Is dereferencing null pointer valid in sizeof operation [duplicate]

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Why does this work?

This works because sizeof is a compile time construct, with the exception of variable length arrays is not evaluated at all. If we look at the C99 draft standard section 6.5.3.4 The sizeof operator paragraph 2 says(emphasis mine):

[…] The size is determined from the type of the operand. The result is an integer. If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.

we also see the following example in paragraph 5 which confirms this:

double *dp = alloc(sizeof *dp);
       ^^^                ^
                          |                                 
                          This is not the use of uninitialized pointer

At compile time the type of the expression with be determined in order to compute the result. We can further demonstrate this with the following example:

int x = 0 ;
printf("%zu\n", sizeof( x++ ));

which won’t increment x, which is pretty neat.

Update

As I note in my answer to Why does sizeof(x++) not increment x? there is an exception to sizeof being a compile time operation and that is when it’s operand is a variable length array(VLA). Although I did not previously point it out the quote from 6.5.3.4 above does say this.

Although in C11 as opposed to C99 it is unspecified whether sizeof is evaluated or not in this case.

Also, note there is a C++ version of this quesiton: Does not evaluating the expression to which sizeof is applied make it legal to dereference a null or invalid pointer inside sizeof in C++?.

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