= Shared Library
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# Introduction
Shows a hello world example which first creates and links a shared library.
This also shows how to create an link:https://cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#alias-targets[alias target]
The files in this tutorial are below:
```
$ tree
.
├── CMakeLists.txt
├── inc
│ └── Hello.h
└── src
├── Hello.cpp
└── main.cpp
```
* link:CMakeLists.txt[] - Contains the CMake commands you wish to run
* link:inc/Hello.h[] - The header file to include
* link:src/Hello.cpp[] - A source file to compile
* link:src/main.cpp[] - The source file with main
# Concepts
## Adding a Shared Library
As with the previous example on static libraries, the +add_library()+ function
is also used to create a shared library from some source files.
This is called as follows:
[source,cmake]
----
set(library_SOURCES
src/Hello.cpp
)
add_library(hello_library SHARED ${library_SOURCES})
----
This will be used to create a shared library with the name libhello_library.so with
the sources from the +library_SOURCES+ variable.
## Alias Target
As the name suggests an link:https://cmake.org/cmake/help/v3.0/manual/cmake-buildsystem.7.html#alias-targets[alias target] is an alternative name for a target that can be used instead of the real target name in read-only contexts.
[source,cmake]
----
add_library(hello::library ALIAS hello_library)
----
As shown below, this allows you to reference the target using the alias name when linking it against other targets.
## Linking a Shared Library
Linking a shared library is the same as linking a static library. When creating your
executable use the the +target_link_library()+ function to point to your library
[source,cmake]
----
add_executable(hello_binary ${binary_SOURCES})
target_link_libraries( hello_binary
hello::library
)
----
This tells CMake to link the hello_library against the hello_binary executable using the alias target name.
An example of this being called by the linker is
```
/usr/bin/c++ CMakeFiles/hello_binary.dir/src/main.cpp.o -o hello_binary -rdynamic libhello_library.so -Wl,-rpath,/home/matrim/workspace/cmake-examples/01-basic/D-shared-library/build
```
# Building the Example
[source,bash]
----
$ mkdir build
$ cd build
$ cmake ..
-- The C compiler identification is GNU 4.8.4
-- The CXX compiler identification is GNU 4.8.4
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/matrim/workspace/cmake-examples/01-basic/D-shared-library/build
$ make
Scanning dependencies of target hello_library
[ 50%] Building CXX object CMakeFiles/hello_library.dir/src/Hello.cpp.o
Linking CXX shared library libhello_library.so
[ 50%] Built target hello_library
Scanning dependencies of target hello_binary
[100%] Building CXX object CMakeFiles/hello_binary.dir/src/main.cpp.o
Linking CXX executable hello_binary
[100%] Built target hello_binary
$ ls
CMakeCache.txt CMakeFiles cmake_install.cmake hello_binary libhello_library.so Makefile
$ ./hello_binary
Hello Shared Library!
----
