Shared Libraries


A shared library is compiled code that can be loaded and used later when running an executable.

When the compiler makes an executable, the basic source files are first turned in to object files. The object files are then linked together to make an executable. A shared library is much like a static library in that it contains object files. But a shared library is also like an executable in that it only gets loaded when the executable is running.

The library is referred to as shared, because the code in the library is loaded by an executable at runtime and can be loaded by more than one executable, even though there might only be one copy of the shared library.

Once the library is made, we can then use the code that it contains just as if we were compiling the source directly with our program.

Shared Library Example
Using Shared Libraries on Windows
Using Shared Libraries on Linux
Executables that export symbols
Loading Shared Libraries Dynamically

Shared Library Example
Following is a simple example of creating a shared library using these three files:

Our library will be a single module providing a single function:

'' mylib.bas
'' compile with: fbc -dll mylib.bas

'' Add two numbers together and return the result
Public Function Add2( ByVal x As Integer, ByVal y As Integer ) As Integer Export
  Return( x + y )
End Function


Compile the library with:
fbc -dll mylib.bas

The -dll option tells the compiler to take the source code, mylib.bas, and turn it in to an object file mylib.o, then store the object file in to a shared library. The name of the shared library will have a .so extension or .dll extension depending on if the platform is the linux or windows version. A library might contain many modules (source files) each with many functions, but for this simple example, it is just one each.

Making a shared library is almost identical to making a static library except for the addition of Export declaration specifier. Export tells the compiler to make the function visible to other executables loading the shared library.

To make use of the library in some other source code, we need some way of telling the compiler what exactly is in the library. A good way to do this is to put the declarations ( also called an interface, or API ) for the library in to a header file.

'' mylib.bi
#inclib "mylib"
Declare Function Add2( ByVal x As Integer, ByVal y As Integer ) As Integer


There is no need to compile the header. We want this in its source form so it can be included with other source files. The #inclib statement will tell the compiler the name of a shared library that we need to link with at runtime running an executable that needs it.

With our library (.dll / .so file) and a header (.bi file) we can try them out in a test program:

'' mytest.bas
'' compile with: fbc mytest.bas
#include once "mylib.bi"
Print Add2(1,2)


The #include statement tells the compiler to include the source code from mylib.bi just as if we had typed it in to the original source. With the way we have written our include file, it tells the compiler everything it needs to know about the library.

We compile this with:
fbc mytest.bas

Then when we run the mytest executable, we should get the result of:
3

More than one source module can be used when making a library. And basic programs can use more than one library by including each needed header. Some libraries are so large that they might use several headers. On very large projects, making shared libraries out of some code modules that seldom change can improve compile times and link times dramatically.

Shared libraries can optionally contain debugging information specified with the -g command line option.

Object files, and therefore shared libraries, are platform specific and in some cases specific to a particular version of the compiler and FreeBASIC runtime library.

Using Shared Libraries on Windows
On Windows, the shared library must be stored in a location where it can be found by the executable that needs it a run-time.

The operating system may search the following directories:

The order in which directories are searched may depend on the Windows version in use, and on what settings that the operating system is configured with.

Using Shared Libraries on Linux
By default, Linux will not normally search the current directory or the directory from which the executable was loaded. You will need to either:

To run the executable ./mytest/ and temporarily tell linux to search the current directory, use the following shell command:
LD_LIBRARY_PATH=.:$LD_LIBRARY_PATH ./mytest


Executables that export symbols
If an executable has symbols that must be available to other shared libraries when those shared libraries are loaded, use the Export procedure declaration specifier, and the -export command line option when making (linking) the executable.

The -export option has no extra effect when used with the -dylib or -dll command line options.

Loading Shared Libraries Dynamically
Shared libraries can be loaded and used at run time by dynamically loading the library and its symbols at runtime.

Procedures in the shared library must use the Export specifier to ensure that the symbols name is placed in the shared library's export table.

'' mydll.bas
'' compile as: fbc -dll mydll.bas
'' This will create mydll.dll (and libmydll.dll.a import library) on Windows,
'' and libmydll.so on Linux.
''
'' Note: libmydll.dll.a is an import library, it's only needed when creating
'' an executable that calls any of mydll's functions, only distribute
'' the DLL files with your apps, do not include the import libraries,
'' they are useless to end-users.

'' Simple exported function; the <alias "..."> disables FB's default
'' all-upper-case name mangling, so the DLL will export AddNumbers() instead of
'' ADDNUMBERS().
Function AddNumbers Alias "AddNumbers"( ByVal a As Integer, ByVal b As Integer ) As Integer Export
    Function = a + b
End Function


'' load.bas: Loads mydll.dll (or libmydll.so) at runtime, calls one of mydll's
'' functions and prints the result. mydll is not needed at compile time.
'' compile as: fbc test.bas
''
'' Note: The compiled mydll.dll (or libmydll.so) dynamic library is expected
'' to be available in the current directory.

'' Note we specify just "mydll" as library file name; this is to ensure
'' compatibility between Windows and Linux, where a dynamic library
'' has different file name and extension.
Dim As Any Ptr library = DyLibLoad( "mydll" )
If( library = 0 ) Then
    Print "Failed to load the mydll dynamic library, aborting program..."
    End 1
End If

'' This function pointer will be used to call the function from mydll, after
'' the address has been found. Note: It must have the same calling
'' convention and parameters.
Dim AddNumbers As Function( ByVal As Integer, ByVal As Integer ) As Integer
AddNumbers = DyLibSymbol( library, "AddNumbers" )
If( AddNumbers = 0 ) Then
    Print "Could not retrieve the AddNumbers() function's address from the mydll library, aborting program..."
    End 1
End If

Randomize Timer

Dim As Integer x = Rnd * 10
Dim As Integer y = Rnd * 10

Print x; " +"; y; " ="; AddNumbers( x, y )

'' Done with the library; the OS will automatically unload libraries loaded
'' by a process when it terminates, but we can also force unloading during
'' our program execution to save resources; this is what the next line does.
'' Remember that once you unload a previously loaded library, all the symbols
'' you got from it via dylibsymbol will become invalid, and accessing them
'' will cause the application to crash.
DyLibFree( library )


See also:
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