
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return ptr.lazy_string (length = len)
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return ptr.string (length = len)
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\end{lstlisting}
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\end{block}
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\begin{itemize}
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\pause
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\item{Present on \verb|libstdcxx| upstream}
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\end{itemize}
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\end{frame}
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\section{Conclusion}
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\begin{frame}[fragile]
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\frametitle{Conclusion}
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\begin{itemize}
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\item{The debugger you knew has changed for the better.}
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\pause
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\item{Things that are coming:}
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\begin{itemize}
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\item{Improvements in the multiprocess feature.}
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\item{Scalability}
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\item{Improvements for \texttt{C++}.}
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\end{itemize}
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\end{itemize}
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\end{frame}
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\section{Thank you's}
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\begin{frame}[fragile]
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\frametitle{Thank you's}
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\begin{itemize}
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\item{Red Hat}
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\item{FSOSS}
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\item{You guys and girls \verb|:-)|}
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\end{itemize}
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\end{frame}
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\section{Questions}
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\begin{frame}
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\frametitle{Questions?}
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\begin{center}
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Shoot! Or send e-mail!
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\end{center}
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\end{frame}
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\end{document}
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