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Many compilers emit code to access things i n the TEB such a s the SEH exception chain directly via the FS register versus making one or more func tion calls and pointer dereferences There's another shortcut you can take Because the FS segmented regis ter has its base set to the TEB itself, you can access fields by specifying off sets The previous snippet works because, if you look at the _NT_T I B data structure above, the S e l f pointer is 24 (Le, axlS) bytes from the start, assuming a 32-bit architecture with 4 byte pointers We can use the same technique to access any of the fields If we want to directly access the stack base and limit, for instance, we can use F S : [ a4h ] for the base and F S : [ aSh ] for the limit
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void * pSt a c k B a s e ; v o i d * pSt a c k Limit ; { mov mov mov mov e a x , f s : [ 04h ] pSt a c k B a s e , eax ea x , fs : [ 0Sh ] pSt a c k L imit , ea x
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} p r i n tf ( " Ba s e %p , L i m i t %p \ r \ n " , pSt a c k B a s e , pSt a c k L imit ) ;
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Unfortunately, the _a s m keyword is not supported on all architectures and isn' t available in managed code, so the above code is only guaran teed to work on X86 VC+ + Furthermore, the hard-coded offsets a4h and aSh are clearly wrong on 64-bit architectures: you need more than 4 bytes to represent a 64-bit pointer Nt C u r r e n t T e b provides access to the TEB without requiring programs to hard-code all of this architecture specific information
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Example Usage: Checking Available Stack Space In some rare cases, it might be useful to query for the remaining stack space on your thread and change behavior based on it As one example, it could enable you to fail gracefully rather than causing a stack overflow A UI that needs to render some very deep XML tree and does so using stack recursion could limit its recursion or show an error message based on this information, as yet another example If the UI program finds that it has insufficient stack space,
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it may decide that it needs to spawn a new thread with a larger stack to perform the rendering Or it may log an error message when testing so that the developers can fine tune the stack size or depend less heavily on stack allocations or so the program can show a dialog box and fail The TEB's St a c k B a s e and St a c k L i mit fields can be used to determine the active stack range The St a c k L i m i t is only updated as you touch pages on the stack and, thus, it's not a reliable way to find out how much uncom mitted stack is left There's an undocumented field, De a l lo c a t i o n St a c k, at exeEec bytes from the beginning of the TEB that will give you this infor mation, but that's undocumented, subject to change in the future, and is too brittle to be reliable The R u n t imeHe l p e r s Probe F o rSuff i c i e n t St a c k function reviewed ear lier may appear promising, but it won't work for this purpose It probes for a fixed number of bytes (48KB on X86 / X64), and, if it finds there isn' t enough, it induces the normal CLR stack overflow behavior That will tear your process down, which is not what we want The same is true of the function shown earlier that uses sta c k a l l o c The good news i s that the V i rt u a lQu e ry Win32 function will provide this information It returns a structure, one field of which is the A l l o c a t i o n B a s e for the original allocation request When Windows reserves a thread's stack, it does so as one contiguous piece of memory The memory manager remembers the base address supplied at creation time, and this is the "end" of the stack; that is, it's the same as the De a l l o c a t i o n St a c k from the TEB If we're in managed code, all we need to do is use P / Invoke to access this information Let's create a new version of the C h e c k F o r S u ff i c i e n t St a c k function using this API Unlike the one earlier, which triggers a stack overflow if there isn't enough stack space, our new function takes a number of bytes as an argument and returns a bool to indicate whether there is enough stack to satisfy the request, enabling the caller to react accordingly
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p u b l i c u n safe stat i c bool Chec k F o rSuffi c ientSt a c k ( long bytes ) { MEMORY_BAS IC_I NFORMATION s t a c k I nfo
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