Code::Blocks and boost 1.55: static library is not used when dynamic library is present

Question

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.

Answer 1

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.

Answer 2

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.