Why can I change a local const variable through pointer casts but not a global one in C?

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It’s a constant and you are using some tricks to change it anyway, so undefined behavior results. The global constant is probably in read-only memory and therefore cannot be modified. When you try to do that you get a runtime error.

The constant local variable is created on the stack, which can be modified. So you get away with changing the constant in this case, but it might still lead to strange things. For example the compiler could have used the value of the constant in various places instead of the constant itself, so that “changing the constant” doesn’t show any effect in these places.

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