[FE training-materials-updates] buildroot-appdev: first part of the lab

[Thomas Petazzoni]

Repository : git://git.free-electrons.com/training-materials.git

On branch  : master
Link       : http://git.free-electrons.com/training-materials/commit/?id=ef792b315d39a0277aaa0cb573ae2b2e022ea7f6

>---------------------------------------------------------------

commit ef792b315d39a0277aaa0cb573ae2b2e022ea7f6
Author: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Date:   Wed Apr 29 15:44:01 2015 +0200

    buildroot-appdev: first part of the lab
    
    Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>


>---------------------------------------------------------------

ef792b315d39a0277aaa0cb573ae2b2e022ea7f6
 labs/buildroot-appdev/buildroot-appdev.tex | 187 ++++++++++++++++++++++++++++-
 1 file changed, 186 insertions(+), 1 deletion(-)

diff --git a/labs/buildroot-appdev/buildroot-appdev.tex b/labs/buildroot-appdev/buildroot-appdev.tex
index 04f4ded..d68757c 100644
--- a/labs/buildroot-appdev/buildroot-appdev.tex
+++ b/labs/buildroot-appdev/buildroot-appdev.tex
@@ -1,4 +1,189 @@
 \subchapter
 {Application development with Buildroot}
-{Objectives:}
+{Objectives:
+  \begin{itemize}
+  \item Build and run your own application
+  \item Remote debug your application
+  \item Use \code{<pkg>_OVERRIDE_SRCDIR}
+  \item Set up Eclipse for Buildroot application development
+  \end{itemize}
+}
 
+\section{Build and run your own application}
+
+Let's create your own little application that we will use for
+demonstration in this lab. Create a folder \code{$HOME/felabs/myapp},
+and inside this folder a single C file called \code{myapp.c} with the
+following contents:
+
+\begin{verbatim}
+#include <stdio.h>
+
+int main(void) {
+	printf("Hello World\n");
+	return 0;
+}
+\end{verbatim}
+
+To build this application, we'll use the cross-compiler generated by
+Buildroot. To make this easy, let's add the Buildroot host directory
+into our PATH:
+
+\begin{verbatim}
+export PATH=$HOME/felabs/buildroot/output/host/usr/bin:$PATH
+\end{verbatim}
+
+Now you can build your application easily:
+
+\begin{verbatim}
+arm-linux-gnueabihf-gcc -o myapp myapp.c
+\end{verbatim}
+
+Copy the myapp binary to your target using scp:
+
+\begin{verbatim}
+scp myapp root at 192.168.0.2:
+\end{verbatim}
+
+And run the \code{myapp} application on your target.
+
+Now, let's extend the application a little bit more to use a library,
+the \code{libconfig} library we've already used in a previous
+lab. Change the source code of the application to the one provided in
+this lab data directory, \code{myapp.c}.
+
+If you try to build this application with just:
+
+\begin{verbatim}
+arm-linux-gnueabihf-gcc -o myapp myapp.c
+\end{verbatim}
+
+It fails to build because it does not link with \code{libconfig}. So
+you can manually do:
+
+\begin{verbatim}
+arm-linux-gnueabihf-gcc -o myapp myapp.c -lconfig
+\end{verbatim}
+
+Since \code{libconfig.so} is in \code{output/staging/usr/lib} and the
+compiler is configured to automatically look in \code{output/staging}
+as its {\em sysroot}, it works fine.
+
+However, there's a better solution: using {\em pkg-config}. Buildroot
+has installed a special version of \code{pkg-config} in
+\code{output/host/usr/bin}, which you can query for libraries
+available for the target. Run:
+
+\begin{verbatim}
+pkg-config --list-all
+\end{verbatim}
+
+And check you have \code{libconfig} mentionned. You can query the
+compiler and linker flags for \code{libconfig}:
+
+\begin{verbatim}
+pkg-config --cflags --libs libconfig
+\end{verbatim}
+
+And use that to build your application:
+
+\begin{verbatim}
+arm-linux-gnueabihf-gcc -o myapp myapp.c $(pkg-config --cflags --libs libconfig)
+\end{verbatim}
+
+In the case of \code{libconfig}, it doesn't simplify a lot because the
+compiler and linker flags are simple, but for some other libraries,
+they are more complicated.
+
+Copy the new version of \code{myapp} to your target, and run
+it. Create a \code{myapp.cfg} config file, and run your application
+again.
+
+\section{Remote debug your application}
+
+Our application is simple and works, but what if you need to debug it?
+So let's set up remote debugging.
+
+In Buildroot \code{menuconfig}, enable
+\code{Copy gdb server to the Target} in the \code{Toolchain}
+menu. Since we don't want to rebuild everything, we will force only
+the reinstallation of the \code{toolchain-external} package. So just
+do: \code{make toolchain-external-reinstall all}. Note that due to a
+minor bug in Buildroot 2015.02, you need to apply the patch
+\code{0001-toolchain-external-fix-rebuild-reinstall-for-Linaro-.patch}
+provided in this lab's data directory, for this
+\code{toolchain-external-reinstall} command to work.
+
+You can reflash your system, or alternatively, just copy
+\code{output/target/usr/bin/gdbserver} to the target \code{/usr/bin/}
+directory using \code{scp}.
+
+To do some appropriate debugging, we need to have debugging symbols
+available. So we need to do two things:
+
+\begin{enumerate}
+
+\item Rebuild our application with the \code{-g} flag.
+
+\item Rebuild the Buildroot system with debugging symbols, so that
+  shared libraries have debugging symbols. However, since we don't
+  want to rebuild the entire Buildroot system now, we'll use a trick
+  and rebuild only the library we need to have the debugging symbols
+  for: \code{libconfig}. To achieve this, first go to Buildroot
+  \code{menuconfig}, and in \code{Build options}, enable
+  \code{build packages with debugging symbols}. Then, do \code{make
+    libconfig-dirclean all} to force the rebuild of just
+  \code{libconfig}.
+
+\end{enumerate}
+
+Now, on your target, start {\em gdbserver} in multi-process mode,
+listening on TCP port 2345:
+
+\begin{verbatim}
+gdbserver --multi localhost:2345
+\end{verbatim}
+
+Back on the host, run the cross-gdb with the \code{myapp} application
+as argument:
+
+\begin{verbatim}
+arm-linux-gnueabihf-gdb myapp
+\end{verbatim}
+
+We need to tell \code{gdb} where the libraries can be found:
+
+\begin{verbatim}
+(gdb) set solib-search-path output/staging/lib:output/staging/lib/arm-linux-gnueabihf/:output/staging/usr/lib:output/staging/usr/lib/arm-linux-gnueabihf/
+\end{verbatim}
+
+And then connect to the target:
+
+\begin{verbatim}
+(gdb) target extended-remote 192.168.0.2:2345
+\end{verbatim}
+
+Let's put a breakpoint on the \code{main} function, and start the
+program:
+
+\begin{verbatim}
+(gdb) break main
+(gdb) run
+\end{verbatim}
+
+It stops on the first line of the \code{main} function, which is the
+call to \code{config_init}, implemented by the \code{libconfig}
+library. If you do the {\em gdb} instruction \code{step}, {\em gdb}
+will step into the function, so you can follow what happens. After
+having done \code{step} once, you can do \code{backtrace} to see that
+you are in the function \code{config_init} called by \code{main}:
+
+\begin{verbatim}
+(gdb) backtrace
+#0  config_init (config=0xbefffc3c) at libconfig.c:725
+#1  0x000106f0 in main () at myapp.c:11
+\end{verbatim}
+
+\section{Use {\tt <pkg>\_OVERRIDE\_SRCDIR}}
+
+\section{Use the Eclipse IDE with Buildroot}