gdb error: Couldn't find method (null)what

Question

 ?58 pinNodes.push_back(node); ?
 ?59 } ?
 ?60 nlb::application::Application::instance().interpreter().eval(QString("unset %1").arg(pinIterListVarName)); ?
 ?61 ?
 ?62 if (!pinNodes.empty()) { ?
 ?63 insert(0, pinNodes.begin(), pinNodes.end()); ?
 ?64 } ?
 ?65 } catch (const std::exception& e) { ?
B+>?66 SA_ASSERT(false, e.what()); ?
 ?67 } ?
 ?68 } 

This is from gdb. I have set breakpoint to failing assert. When I am trying to print the reason of failure using p e.what() gdb prints Couldn't find method (null)what

Printing e gives that is is incomplete.

(gdb) p e
4ドル = (const struct std::exception &) @0x58f1990: <incomplete type>

How I can print it?

If SA_ASSERT's condition is false it calls this

void printTrace(unsigned int startDepth, QString& buff)
{
 const size_t maxDepth = 200;
 size_t stackDepth;
 void *stackAddrs[maxDepth];
 char **stackStrings;
 stackDepth = backtrace(stackAddrs, maxDepth);
 stackStrings = backtrace_symbols(stackAddrs, stackDepth);
 buff += "Call stack\n";
 unsigned int line = 1;
 for (size_t i = startDepth; i < stackDepth; i++) {
 size_t sz = 500; // just a guess, template names will go much wider
 char *function = reinterpret_cast<char*>(malloc(sz));
 char *begin = 0, *end = 0;
 // find the parentheses and address offset surrounding the mangled name
 for (char *j = stackStrings[i]; *j != 0; ++j) {
 if (*j == '(') {
 begin = j;
 } else if (*j == '+') {
 end = j;
 }
 }
 if (begin && end) {
 begin++;
 *end = 0;
 // found our mangled name, now in [begin, end)
 int status;
 char *ret = abi::__cxa_demangle(begin, function, &sz, &status);
 if (ret) {
 // return value may be a realloc() of the input
 function = ret;
 } else {
 // demangling failed, just pretend it's a C function
 // with no args
 std::strncpy(function, begin, sz);
 std::strncat(function, "()", sz);
 function[sz-1] = ' ';
 }
 buff += QString::number(line++) + QString(") ") + function + "\n";
 } else {
 // didn't find the mangled name, just print the whole line
 buff += QString::number(line++) + QString(") ") + stackStrings[i] + "\n";
 }
 free(function);
 }
 free(stackStrings); // malloc()ed by backtrace_symbols
}

The gdb version

GNU gdb 6.8
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-unknown-linux-gnu".

• Djon: I think e is null because you do not enter the catch block?
• Djon: Nevermind, what does SA_ASSERT() do?
• Ashot: Before SA_ASSERT, why is e incomplete ?
• Djon: Well this just got way out of my reach, sorry.
• jplatte: I would assume that somewhere in the code, a reference to NULL / nullptr is formed and throwed, like throw *((const std::exception*) nullptr);. Try throwing nullptr this way and see if you get the same error.

Answer 1

An incomplete type means you are missing some debuginfo. Without it, gdb can't do much. For example it can't print an object of that type, because it doesn't know how the type is laid out.

Version 6.8 is quite old. You should upgrade if you can. There have been many, many bug fixes to C++ support since then.

The "(null)" looks like a gdb bug. This is what happens when you try to printf NULL (at least with glibc). However, it is probable that this bug has been fixed sometime in the last 5 years.

• Tom Tromey: I don't know why that would happen. A bug report would be nice, if you have a simple way to reproduce. If you don't fix the incomplete type problem, though, you shouldn't expect much success.