strict aliasing in C

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They both violate the strict aliasing rule, I am going to quote my answer here which says (emphasis mine going forward):

code violates the strict aliasing rules which makes it illegal to access an object through a pointer of a different type, although access through a char * is allowed. The compiler is allowed to assume that pointers of different types do not point to the same memory and optimize accordingly.

gcc is a little more detailed in the documetation of -Wstrict-aliasing=n here which says:

This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

and describes each level as follows:

  • Level 1: Most aggressive, quick, least accurate. Possibly useful when
    higher levels do not warn but -fstrict-aliasing still breaks the code,
    as it has very few false negatives. However, it has many false
    positives. Warns for all pointer conversions between possibly
    incompatible types, even if never dereferenced. Runs in the front end
    only.

  • Level 2: Aggressive, quick, not too precise. May still have many false
    positives (not as many as level 1 though), and few false negatives
    (but possibly more than level 1). Unlike level 1, it only warns when
    an address is taken. Warns about incomplete types. Runs in the front
    end only.

  • Level 3 (default for -Wstrict-aliasing): Should have very few false
    positives and few false negatives. Slightly slower than levels 1 or 2
    when optimization is enabled. Takes care of the common pun+dereference
    pattern in the front end: *(int*)&some_float.     If optimization is
    enabled, it also runs in the back end, where it deals with multiple
    statement cases using flow-sensitive points-to information. Only warns
    when the converted pointer is dereferenced. Does not warn about
    incomplete types.

So it is not guaranteed to catch all instances and different levels have different degrees of accuracy.

Typically the effect you are looking for can be accomplished using type punning through a union, which I cover in my linked answer above and gcc explicitly supports.

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