Feeds

Binary interfaces in component development

A template class with static data - how can that possibly go wrong?

Beginner's guide to SSL certificates

In the next example (see Figure 2) the code is logically and textually identical apart from the usual dllimport and dllexport specifiers. Later in this article we’ll see more about these and how they work, but for this example they’re not important. Instead, we want to focus on the behaviour change; our singleton is no longer living up to its name. Two singleton objects are created and the Gang of Four are already drafting a blog entry, complaining about our corruption of their pattern.

Figure 2 – Shows the dynamic version, where our Singleton is constructed twice.

So what has happened here? Earlier we noted that a C++ compiler would (with few exceptions) instantiate a class template and its member functions when it’s specialised and the template definitions for the member functions are available. The Windows behaviour is that both A and B are dlls, making calls to member functions in our singleton class. The template definitions are in the header file and hence always available (they are always instantiated). As a result, both A.dll and B.dll contain a definition for our static variable. However, if we try to confirm this by using the dumpbin command that comes with Visual Studio, there’s no trace of our static variable anywhere in the dynamic libraries (see Figure 3). In the linker map output (see Figure 4), we can see that the symbols are still in the library but the identifying names have been eliminated.

Figure 3: Dumpbin of all dll information 
(N.B. no mention of our static variable!).

D:\ …\code\somemodularexe\Release>dumpbin /all DynLibUsingSingletonA.dll 

SECTION HEADER #1
   .text name
   119ED virtual size
    1000 virtual address (10001000 to 100129EC)
   12000 size of raw data
    1000 file pointer to raw data (00001000 to 00012FFF)
       0 file pointer to relocation table
       0 file pointer to line numbers
       0 number of relocations
       0 number of line numbers
60000020 flags
         Code
         Execute Read

RAW DATA #1
  10001000: C7 01 B4 31 01 10 E9 17 2D 00 00 CC CC CC CC CC  Ç.´1..é.-..ÌÌÌÌÌ
  10001010: 56 8B F1 C7 06 B4 31 01 10 E8 04 2D 00 00 F6 44  V.ñÇ.´1..è.-..öD
  10001020: 24 08 01 74 09 56 E8 D9 2D 00 00 83 C4 04 8B C6  $..t.VèÙ-...Ä..Æ
  10001030: 5E C2 04 00 CC CC CC CC CC CC CC CC CC CC CC CC  ^Â..ÌÌÌÌÌÌÌÌÌÌÌÌ
  10001040: 8B 44 24 10 8B 4C 24 0C 8B 54 24 08 56 8B 74 24  .D$..L$..T$.V.t$
                                …       ….      …

Figure 4: Linker map output showing variable definition.

0004:00001b60 ?myNsExportedVar@A@@3HA    100b2b60     A.obj
0004:00001b64 ?m_instance@?$GenericSingleton@VMySingletonObject@@@@0VMySingletonObject@@A

The reason we can’t see the static objects in the dumpbin report is that in our example we haven’t tagged the instantiated objects of static duration as dllexport. As a result, in Microsoft’s portable executable format, the linker doesn’t add them to either the export section of the dll - or even preserve the symbol names. After all, if a symbol isn’t exported there’s no need to put its name into the dll; as all references to it are known at link time. However, under such a system we cannot expect our static data at runtime to behave as C++ indicates that they should, with a single instance across the program.

Remote control for virtualized desktops

More from The Register

next story
Download alert: Nearly ALL top 100 Android, iOS paid apps hacked
Attack of the Clones? Yeah, but much, much scarier – report
NSA SOURCE CODE LEAK: Information slurp tools to appear online
Now you can run your own intelligence agency
Microsoft: Your Linux Docker containers are now OURS to command
New tool lets admins wrangle Linux apps from Windows
Microsoft adds video offering to Office 365. Oh NOES, you'll need Adobe Flash
Lovely presentations... but not on your Flash-hating mobe
You stupid BRICK! PCs running Avast AV can't handle Windows fixes
Fix issued, fingers pointed, forums in flames
HTML5 vs native: Harry Coder and the mudblood mobile app princes
Developers just want their ideas to generate money
prev story

Whitepapers

Go beyond APM with real-time IT operations analytics
How IT operations teams can harness the wealth of wire data already flowing through their environment for real-time operational intelligence.
5 critical considerations for enterprise cloud backup
Key considerations when evaluating cloud backup solutions to ensure adequate protection security and availability of enterprise data.
Getting started with customer-focused identity management
Learn why identity is a fundamental requirement to digital growth, and how without it there is no way to identify and engage customers in a meaningful way.
Reg Reader Research: SaaS based Email and Office Productivity Tools
Read this Reg reader report which provides advice and guidance for SMBs towards the use of SaaS based email and Office productivity tools.
Simplify SSL certificate management across the enterprise
Simple steps to take control of SSL across the enterprise, and recommendations for a management platform for full visibility and single-point of control for these Certificates.