Playing Fair on the Stack

In any given function, you must keep stack usage to a minimum. There is no hard and fast rule, but you should keep the sum of all local (that is, automatic) variables in a particular function to a maximum of a couple hundred bytes. Performing a large static allocation on the stack, such as of a large array or structure, is dangerous. Otherwise, stack allocations are performed in the kernel just as in user-space. Stack overflows occur silently and will undoubtedly result in problems. Because the kernel does not make any effort to manage the stack, when the stack overflows, the excess data simply spills into whatever exists at the tail end of the stack. The first thing to eat it is the tHRead_info structure. (Recall from Chapter 3 that this structure is allocated at the end of each process's kernel stack.) Beyond the stack, any kernel data might lurk. At best, the machine will crash when the stack overflows. At worst, the overflow will silently corrupt data.

Therefore, it is wise to use a dynamic allocation scheme, such as one of those discussed earlier in this chapter for any large memory allocations.

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