[CMake] Programs linked to .a or .so libraries?

[gga]

Philip Lowman wrote:
> I've never read that shared libraries are faster than static
> executables.  One would think that, if anything, all of the hoops the OS
> has to run through to map all of the symbols would cause applications
> that use shared libraries to be slower to startup then statically built
> executables.

The mapping happens pretty much automatically as it is usually done by 
an indirect table.
This indirect table does add some overhead to runtime execution, but 
with current machines this is negligible.

Loading all the code for a library can take some time (some seconds or 
minutes), even with today's machines.

If the same shared library is used by multiple programs, loading the 
code in memory needs to happen only once.
With static libraries, you always have to load the code regardless and 
as you say, you keep several copies of the same or similar code in memory.

Most modern OSes also support the concept of delay loaded dsos.  That 
is, shared dso files which are loaded only when a function to them is 
referenced, not just at startup of the executable.  This allows an 
executable to be linked to a shared library but not have to load it 
unless the user actually uses a function belonging to the dso.  There's 
no way to do that with static linking.

Good OSes like linux will also often cache the access to shared 
libraries, making reopening the same .so several times almost immediate. 
  Static executables are hardly ever cached.

If you are also loading a library already used by your window manager 
(usually some KDE or Gnome library on linux these days, but also some 
other common libs like libz, libpng, etc), the shared library will 
probably not even need to be loaded at all, as the window manager 
already did it for you.   Again, with static linking you loose that.

All those benefits can lead a program using shared libraries to start up 
faster than if it had been linked statically.  Of course, this does not 
mean it will indeed do so, just that it may.


-- 
Gonzalo Garramuño
ggarra at advancedsl.com.ar

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