*** glibc detected *** free(): invalid pointer: 0x0804b094 ***

lmvent · 02-03-2007, 05:08 PM

I get this error when running a function that calls delete on pointers. Everything compiles fine. I checked the addresses of the pointers and they're all different. This is in the dtor:

Code:

void Queue::~Queue()
{
	for(unsigned int i = 0; i < count; i++)
	{
		delete &array[head];
		head++;
		if(head == array.Length())
		{
			head = 0;
		}
	}
	count = 0;
}

If I have 2 things in the queue, I get this error when it calls delete on the second one but the 2 addresses are different. Anyone know how to fix this?

tuxdev · 02-03-2007, 06:35 PM

What are you trying to do? Some general pointers:
1. Learn to love the STL
2. Learn to love the RAII idiom
3. Learn to love gdb
4. Learn to love the C++ FAQ

lmvent · 02-03-2007, 06:42 PM

Well I'm not allowed to use STL in this class

and I tried gdb but it doesn't give me much information and I'm not sure how to interpret what it does give me. What's the RAII idiom? This is what I got in gdb:

Code:

GNU gdb Red Hat Linux (6.3.0.0-1.132.EL4rh)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu"...Using host libthread_db library "/lib64/tls/libthread_db.so.1".

(gdb) run
Starting program: /afs/engin.umich.edu/u/l/m/lmvent/eecs-281/proj1/a.out 
Reading symbols from shared object read from target memory...done.
Loaded system supplied DSO at 0xffffe000
*** glibc detected *** free(): invalid pointer: 0x0804b094 ***

Program received signal SIGABRT, Aborted.
0xffffe405 in __kernel_vsyscall ()
(gdb) backtrace
#0  0xffffe405 in __kernel_vsyscall ()
#1  0xf7dd57a5 in raise () from /lib/tls/libc.so.6
#2  0xf7dd7209 in abort () from /lib/tls/libc.so.6
#3  0xf7e0971a in __libc_message () from /lib/tls/libc.so.6
#4  0xf7e0ffbf in _int_free () from /lib/tls/libc.so.6
#5  0xf7e1033a in free () from /lib/tls/libc.so.6
#6  0xf7fa09f1 in operator delete () from /usr/lib/libstdc++.so.6
#7  0x08048c1c in Queue::Purge (this=0xffffd7a0) at queue.cpp:26
#8  0x08048cac in ~Queue (this=0xffffd7a0) at queue.cpp:38
#9  0x0804899d in main () at test.cpp:29

tuxdev · 02-03-2007, 09:06 PM

The most important info gdb can give you with a crash is where it is crashing.

Some more RAII links:
http://en.wikipedia.org/wiki/Resourc...Initialization
http://www.parashift.com/c++-faq-lit...html#faq-17.12

Essentially, it just means make the object clean up after itself in the destructor. And remember that objects are destroyed in the reverse order of construction (so member objects should not be cleaned up in its owner object, but automatically by the compiler.

graemef · 02-03-2007, 09:51 PM

My guess is that you are deleting an element that has already been deleted.

Is the memory deleted anywhere else?
To help yourself could re-initialise the element
[code]delete &array[head];
array[head] = 0;[code]
and check that array[head] is not null before you try and delete it.