Experimental Physics and
Industrial Control System

"Mark Rivers" <[email protected]> · Tue, 29 Apr 2008 10:22:44 -0500

Folks,

I'm having a simple problem with linking C++ on Linux that I wonder if
you can help with. 

Here's the problem.

- I have a C++ base class and a number of derived classes.  If the base
class is in the same library as the derived classes everything is fine.
However, if the base class is in a different library from the derived
classes, then it compiles fine (because it finds the header file), but
it fails at link time.  

My base class has methods like the following:

/* Constructor */
asynParamBase::asynParamBase(const char *portName, int maxAddr, int
paramTableSize)
    : maxAddr(maxAddr){ 

Since that is not declared static, if it were a C function the linker
would find it even if it were in a different library.  However, when I
try to link this base class which is used by a derived class in another
library I get the following error:

../NDPluginBase.cpp:454: undefined reference to
`asynParamBase::asynParamBase[not-in-charge](char const*, int, int)
'../../../lib/linux-x86//libADPlugin.a
(NDPluginBase.o)(.text+0xfff): In function
`NDPluginBase::NDPluginBase[in-charge](char const*, int, int, char
const*, int, int, int)':

However, if I force the linker to include the module that includes the
base class source code by adding the following lines to the beginning of
it:

/* This is for testing */
#include <epicsExport.h>
int asynParamBaseDebug=0;
epicsExportAddress(int, asynParamBaseDebug);

and then add this line to the .dbd file:

variable(asynParamBaseDebug,int)

then it links OK.

How do I get the linker to pull in the module with the base class
definition without having to force it with that global definition?

Andrew Johnson suggested putting the keyword epicsShareClass before the
class definition.  I tried that, but interestingly it links fine on
Windows with or without that keyword, but fails on Linux with or without
the keyword.

Thanks,
Mark

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System