Re: [Mingw-users] strange error

[Greg C. <chi...@mi...>]

Franco Bez wrote:
> 
> Am Sonntag,  6. Mai 2001 03:40 schrieb Greg Chicares:
> > Michael Kristofik wrote:
> > This program segfaults even if I comment out the delete line:
> 
> No wonder - you reserve only 2Byte of storage in a static array.

Yes. The program was designed to illustrate undefined behavior.

> Also You MUST NOT delete static arrays.
[snip]
> > int main()
> > {
> >   char a[2];
> >   a[1000] = 'x'; // <-- segfault
> > //  delete[] a;
> > }

There was a defect in the comment. Attempting to delete an auto
array was not the undefined behavior I wanted to show. Here's a
corrected version with no unintended defect:

// Demonstration that a program that manipulates char arrays
// created on the freestore can crash with a segfault, even
// when no such array is deleted. Warning--this program
// deliberately contains one intentional defect.
//
int main()
{
  char* a = new char[2];
  a[100000] = 'x'; // <-- segfault here (intentional defect)...
  delete[] a;      // <-- ...so removing this line doesn't help
}

Here's an error log produced by a proprietary compiler package:

Error 00001. 0x130610 (Thread 0xFFE5062F):
Access overrun: Attempt to access 1 byte(s) at 0x00B82FDC+100000, that is at
 offset 0+100000 in heap block 0x00B82FDC which is only 2 bytes long.
| crash.cpp line 4:
| {
|   char* a = new char[2];
|>  a[100000] = 'x'; // <-- segfault
|   delete[] a;
| }
Call Tree:
   0x0040112D(=CRASH.EXE:0x01:00012D) crash.cpp#4
   0x004385E9(=CW3230.DLL:0x01:0275E9)

The object array (0x00B82FDC) [size: 2 bytes] was created with new[]
| crash.cpp line 3:
| int main()
| {
|>  char* a = new char[2];
|   a[100000] = 'x'; // <-- segfault
|   delete[] a;
Call Tree:
   0x0040111F(=CRASH.EXE:0x01:00011F) crash.cpp#3
   0x004385E9(=CW3230.DLL:0x01:0275E9)

------------------------------------------
Error 00002. 0x400000 (r) (Thread 0xFFE5062F):
Exception 0xC0000005: Access violation at 0xB9B67C.
| crash.cpp line 4:
| {
|   char* a = new char[2];
|>  a[100000] = 'x'; // <-- segfault
|   delete[] a;
| }
Call Tree:
   0x0040112D(=CRASH.EXE:0x01:00012D) crash.cpp#4
   0x004385E9(=CW3230.DLL:0x01:0275E9)

I know of no tool that does all that for mingw, but the
original poster might use gdb to find his problem, which
I suspect is not in the tools. For the program above,
for instance, gdb gives:

(gdb) bt
Error: #0  0x401235 in main () at crash.cpp:4
#1  0x4011c1 in __mingw_CRTStartup ()
#2  0x4011eb in mainCRTStartup ()
#3  0xbff88e93 in _size_of_stack_reserve__ ()
#4  0xbff88d41 in _size_of_stack_reserve__ ()
#5  0xbff87759 in _size_of_stack_reserve__ ()
#6  0x6 in ?? ()
#7  0x650de100 in ?? ()
Cannot access memory at address 0x16007004