I have a problem linking C++ libraries and I could really use some advice.
I have two custom .so libraries, which I am using in my cpp project, which I am then linking to Java using JNI.
In my cpp project I am including the header files for the two libraries and JNI. Then I am creating an object file, like this:
g++ -I/usr/lib/jvm/java-7-openjdk-amd64/include -I/usr/lib/jvm/java-7-openjdk-amd64/include/linux -std=c++11 -fPIC -c my_file.cpp
The two includes specify the path to JNI libraries. I am using cpp11 and then a switch -fpic I found that may be necessary during creation of shared libraries.
Next I tried to link my object file: my_file.o with the two shared libraries like this:
g++ -fpic -L/<absolute path>/library1.so -L/<absolute path>/library2.so my_file.o -shared -o my_file.so
I am building a shared library which will be called from Java using JNI. I tried several different variations of assembling the linking parameters but none worked. I am further calling System.load() in Java on an absolute paths to my_file.so and library1.so
Currently I am receiving an error:
/usr/lib/jvm/java-7-openjdk-amd64/bin/java: symbol lookup error: /<absoulte path>/my_file.so: undefined symbol: <method from library 1>
I have read several post here about linking and required parameter order of libraries in order to ensure the correct linking and I have gone through much of g++ documentation, yet no matter how I order them and no matter what parameters I use, it does not seem to work. I will be greatfull for any advice as to how to link these files.
Figured it out with some help.
Firstly, for those who do not know about the command "ldd" on linux, it shows the linked symbols (linked library dependencies). When I saw that my libraries weren't there I tried to rearrange the linking command.
I had to place the object file my_file.o before the other two libraries. Then I saw that ldd pointed out that the libraries were not found. I moved them to a system library and with some more symlinks everything worked. Thanks to everyone for their effort
Related
I am relatively new to Linux development, having been using Windows for a while now. Anyway, I am compiling a C++ game using g++ on both Windows and Linux (using mingw32 when needed), and am linking against SDL2 and SDL2_mixer. On Windows, one would only need to put the DLL files in the same folder as the executable and everything would run fine. On Linux however, although the code compiled just fine with not even a single warning, I get this at runtime :
./nKaruga: error while loading shared libraries: libSDL2_mixer-2.0.so.0: cannot open shared object file: No such file or directory
although said shared lib is in the same folder. I looked up several similar cases on Stack Overflow, all of them involving the use of LD_LIBRARY_PATH, and tried it but to no avail.
% LD_LIBRARY_PATH=pwd
% export LD_LIBRARY_PATH
% ./nKaruga
./nKaruga: error while loading shared libraries: libSDL2_mixer-2.0.so.0: cannot open shared object file: No such file or directory
I want to distribute this program on systems that do not necessarily have admin rights to install dependencies, hence why I am putting the SO in the same folder as the executable.
Thanks by advance !
LD_LIBRARY_PATH is a quick ad-hoc hack to specify alternate library loading search paths. A more permanent and cleaner solution is to specify the specific sets of paths in which libraries shall be searched specific for your particular binary. This is called the rpath (Wikipedia article on it: https://en.wikipedia.org/wiki/Rpath). There are a number of "variables" that can be specified in the binary rpath that get substituted. In your case the rpath variable ${ORIGIN} would be the most interesting for you. ${ORIGIN} tells the dynamic linker to look for libraries within the very same directory in which also the binary resides.
The rpath can be set at link time with the -rpath linker option, i.e. when invoked through GCC the option would be -Wl,-rpath='${ORIGIN}', i.e.
gcc -o program_binary -Wl,-rpath='${ORIGIN}' -lSDL2_mixer a.o b.o …
For an existing binary the rpath can be set post-hoc using the chrpath or patchelf tools; it's better to set it at link time proper, though.
I have built the boost 1.55 serialzation library with the following command:
b2 --build-dir=build toolset=gcc --with-serialization --layout=tagged link=static threading=multi stage
and got libboost_serialization-mt.a and libboost_wserialization-mt.a in my stage/lib directory - fine. Then I added boost_serialization to my C::B project's linker library list and compiled the boost serialization example and it ran fine from the command line. I then built the dynamic and single thread variants additionally using
b2 --build-dir=build toolset=gcc --with-serialization --layout=tagged link=static,shared threading=multi,single stage
and got more libraries in my stage/lib directory, as expected. What puzzles me is that there is a .so file for every library, even those that should be static. Why is it there? What is it needed for?
When I now compile the project, the executable complains:
error while loading shared libraries: libboost_serialization.so.1.55.0: cannot open shared object file: No such file or directory
The library is definitely there and I might just need to add the path to it to LD_LIBRARY_PATH, but I want to link statically for now. How can I do that?
I also don't quite understand the library naming: I have some libboost_wserialization... libraries in my lib folder and the w prefix to serialization is not described in the library naming section of the current boost getting started page.
Your answers gave me a better understanding of what was going on - now I know where the boost_wserialization libraries came from. I turned out that after doing the second build, all present libraries were shared, and the static libraries were overwritten. That's why I got confused by the "extra" .so files for those libraries which previously were indeed static.
Ok, first question:
Why is there a boost_serialization and boost_wserialization library?
the wserialization library is wchar_t oriented. Put into a separate library because it may not actually be needed.
Why are there multiple shared/static libraries?
The reason you're seeing all those extra shared libraries is because you're invoking b2 with link=static,shared, which instructs boost to build the shared libraries as well as the static libraries. Additionally, adding in the thread=multi causes the building of the mt libraries, which are libraries that should be used when linking to multi-threaded applications.
Why am I getting the run-time link error about libboost_serialization.so.1.55.0?
By default, most unix/linux systems will prefer the use of shared libraries over static libraries when linking, so when you try to link it will prefer to use the shared libraries over the static ones. If you want to force the link of the static library rather than the shared one, you tell the compile-time linker to do that, using:
-Wl,-Bstatic -lboost_serialization -Wl,-Bdynamic
This will cause the linker to look for the static variant of the boost_serialization library, rather than the dynamic one.
Now, because you're using code::blocks, I'd have to look up how to specify these flags on a case-by-case basis, but the most sensible thing to do is to clean the boost build using ./b2 clean and then rebuild, specifying only link=static, then you should end up with only .a files, which will produce stand-alone executables again.
If you want to specify this option for code::blocks, you would need to put them into the Build Settings -> Linker settings -> Other Linker Options field for the project. Simply specifying the library in the libraries field will not work for this case. Additionally, forgetting to pass in the -Wl,-Bdynamic option will cause it to try to link in static versions of some platform libraries, which can lead to build failure if the library in question is not present.
If you want to avoid having to set LD_LIBRARY_PATH to run the binary, you can add the option -Wl,-rpath,/path/to/boost/libraries, to the linker flags which will cause the compiled program to search that directory when trying to resolve the location of libraries.
What puzzles me is that there is a .so file for every library, even
those that should be static. Why is it there? What is it needed for?
You apparently are using someone else's make file. I wrote my own. My build command does not create a ".so" (shared object library). It only creates the ".a" (archive library). The linker knows how to use either.
See man ar. The utility ar builds archives.
See man ld. The utility ld can build shared objects.
You might look for these utility invocations in your build sequence, or ask someone where they are and comment out the use of ld, as you most likely do not need both (and building both will extend your build time unnecessarily). Alternatively, you might temporarily rename the ld command, and try your build. When it can not find the ld command, you might get a useful hint as to where the ld is invoked.
In my make file, the commands look like the following. The comment char is a # at beginning of line. (The string expansions $(AR) and $(LD) allow the use of non-standard utilities.)
$(TARGET_ARCHIVE): $(OBJ)
#echo R10: $(TARGET_ARCHIVE) :from: $(OBJ)
$(AR) crs $(TARGET_ARCHIVE) $(OBJ)
# $(TARGET_OLB) : $(OBJ)
# #echo R00: $(TARGET_OLB) :from: $(OBJ)
# $(LD) -o $(TARGET_OLB) -r $(OBJ)
The archive (.a), when used, is linked directly to and included in your executable. When the executable is loaded, all the referenced symbols of the .a are already in it. (un-referenced symbols and code are not linked in)
The shared object (.so) is not directly linked, but rather your executable gets a handle (or perhaps a file name) to the .so. It is my belief that when your executable is loaded, the .so is not immediately loaded. The .so does not load until the first time your executable references a symbol that is in the .so. At that loading, your app will experience a delay, but probably this late loading is reasonable for most applications.
It is also possible that the .so is already loaded in system memory before you activated your process. In that case, when your executable first references a symbol in the .so, some system code will 'map' the existing in-memory .so to your application -- probably faster than loading it, but I suppose the big benefit is that a .so that is used / referenced by many processes need only be loaded once, saving memory space. The loaded .so has all of its symbols, even if your app does not need all of them.
In either case, your executable will be smaller with .so's, bigger with .a's, but the .so's have some small performance hit for each .so that needs to be loaded or mapped in. With 4 GB in my desktop, the desktop has never felt 'crowded'. It's swap has never been used (afaik). So I generally use .a's.
NOTE: When the linker has access to both an archive (.a) and a shared object (.so) file, the linker will use the .so (and ignore the .a). Probably you can override that preference, but I have not tried. I find it easier to simply move the archive (.a) into a separate (from the .so's) directory, and inform the linker via the -L build option.
I have been trying to get this working for a while now and am unable to find an answer elsewhere, here is my problem.
When I make a static library in Visual C++ any dependencies that this library uses are carried on to the executable program that I link the library to. Here is an example
Test.lib depends on
- SDL
- OpenGL
TestApp.exe links Test.lib
In Visual C++ I do not have to add the dependencies for Test.lib in my TestApp executable, they are carried over and it works great.
However when I port my code to Linux g++ (With the Code::Blocks IDE), if I make a Static Library which is .a in g++, and I make a TestApp that links the library, it gets undefined references to the dependencies.
Is g++ able to do this, and if so what am I missing to have the dependencies carried over to my executable?
As for my settings for my library I simply use the IDE's Static Library setting.
With Microsoft's compiler, header files can have library dependency information in them (source files, too, but this is typically done in headers); this gets compiled into the object file, and the linker understands and applies that information. That can be handy: you don't have to remember long, funky names, and if you compile two source files with incompatible options you may get a library name conflict that the linker will complain about. Most compilers and linkers don't do this kind of thing, and you have to tell the linker explicitly which libraries you want to link with.
Static libraries do not statically link with other libraries. Does that sound right?
However you can pack many object files together with a tool called ar.
What happens on windows is probably because you have the (SDL, opengl32) dlls somewhere in a system env path.
I've been using libarchive in my project for some time now and it's working great, at the moment I am dynamically linking to it, so on Windows the libarchive.dll file has to present on the system.
I would now like to statically link to the library so I don't have to bother distributing the DLL, but I'm having real trouble trying to achieve this!
Currently, in my make file, I have something like this:
-Lpath/to//libarchive/ -larchive
And this works, but it does a dynamic link. I don't know how to enforce a static link.
I can see in the libarchive directory there are two a files, libarchive.dll.a and libarchive_static.a. I suppose I want to link to libarchive_static.a but I can't seem to specify this, doing -larchive_static in the make file results in linker errors.
I was under the impression that static libraries under windows are lib files, but I get no such file type when I build libarchive.
How can I make a .lib file from libarchive. Also, as an extra question, what is the difference between an a file and a lib file?
Update
To statically link to libarchive, your library command for make should contain:
-Lpath/to//libarchive/ -larchive_static
This will link to the libarchive_static.a file. However, you also need to define LIBARCHIVE_STATIC in your code.
Now the problem is that libarchive depends on the bzip2 libraries (as well as others), and if you do not have a static build of them you will get linker errors something like:
undefined reference to `BZ2_bzCompressInit'
You need a static build of the dependent libraries and a similar command to the linker after the libarchive command:
-Lpath/to/bzip2/ -lbzip2
You can either build bzip2 from source, or do it the easy way and get a pre-built binary from the Gnu32Win project here: http://gnuwin32.sourceforge.net/packages.html
Just add libarchive_static.a explicitly to your link command.
gcc -o YourApp.exe $(OBJS) path/to/libarchive_static.a $(OtherLibs)
".lib" files differ from compiler to compiler (Borland, Microsoft etc.), ".a" is an old "archive" format from UNIX's ar tool. It is now used only for the bundling of static libraries.
Currently, in my make file, I have something ...
And this works, but it does a dynamic link
The .a file actually contains some code for dynamic linking to the .dll file, not the libarchive itself. On the startup the pointers to functions are allocated and dynamic linking is done.
I have some project with file:
src/Test.cpp.
Now I make subproject for it with another file src/TestWrapper.cpp which includes src/Test.hpp (related to Test.cpp).
Sure, to build successful shared library with wrapper I have to link Test.cpp too.
Everything works. I have project with Test.cpp linked and subproject (shared library) which linkes Test.cpp too.
The problem: that shared library would be linked to the main project. Will I have any errors due to this?
You should not put Test.cpp in your shared library. You can link your shared library without Test.cpp, it will just have some undefined symbols, which is perfectly normal for a shared library.
What you did will probably work but is dangerous and will make your software unmaintainable.