Is it legal to use memset(…, 0, …) on an array of doubles?

Question

Is it legal to zero the memory of an array of doubles (using memset(…, 0, …)) or struct containing doubles?

The question implies two different things:

Answer 1

David Heffernan has given a good answer for part (2) of your question. For part (1):

The C99 standard makes no guarantees about the representation of floating-point values in the general case. §6.2.6.1 says:

The representations of all types are unspecified except as stated in this subclause.

...and that subclause makes no further mention of floating point.

You said:

(on a fixed platform, how can this UB ... it just depends of floating representation that's all ...)

Indeed - there a difference between "undefined behaviour", "unspecified behaviour" and "implementation-defined behaviour":

• "undefined behaviour" means that anything could happen (including a runtime crash);
• "unspecificed behaviour" means that the compiler is free to implement something sensible in any way it likes, but there is no requirement for the implementation choice to be documented;
• "implementation-defined behaviour" means that the compiler is free to implement something sensible in any way it likes, and is supposed to document that choice (for example, see here for the implementation choices documented by the most recent release of GCC);

and so, as floating point representation is unspecified behaviour, it can vary in an undocumented manner from platform to platform (where "platform" here means "the combination of hardware and compiler" rather than just "hardware").

(I'm not sure how useful the guarantee that a double is represented such that all-bits-zero is +0.0 if __STDC_IEC_559__ is defined, as described in Matteo Italia's answer, actually is in practice. For example, GCC never defines this, even though is uses IEEE 754 / IEC 60559 on many hardware platforms.)