Shared Libraries (Preview)#

Different Flavors Of Library#

  • STATIC

    • Simple bag of objects

    • At executable link time: linker copies needed object into executable

    • Pro

      • Executable is standalone (“statically linked against that library”)

    • Con

      • Library update (bug fix?) requires rebuild of all executables that depend on it

      • Large executables

  • SHARED

    • At executable link time: linker just verifies all is there, but code remains in library

    • Loaded at executable startup time

    • ⟶ Loader (usually /lib64/ld-linux-x86-64.so.2)

    • More: Shared Libraries (Shared Objects)

    • Pro

      • Small executable

      • Library update is just a matter of copying the new library version

      • “More professional”

    • Con

      • Rather complex

      • Requires understanding of library versioning

      • Not quite easy during development; roundtrips are longer and more error prone

  • MODULE

    • Plugin that is loaded explicitly by the program, at an arbitrary time in its lifetime

Building Shared Libraries#

  • In the CMakeLists.txt, using SHARED tag

  • ⟶ Library is always built as a shared library

    add_library(hello SHARED hello.c hello-name.c) # <-- shared library only

  • Switchable at build/config time

  • Convenient during development

  • cmake -DBUILD_SHARED_LIBS=ON|OFF ...

    add_library(hello hello.c hello-name.c)        # <-- defaults to STATIC

    $ cmake -DBUILD_SHARED_LIBS=ON ...

More About Loading#

  • Program interpreter ⟶ hash bang (e.g. #!/bin/sh) in scripts

  • For executables: /lib64/ld-linux-x86-64.so.2

  • Library SONAME and NEEDED tags ⟶ look in /usr/lib64/

    $ rpm -qf /usr/lib64/libboost_regex.so.1.76.0
$ sudo dnf install boost-devel

  • A-ha: development versions

    $ ls -l /usr/lib64/libboost_regex*
lrwxrwxrwx. 1 root root     24 Jul 14  2022 /usr/lib64/libboost_regex.so -> libboost_regex.so.1.76.0
-rwxr-xr-x. 1 root root 289264 Jul 14  2022 /usr/lib64/libboost_regex.so.1.76.0

  • Temporarily add

    set_property(TARGET hello PROPERTY VERSION "1")``

    and modify to

    project(Demo VERSION "1.0.0")
set_property(TARGET hello PROPERTY VERSION ${PROJECT_VERSION})

  • Discuss ABI versions ⟶ rarely project versions can be used as ABI versions

  • ⟶ can of worms: installation/deployment