I maintain rawdraw, and rawdraw uses global symbols to callback, for creation, input, and closure. I would like automatically use one copy of a symbol, say, a no-op if the user has not defined one, or if the user has defined one, I would want to use it.
This isn't a problem for most platforms, but for Android it's incredibly annoying because you can't identify linker issues until the app is running on-device.
The issue is this is in the header. So what I want is a way to, from the same C file, allow a function call to be overridden, but not require it.
I've tried both weak and weak,aliasof(...) attributes and neither seem to allow overriding of a symbol in the same C file.
For example, in this case, a user wants to allow detection of window termination, but many users of this library don't care.
cnfg.h
void HandleWindowTermination();
...
case APP_CMD_TERM_WINDOW:
...
HandleWindowTermination();
break;
Would require the user to implement HandleWindowTermination() - but I would like to have an optional implementation of it.
void DefaultHandleWindowTermination() { } // Do nothing
That is only used if HandleWindowTermination() is never defined.
GCC "weak" attribute is not available for all targets. From the GCC manual:
This attribute requires assembler and object file support, and may not be available on all targets.
Hence you may need to use a work-around. One possibility could be to define a function pointer to an optional implementation:
typedef void (* HandleWindowTerminationFuncType) ();
HandleWindowTerminationFuncType HandleWindowTerminationFunction = NULL;
static void HandleWindowTermination()
{
if(HandleWindowTerminationFunction != NULL)
{
HandleWindowTerminationFunction();
}
else
{
// Default action
}
}
Another module may then implement a HandleWindowTermination() function and set the HandleWindowTerminationFunction pointer to point at that implementation (you may implement a "setter" function to do that).