stack traces with any exception class

Define THROW_EXCEPTION(E) which takes an exception class instance,
and uses it to construct an instance of a class which is a subclass
of E and ExceptionMixin.  The original instance is discarded, and
the newly constructed sub-class is thrown.  Equivalent to
"throw E;".

Define THROW_EXCEPTION2(ETYPE,MSG) which takes an exception class
type, and argument.  Directly constructs a ExceptionMixin sub-class
with the given message argument.  Equivalent to
"throw ETYPE(MSG);".

Define PRINT_EXCEPTION2(E, FP) If E is a instance of a sub-class of
ExceptionMixin then write information to FP (FILE*).

Define SHOW_EXCEPTION(E) If E is a instance of a sub-class of
ExceptionMixin then return a std::string with information.

pvDataApp/misc/epicsException.cpp

@@ -0,0 +1,87 @@
+/**
+ * Copyright - See the COPYRIGHT that is included with this distribution.
+ * EPICS pvDataCPP is distributed subject to a Software License Agreement found
+ * in file LICENSE that is included with this distribution.
+ */
+
+#include "epicsException.h"
+
+#include <sstream>
+
+#include <cstdio>
+#include <cstring>
+
+namespace epics{namespace pvData{
+
+void
+ExceptionMixin::print(FILE *fp) const
+{
+    fprintf(fp, "On line %d of %s\n",m_line,m_file);
+
+#if defined(EXCEPT_USE_BACKTRACE)
+    if(m_depth>0) {
+        fflush(fp); // must flush before using raw handle
+        backtrace_symbols_fd(m_stack, m_depth, fileno(fp));
+        fprintf(fp, "To translate run 'addr2line -e execname 0xXXXXXXX ...'\n"
+                "  Note: Must be compiled with debug symbols\n");
+    }
+#endif
+}
+
+std::string
+ExceptionMixin::show() const
+{
+    std::ostringstream out;
+
+    out<<"On line "<<m_line<<" of "<<m_file<<"\n";
+
+#if defined(EXCEPT_USE_BACKTRACE)
+    if (m_depth>0) {
+
+        char **symbols=backtrace_symbols(m_stack, m_depth);
+
+        for(int i=0; i<m_depth; i++) {
+            out<<symbols[i]<<"\n";
+        }
+
+        free(symbols);
+    }
+
+#endif
+    return out.str();
+}
+
+
+const char*
+BaseException::what() const throw()
+{
+    try{
+        if (base_msg.size()==0) {
+            const char *base=std::logic_error::what();
+            std::string out, stack;
+
+            const ExceptionMixin *info=dynamic_cast<const ExceptionMixin*>(this);
+            if(info) {
+                stack=info->show();
+            }
+
+            out.reserve(strlen(base)+1+stack.size()+1);
+
+            out+=base;
+            out+="\n";
+            if(info) {
+                out+=stack;
+                out+="\n";
+            }
+
+            base_msg.swap(out);
+        }
+        return base_msg.c_str();
+    } catch(std::bad_alloc&) {
+        return "BaseException::what - Insufficient memory to construct message";
+    } catch(...) {
+        return "BaseException::what - Unknown error when constructing message";
+    }
+}
+
+}}