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Contracts: document use of __CPROVER_loop_entry with arrays #8470

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Sep 27, 2024
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Contracts: document use of __CPROVER_loop_entry with arrays #8470

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7 changes: 7 additions & 0 deletions src/goto-instrument/contracts/doc/user/contracts-history-variables.md
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Expand Up @@ -53,6 +53,13 @@ __CPROVER_loop_entry(*identifier*)

### Semantics
`__CPROVER_loop_entry` takes a snapshot of the variable value right before the **first iteration** of the loop.
Caveat: to create a snapshot of an array, cast the array variable (which is a
pointer per C's type system) to a pointer-to-array, and then dereference.
```c
typedef int array_type[2];
array_type var;
__CPROVER_loop_invariant(__CPROVER_loop_entry(*(array_type*)var)[0] <= 42)
```

### Example
In this example the loop invariant asserts that `x <= 200` is upheld before and after every iteration of the `while` loop:
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