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threads, running finally blocks, o r running finalizers A s with any normal unhand led exception, a debugger will be given a first and second chance to debug the process Previously, in 1 0 and 1 1 , a St a c kOverflowException was generated, and could be caught The new behavior ensures that subtle problems caused by the inability of a component to react to stack overflow are not permitted to run rampant, which would otherwise possibly trigger silent data corruption CLR hosts such as SQL Server can override this policy, but when they do so they assume all of the responsibility for containing the possible damage Unmanaged code can catch a stack overflow exception using an SEH try / catch clause
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But the same caveats mentioned before still apply It is extremely difficult to determine when it is or isn't safe to proceed running any code in the process at all Because the decision is not enforced by a runtime, as is the case with managed code, native applications and libraries are all over the map when it comes to responding to stack overflow Some Win32 APls and COM compo nents actually catch stack overflow and try to continue running, for instance An overflow due to the first cause above (running out of reserved space) actually happens before the last reserved page is committed On X86 and X64 platforms, the two last pages, and on IA64, the last three pages, are never used for guard page usage Instead, they are reserved for executing necessary stack overflow exception handling should the guard ever reach them For most applications, this still isn't sufficient, however, which is why the CLR uses SetTh r e a d St a c kG u a r a n t e e as noted earlier The CLR goes a step further and doesn't have to worry about the second cause of stack overflow mentioned earlier Because the CLR pre-commits all managed thread stacks, stack overflow due to inability to back stacks in the pagefile is simply not possible These situations are effectively turned into
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OutOfMemo ry E x c e pt i o n s during thread creation This technique is not
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without flaws: namely, it puts quite a bit of pressure on the pagefile For instance, if you create 1 ,000 threads in a process, you will need 1 G B of pagefile space just for their stacks alone This doesn' t eat up physical memory until the pages are written to and faulted into RAM, but managed programs end up using more disk space than their native counterparts If a program decides to continue running after a stack overflow has occurred, it is imperative that the guard page is reset When a stack over flow has occurred, it means there is no longer a page in the stack region of memory with the PAG E_GUARD attribute on it Resetting the guard region can be done manually via the virtual memory Win32 functions (Le, Vi rt u a lA l l o c ) or the C RT's J e s e t s t koflw function If the stack overflow logic attempts to commit beyond the last page-or if a bug prevents the guard page from being restored and subsequent code overflows the stack again an access violation exception will occur This is done to prevent an error in stack overflow from overwriting arbitrary memory below the stack, which could result in security problems Due to exhaustion of all stack space, this access violation will probably not be handled gracefully Windows needs user-mode stack space to dispatch exceptions, so if the stack has grown to the point where an access violation happens, it may not be able to do so Windows detects this and responds by abruptly terminating the process No error dialog will be shown, no warning is issued, and the process just disappears
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Stack Probes and Reliability The CLR's policy of failing a process in response to stack overflow without running finally blocks or finalizers could lead to problems for some code If managed code was amidst a multistep update to some machine-wide persistent state (such as the registry) when an overflow tore down the process, it could lead to corruption In some cases, corruption is limited to a single process In others, it may affect the entire system, but will be cleared up with a reboot In yet other cases, the situation could be more severe In any case, the user of an end application is likely to be left dissatisfied with the experience, and so we'd like to ensure our software minimizes the probability and rate of such occurrences Instead of
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