Question

I have a piece of code looking like this :

TAxis *axis = 0; if (dynamic_cast<MonitorObjectH1C*>(obj))    axis = (dynamic_cast<MonitorObjectH1C*>(obj))->GetXaxis(); 

Sometimes it crashes :

Thread 1 (Thread -1208658240 (LWP 11400)): #0  0x0019e7a2 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2 #1  0x048c67fb in __waitpid_nocancel () from /lib/tls/libc.so.6 #2  0x04870649 in do_system () from /lib/tls/libc.so.6 #3  0x048709c1 in system () from /lib/tls/libc.so.6 #4  0x001848bd in system () from /lib/tls/libpthread.so.0 #5  0x0117a5bb in TUnixSystem::Exec () from /opt/root/lib/libCore.so.5.21 #6  0x01180045 in TUnixSystem::StackTrace () from /opt/root/lib/libCore.so.5.21 #7  0x0117cc8a in TUnixSystem::DispatchSignals ()    from /opt/root/lib/libCore.so.5.21 #8  0x0117cd18 in SigHandler () from /opt/root/lib/libCore.so.5.21 #9  0x0117bf5d in sighandler () from /opt/root/lib/libCore.so.5.21 #10 <signal handler called> #11 0x0533ddf4 in __dynamic_cast () from /usr/lib/libstdc++.so.6 

I have no clue why it crashes. obj is not null (and if it was it would not be a problem, would it ?).

What could be the reason for a dynamic cast to crash ?

If it can’t cast, it should just return NULL no ?

Answer 1

Some possible reasons for the crash:

• obj points to an object with a non-polymorphic type (a class or struct with no virtual methods, or a fundamental type).
• obj points to an object that has been freed.
• obj points to unmapped memory, or memory that has been mapped in such a way as to generate an exception when accessed (such as a guard page or inaccessible page).
• obj points to an object with a polymorphic type, but that type was defined in an external library that was compiled with RTTI disabled.

Not all of these problems necessarily cause a crash in all situations.