Issues in c++ compilation for embedded system

Question

I have a few related questions about my issues with compilation for embedded system. My questions are not only about HOW to do something, but more about WHY, because I have solutions for my problems (but maybe there are better ones?), but have no idea why some things works in some conditions, and does not work in others. I already spent some time with this, but until yesterday I was doing things a little blindly, with trials and errors, and without knowing what I was doing. Time to stop that! Please, help.

Scenario

I want to develop an application for Xilinx’s Zynq ARM processor, on Zedboard. The app will involve multithreading, some audio manipulation, and httpserver. So I will need pthread, alsa, sndfile and microhttpd libraries. I created rootfs with yocto. In original conf.local file I added/modified these lines:

BB_NUMBER_THREADS ?= "${@oe.utils.cpu_count()}"
PARALLEL_MAKE ?= "-j ${@oe.utils.cpu_count()}"
MACHINE ?= "zedboard-zynq7"
PACKAGE_CLASSES ?= "package_deb"
EXTRA_IMAGE_FEATURES = "debug-tweaks eclipse-debug"
IMAGE_INSTALL_append = "libgcc alsa-utils mpg123 libstdc++ sthttpd libmicrohttpd libsndfile1"
LICENSE_FLAGS_WHITELIST = "commercial_mpg123"

I also had to add some additional layers to bblayers.conf (and of course downloaded them):

meta-xilinx
meta-multimedia (from meta-openembedded)
meta-oe         (from meta-openembedded)
meta-webserver  (from meta-openembedded)

Lastly, I generated core-image-minimal with bitbake.

This, together with Linux kernel, and other stuff compiled separately, boots and works fine.

Problems

1. Simple app with this rootfs

It is app for Zynq, so I use XSDK, which is SDK from Xilinx, based on Eclipse. I created new Application project. In dialog window I chose Linux as platform, C++ as language, and I provided path to my unpacked rootfs (excactly the one that system boots with, via NFS). My rootfs path is /home/stas/ZedboardPetalinuxFS (it is not Petalinux, I just used to use it, and this folder name is still the same). This sets proper paths for library and headers search in rootfs. I started with something very simple:

#include <pthread.h>

int main()
{
    int i;
    i = 1;
    return 0;
}

I also added pthread library for linker (in Eclipse settings). Linking command at this point:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" -o "test.elf"  ./src/main.o   -lpthread

At this point it compiles. But it stops, when I add sndfile library

#include <sndfile.h>

This is reasonable, because this rootfs does not have all headers. I need to add another path for searching for headers. So I added path in yocto tmp folder, that has all the headers, that was needed for building rootfs. After I add it, it compiles again successfully. But problems started, when I added sndfile library for linking. Here is linking command and error:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" -o "test.elf"  ./src/main.o   -lpthread -lsndfile
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lsndfile

I looked to usr/lib to check if libsndfile.so is there, and I found only libsndfile.so.1 and ibsndfile.so.1.27. But it is also the case for pthread, and linker does not complain for that. I decided to create libsndfile.so by hand (I linked it to libsndfile.so.1). Linker stopped complaining about it, but started complaining about it’s dependencies. So I also creaded .so files for all the dependencies, and their dependencies, and added them for linking. Then it succeeded. At the end, linking command looked like this:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" -o "test.elf"  ./src/main.o   -lpthread -lvorbisenc -lvorbis -logg -lFLAC -lsndfile

So here goes the first question – why I did not needed .so file for pthread, but needed it for all other libraries? Or more general – when do I need .so file, and when .so.X file is enough?

2. Simple app - another approach

After the first try, I thought I should make another image, this time more suitable for development. Luckily, in Yocto it is quite easy – I just had to modify one line:

EXTRA_IMAGE_FEATURES = "debug-tweaks eclipse-debug dev-pkgs"

dev-pkgs option adds -dev packages for all installed packages. So now I have rootfs with all needed headers, and .so files pointing where they should.
Before compilation, I removed unnecessary Include path, leaving only the one from rootfs, and removed all the libraries, except pthread, and sndfile. But then I get new errors:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" -o "test.elf"  ./src/main.o   -lsndfile -lpthread
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find /lib/libpthread.so.0
makefile:48: polecenia dla obiektu 'test.elf' nie powiodły się (commands for ‘test.elf’ did not succeed)
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find /usr/lib/libpthread_nonshared.a

I spotted, that it looks for libraries in my root folder. Quick search in Google (and SO:)) told me that I should set –-sysroot variable. So I added it to Eclipse option (in Miscelenious card in Linker options) like that:

--sysroot=/home/stas/ZedboardPetalinuxFS

So now linker command looked like this:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" --sysroot=/home/stas/ZedboardPetalinuxFS -o "test.elf"  ./src/main.o   -lsndfile -lpthread

And all succeed! I also wrote simple example that uses pthreads, and sndfile, and it also worked. But WHY? This leads me to second question: Why do I need --sysroot option in this case? When do I need to use this option in general? And why this time I didn't have to add all the dependencies to linking command?

3. Another idea

At this point, I had an idea, to check what will happen, if I add --sysroot option having rootfs populated with old, non development image. But this gave me new errors:

arm-linux-gnueabihf-g++ -L"/home/stas/ZedboardPetalinuxFS/usr/lib" -L"/home/stas/ZedboardPetalinuxFS/lib" --sysroot=/home/stas/ZedboardPetalinuxFS -o "test.elf"  ./src/main.o   -lpthread -lvorbisenc -lvorbis -logg -lFLAC -lsndfile
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find crt1.o: No such file or directory
makefile:48: polecenia dla obiektu 'test.elf' nie powiodły się
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find crti.o: No such file or directory
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lpthread
/opt/Xilinx/SDK/2016.4/gnu/aarch32/lin/gcc-arm-linux-gnueabi/bin/../lib/gcc/arm-linux-gnueabihf/5.2.1/../../../../arm-linux-gnueabihf/bin/ld: cannot find -lm

So third question – what does this errors mean?

Thanks very much in advance!

Answer 1

You are doing several things wrong:

1. looks like you are not using environment variables, but calling cross-compiler directly. So, instead of compiling with arm-linux-gnueabihf-g++ ..., you should do $CXX .... The CXX is the environment variable set by the yocto script to set environment for cross compilation. Using CXX, you do not need to manually pass --sysroot
2. You should not link directly to pthread library with -lpthread. You should use -pthread

Answer 2

why I did not needed .so file for pthread, but needed it for all other libraries?

A cross compiler will normally come with a C Runtime (including pthread), typically in a directory that is part of the cross compiler installation.

The linker has built in search paths for libraries. These are in respect to the sysroot, which would by default be set to search the cross compiler's own included target C Runtime. If you added any -L options it would search those first and then move on to these pre-defined directories.

When you linked against pthread it would have found at least libpthread.a in the cross compiler's library directory.

Or more general – when do I need .so file, and when .so.X file is enough?

Shared libraries in Linux typically have a major and a minor version number. Libraries are ABI compatible between different minor versions with the same major version, but not between major versions. Sometimes there are three levels of versions but the principal is similar.

When installing libraries it is common to install the actual file with the full name, eg. libmy.so.1.2, then provide symlinks to libmy.so.1 and libmy.so.

If you are linking an application can work with any library version then you would just specify the name, eg. -lmy. In that case you would need symlinks from libmy.so to libmy.so.1.

If you required a specific version you would put -l:libmy.so.1. The ':' indicates a literal file name.

Linker scripts may affect things and may result in specific versions being selected even when you do specify the short name.

Why do I need --sysroot option in this case? When do I need to use this option in general?

What --sysroot does is prepend the given path onto all the search directories which would normally be used to search for includes and libraries. It is most useful when cross compiling (as you are doing now) to get the compiler and the linker to search inside the target root instead of the build host's own root.

If you have specified a sysroot you probably do not need to specify include paths via -I or linker paths via -L, assuming that the files are within their normal spots inside your target root.

And why this time I didn't have to add all the dependencies to linking command?

One possible scenario is that the first time, sndfile for statically rather than dynamically linked. This would happen if your first root image had only sndfile.a in the lib dir, or elsewhere on the search path. To then satisfy the requirements of sndfile.a you would also need to link the other libs.

When linking against sndfile.so the dependencies will automatically get loaded via the dynamic linking process.

That's just a working theory at present.

So third question – what does this errors mean?

They mean it cannot find even the C runtime library to link.

As described for the first question, it was previously finding the C runtime in the pre-defined search path (relative to the predefined sysroot) which located the C runtime supplied by the cross compiler.

You disturbed this by supplying your own sysroot. It was now only searching the target root. Since this target root filesystem did not have development libs installed, there was no C runtime there to find.

Answer 3

1. "why I did not needed .so file for pthread, but needed it for all other libraries?"

   Actually you do need pthread.so file. You included pthread.h but didn't link with -lpthread. So it's normal you don't see any linker errors.

   "when do I need .so file, and when .so.X file is enough"

   When you give "-lNAME" parameter to g++, the compiler tells the linker to find libNAME.so within library search paths. Since there may exist multiple versions of the same library(libNAME.so.1, libNAME.so.1.20), *.so files link to desired actual library file. (Versioning of shared objects, ld man pages)

2. "Why do I need --sysroot option in this case? When do I need to use this option in general? And why this time I didn't have to add all the dependencies to linking command?"

   The "dev-pkgs" in EXTRA_IMAGE_FEATURES changes your sysroot implicitly to let you link against the dev packages(yoctoproject image-features). That's why you need -sysroot option. You generally need this option when cross compiling to provide a root for standard search paths for headers and libraries. You didn't need it because you didn't have dev-pkgs image feature that changes your sysroot

3. "So third question – what does this errors mean?"

   Even your the most basic hello world code gets linked with standard c library(if you didn't specify otherwise). libm.so, libpthread.so and crt1.o files are parts of libc library and come with libc dev package. So the linker can't see the standard library directories when it looks from your old sysroot