External variables in shared library constructor code

External variables in shared library constructor code

[smokyboy]

Hi,

I have a problem using external variables in the constructor code of my
shared library. The constructor code is an init() function labeled by gcc
__attribute__((constructor)). In that function I make a reference to an
extern variable X provided by the driving app. The problem is that X is not
yet initialized at the moment init() is invoked, which is before main() is
entered.
A solution might be to introduce a init flag and move the initialization
code to some other exported function, but this is not really elegant. I will
appreciate any alternative suggestion.
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Re: External variables in shared library constructor code

[Eljay Love-Jensen]

Hi smokyboy,

There are no guarantees as to the order of initialization in the C/C++
standards.

Some compilers provide initialization ordering extensions, but those are
usually within a package (such as a DLL), and not across packages (DLLs).

One solution is to hide the external variable within a global function call:

// C++
extern int& GlobalIntValue();

// C
extern int* GlobalIntValue();

That way, the function call can guarantee the variable is initialized on
demand, and that the global variable is only initialized once.

The routine will look like this, in C++:

int& GlobalIntValue()
{
  static int foo = 77;
  return foo;
}

And in C:

int* GlobalIntValue()
{
  static int foo = 77;
  return &foo;
}

HTH,
--Eljay