null pointer dereference

I was looking at the container_of macro, and this piece of code confuses me a bit:

Code:

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const typeof( ((type *)0)->member ) *__mptr = (ptr);

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Why is it legal to cast 0 (null pointer?) into some (type *) and then deference it to get the structure's member field? Shouldn't it cause some kind of segmentation violation?

Quote:

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Originally Posted by qwertylurker

I was looking at the container_of macro, and this piece of code confuses me a bit:

Code:

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const typeof( ((type *)0)->member ) *__mptr = (ptr);

---

Why is it legal to cast 0 (null pointer?) into some (type *) and then deference it to get the structure's member field? Shouldn't it cause some kind of segmentation violation?

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No actual dereferencing occurs. The -> operator is only being used in this case to name a member of 'type' for the typeof() operator to operate on.

Yikes. That is as ugly as old Yog.

Assuming `typeof' is the GCC variation, the line is never compiled, executed, or even "seen" by the generation side of the compiler.

Soma

typeof is gcc extension. I think it's just like sizeof(*p) . Where p is NULL. Actual deferencing is never made.