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### 6.53 Stack scrubbing internal interfaces

Stack scrubbing involves cooperation between a `strub` context, i.e., a function whose stack frame is to be zeroed-out, and its callers. The caller initializes a stack watermark, the `strub` context updates the watermark according to its stack use, and the caller zeroes it out once it regains control, whether by the callee’s returning or by an exception.

Each of these steps is performed by a different builtin function call. Calls to these builtins are introduced automatically, in response to `strub` attributes and command-line options; they are not expected to be explicitly called by source code.

The functions that implement the builtins are available in libgcc but, depending on optimization levels, they are expanded internally, adjusted to account for inlining, and sometimes combined/deferred (e.g. passing the caller-supplied watermark on to callees, refraining from erasing stack areas that the caller will) to enable tail calls and to optimize for code size.

Built-in Function: void __builtin___strub_enter (void **wmptr)

  This function initializes a stack watermark variable with the current top of the stack. A call to this builtin function is introduced before entering a `strub` context. It remains as a function call if optimization is not enabled.

Built-in Function: void __builtin___strub_update (void **wmptr)

  This function updates a stack watermark variable with the current top of the stack, if it tops the previous watermark. A call to this builtin function is inserted within `strub` contexts, whenever additional stack space may have been used. It remains as a function call at optimization levels lower than 2.

Built-in Function: void __builtin___strub_leave (void **wmptr)

  This function overwrites the memory area between the current top of the stack, and the watermarked address. A call to this builtin function is inserted after leaving a `strub` context. It remains as a function call at optimization levels lower than 3, and it is guarded by a condition at level 2.

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