Help: Add cmake-buildsystem(7) subsections for each binary target type

Re-organize the "Binary Targets" section of the manual to have a
dedicated subsection for each kind of binary target.  Initialize each
subsection by documenting the object file and link semantics of the
corresponding target type.

Help/command/add_executable.rst

@@ -14,8 +14,8 @@ Normal Executables
   add_executable(<name> <options>... <sources>...)
   :target: normal
 
-  Add an executable target called ``<name>`` to be built from the source
+  Add an :ref:`executable <Executables>` target called ``<name>`` to
-  files listed in the command invocation.
+  be built from the source files listed in the command invocation.
 
   The options are:
 

Help/command/add_library.rst

@@ -20,14 +20,17 @@ Normal Libraries
   The optional ``<type>`` specifies the type of library to be created:
 
   ``STATIC``
-    An archive of object files for use when linking other targets.
+    A :ref:`Static Library <Static Libraries>`:
+    an archive of object files for use when linking other targets.
 
   ``SHARED``
-    A dynamic library that may be linked by other targets and loaded
+    A :ref:`Shared Library <Shared Libraries>`:
+    a dynamic library that may be linked by other targets and loaded
     at runtime.
 
   ``MODULE``
-    A plugin that may not be linked by other targets, but may be
+    A :ref:`Module Library <Module Libraries>`:
+    a plugin that may not be linked by other targets, but may be
     dynamically loaded at runtime using dlopen-like functionality.
 
   If no ``<type>`` is given the default is ``STATIC`` or ``SHARED``

Help/command/target_link_libraries.rst

@@ -238,6 +238,8 @@ is not ``NEW``, they are also appended to the
 ``general`` (or without any keyword) are treated as if specified for both
 ``debug`` and ``optimized``.
 
+.. _`Linking Object Libraries`:
+
 Linking Object Libraries
 ^^^^^^^^^^^^^^^^^^^^^^^^
 

Help/manual/cmake-buildsystem.7.rst

@@ -37,10 +37,13 @@ is defined as an executable formed by compiling and linking ``zipapp.cpp``.
 When linking the ``zipapp`` executable, the ``archive`` static library is
 linked in.
 
-.. _`Binary Executables`:
+.. _`Executables`:
 
-Binary Executables
+Executables
-------------------
+-----------
+
+Executables are binaries created by linking object files together,
+one of which contains a program entry point, e.g., ``main``.
 
 The :command:`add_executable` command defines an executable target:
 
@@ -48,56 +51,101 @@ The :command:`add_executable` command defines an executable target:
 
   add_executable(mytool mytool.cpp)
 
+CMake generates build rules to compile the source files into object
+files and link them into an executable.
+
+Link dependencies of executables may be specified using the
+:command:`target_link_libraries` command.  Linkers start with the
+object files compiled from the executable's own source files, and
+then resolve remaining symbol dependencies by searching linked libraries.
+
 Commands such as :command:`add_custom_command`, which generates rules to be
 run at build time can transparently use an :prop_tgt:`EXECUTABLE <TYPE>`
 target as a ``COMMAND`` executable.  The buildsystem rules will ensure that
 the executable is built before attempting to run the command.
 
-Binary Library Types
+.. _`Static Libraries`:
---------------------
 
-.. _`Normal Libraries`:
+Static Libraries
+----------------
 
-Normal Libraries
+Static libraries are archives of object files.  They are produced by an
-^^^^^^^^^^^^^^^^
+archiver, not a linker.  `Executables`_, `Shared Libraries`_, and
+`Module Libraries`_ may link to static libraries as dependencies.
+Linkers select subsets of object files from static libraries as needed
+to resolve symbols and link them into consuming binaries.  Each binary
+that links to a static library gets its own copy of the symbols, and
+the static library itself is not needed at runtime.
 
-By default, the :command:`add_library` command defines a ``STATIC`` library,
+The :command:`add_library` command defines a static library target
-unless a type is specified.  A type may be specified when using the command:
+when called with the ``STATIC`` library type:
-
-.. code-block:: cmake
-
-  add_library(archive SHARED archive.cpp zip.cpp lzma.cpp)
 
 .. code-block:: cmake
 
   add_library(archive STATIC archive.cpp zip.cpp lzma.cpp)
 
-The :variable:`BUILD_SHARED_LIBS` variable may be enabled to change the
+or, when the :variable:`BUILD_SHARED_LIBS` variable is false, with no type:
-behavior of :command:`add_library` to build shared libraries by default.
-
-In the context of the buildsystem definition as a whole, it is largely
-irrelevant whether particular libraries are ``SHARED`` or ``STATIC`` --
-the commands, dependency specifications and other APIs work similarly
-regardless of the library type.  The ``MODULE`` library type is
-dissimilar in that it is generally not linked to -- it is not used in
-the right-hand-side of the :command:`target_link_libraries` command.
-It is a type which is loaded as a plugin using runtime techniques.
-If the library does not export any unmanaged symbols (e.g. Windows
-resource DLL, C++/CLI DLL), it is required that the library not be a
-``SHARED`` library because CMake expects ``SHARED`` libraries to export
-at least one symbol.
 
 .. code-block:: cmake
 
-  add_library(archive MODULE 7z.cpp)
+  add_library(archive archive.cpp zip.cpp lzma.cpp)
+
+CMake generates build rules to compile the source files into object
+files and archive them into a static library.
+
+Link dependencies of static libraries may be specified using the
+:command:`target_link_libraries` command.  Since static libraries are
+archives rather than linked binaries, object files from their link
+dependencies are not included in the libraries themselves (except for
+`Object Libraries`_ specified as *direct* link dependencies).
+Instead, CMake records static libraries' link dependencies for
+transitive use when linking consuming binaries.
+
+.. _`Shared Libraries`:
+
+Shared Libraries
+----------------
+
+Shared libraries are binaries created by linking object files together.
+`Executables`_, other shared libraries, and `Module Libraries`_ may link
+to shared libraries as dependencies.  Linkers record references to shared
+libraries in consuming binaries.  At runtime, a dynamic loader searches
+for referenced shared libraries on disk and loads their symbols.
+
+The :command:`add_library` command defines a shared library target
+when called with the ``SHARED`` library type:
+
+.. code-block:: cmake
+
+  add_library(archive SHARED archive.cpp zip.cpp lzma.cpp)
+
+or, when the :variable:`BUILD_SHARED_LIBS` variable is true, with no type:
+
+.. code-block:: cmake
+
+  add_library(archive archive.cpp zip.cpp lzma.cpp)
+
+CMake generates build rules to compile the source files into object
+files and link them into a shared library.
+
+Link dependencies of shared libraries may be specified using the
+:command:`target_link_libraries` command.  Linkers start with the
+object files compiled from the shared library's own source files, and
+then resolve remaining symbol dependencies by searching linked libraries.
+
+.. note::
+
+  CMake expects shared libraries to export at least one symbol.  If a library
+  does not export any unmanaged symbols, e.g., a Windows resource DLL or
+  C++/CLI DLL, make it a `Module Library <Module Libraries>`_ instead.
 
 .. _`Apple Frameworks`:
 
 Apple Frameworks
-""""""""""""""""
+----------------
 
-A ``SHARED`` library may be marked with the :prop_tgt:`FRAMEWORK`
+`Shared Libraries`_ and `Static Libraries`_ may be marked with the
-target property to create an macOS or iOS Framework Bundle.
+:prop_tgt:`FRAMEWORK` target property to create a macOS or iOS Framework.
 A library with the ``FRAMEWORK`` target property should also set the
 :prop_tgt:`FRAMEWORK_VERSION` target property.  This property is typically
 set to the value of "A" by macOS conventions.
@@ -113,46 +161,80 @@ and it uniquely identifies the bundle.
     MACOSX_FRAMEWORK_IDENTIFIER org.cmake.MyFramework
   )
 
+.. _`Module Libraries`:
+
+Module Libraries
+----------------
+
+Module libraries are binaries created by linking object files together.
+Unlike `Shared Libraries`_, module libraries may not be linked by other
+binaries as dependencies -- do not name them in the right-hand side of
+the :command:`target_link_libraries` command.  Instead, module libraries
+are plugins that an application can dynamically load on-demand at runtime,
+e.g., by ``dlopen``.
+
+The :command:`add_library` command defines a module library target
+when called with the ``MODULE`` library type:
+
+.. code-block:: cmake
+
+  add_library(archivePlugin MODULE 7z.cpp)
+
+CMake generates build rules to compile the source files into object
+files and link them into a module library.
+
+Link dependencies of module libraries may be specified using the
+:command:`target_link_libraries` command.  Linkers start with the
+object files compiled from the module library's own source files, and
+then resolve remaining symbol dependencies by searching linked libraries.
+
 .. _`Object Libraries`:
 
 Object Libraries
-^^^^^^^^^^^^^^^^
+----------------
 
-The ``OBJECT`` library type defines a non-archival collection of object files
+Object libraries are collections of object files created by compiling
-resulting from compiling the given source files.  The object files collection
+source files without any archiving or linking.  The object files may be
-may be used as source inputs to other targets by using the syntax
+used when linking `Executables`_, `Shared Libraries`_, and
-:genex:`$<TARGET_OBJECTS:name>`.  This is a
+`Module Libraries`_, or when archiving `Static Libraries`_.
-:manual:`generator expression <cmake-generator-expressions(7)>` that can be
+
-used to supply the ``OBJECT`` library content to other targets:
+The :command:`add_library` command defines an object library target
+when called with the ``OBJECT`` library type:
 
 .. code-block:: cmake
 
-  add_library(archive OBJECT archive.cpp zip.cpp lzma.cpp)
+  add_library(archiveObjs OBJECT archive.cpp zip.cpp lzma.cpp)
 
-  add_library(archiveExtras STATIC $<TARGET_OBJECTS:archive> extras.cpp)
+CMake generates build rules to compile the source files into object files.
 
-  add_executable(test_exe $<TARGET_OBJECTS:archive> test.cpp)
+Other targets may specify the object files as source inputs by using the
-
+:manual:`generator expression <cmake-generator-expressions(7)>` syntax
-The link (or archiving) step of those other targets will use the object
+:genex:`$<TARGET_OBJECTS:name>`:
-files collection in addition to those from their own sources.
-
-Alternatively, object libraries may be linked into other targets:
 
 .. code-block:: cmake
 
-  add_library(archive OBJECT archive.cpp zip.cpp lzma.cpp)
+  add_library(archiveExtras STATIC $<TARGET_OBJECTS:archiveObjs> extras.cpp)
+
+  add_executable(test_exe $<TARGET_OBJECTS:archiveObjs> test.cpp)
+
+The consuming targets are linked (or archived) using object files
+both from their own sources and from the named object libraries.
+
+Alternatively, object libraries may be specified as link dependencies
+of other targets:
+
+.. code-block:: cmake
 
   add_library(archiveExtras STATIC extras.cpp)
-  target_link_libraries(archiveExtras PUBLIC archive)
+  target_link_libraries(archiveExtras PUBLIC archiveObjs)
 
   add_executable(test_exe test.cpp)
-  target_link_libraries(test_exe archive)
+  target_link_libraries(test_exe archiveObjs)
 
-The link (or archiving) step of those other targets will use the object
+The consuming targets are linked (or archived) using object files
-files from ``OBJECT`` libraries that are *directly* linked.  Additionally,
+both from their own sources and from object libraries specified as
-usage requirements of the ``OBJECT`` libraries will be honored when compiling
+*direct* link dependencies by :command:`target_link_libraries`.
-sources in those other targets.  Furthermore, those usage requirements
+See :ref:`Linking Object Libraries`.
-will propagate transitively to dependents of those other targets.
 
 Object libraries may not be used as the ``TARGET`` in a use of the
 :command:`add_custom_command(TARGET)` command signature.  However,
@@ -386,14 +468,15 @@ for linking a target.
 
 :prop_tgt:`LINK_LIBRARIES`
   List of link libraries for linking the target, if it is an executable,
-  shared library, or module library.  Entries for `Normal Libraries`_ are
+  shared library, or module library.  Entries for `Static Libraries`_
-  passed to the linker either via paths to their link artifacts, or
+  and `Shared Libraries`_ are passed to the linker either via paths to
-  with ``-l`` flags or equivalent.  Entries for `Object Libraries`_ are
+  their link artifacts, or with ``-l`` flags or equivalent.  Entries for
-  passed to the linker via paths to their object files.
+  `Object Libraries`_ are passed to the linker via paths to their object
+  files.
 
   Additionally, for compiling and linking the target itself,
   `usage requirements <Target Usage Requirements_>`_ are propagated from
-  ``LINK_LIBRARIES`` entries naming `Normal Libraries`_,
+  ``LINK_LIBRARIES`` entries naming `Static Libraries`_, `Shared Libraries`_,
   `Interface Libraries`_, `Object Libraries`_, and `Imported Targets`_,
   collected over the transitive closure of their
   :prop_tgt:`INTERFACE_LINK_LIBRARIES` properties.
@@ -581,8 +664,8 @@ linking consumers.
   Additionally, for compiling and linking the target's consumers,
   `usage requirements <Target Usage Requirements_>`_ are collected from
   the transitive closure of ``INTERFACE_LINK_LIBRARIES`` entries naming
-  `Normal Libraries`_, `Interface Libraries`_, `Object Libraries`_,
+  `Static Libraries`_, `Shared Libraries`_, `Interface Libraries`_,
-  and `Imported Targets`_,
+  `Object Libraries`_, and `Imported Targets`_,
 
 :prop_tgt:`INTERFACE_LINK_DIRECTORIES`
   .. versionadded:: 3.13

Help/prop_tgt/IMPORTED_CONFIGURATIONS.rst

@@ -9,8 +9,7 @@ must be specified in other target properties:
 
 * :prop_tgt:`IMPORTED_LOCATION_<CONFIG>`, or
 * :prop_tgt:`IMPORTED_IMPLIB_<CONFIG>` (on DLL platforms, on AIX for
-  :ref:`Executables <Binary Executables>` or on Apple for
+  :ref:`Executables` or on Apple for :ref:`Shared Libraries`), or
-  :ref:`Shared Libraries <Normal Libraries>`), or
 * :prop_tgt:`IMPORTED_OBJECTS_<CONFIG>` (for :ref:`Object Libraries`), or
 * :prop_tgt:`IMPORTED_LIBNAME_<CONFIG>` (for :ref:`Interface Libraries`).