Alex Rivera | Logout

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

Asked 2011-01-07T20:15:56.170
44

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:

  1. From the point of view of C standard: Is this undefined behavior of not? (On any particular platform, I presume, this cannot be undefined behavior, as it just depends on the in-memory representation of floating-point numbers—that’s all.)

  2. From practical point of view: Is it OK on Intel platform? (Regardless of what the standard is saying.)

arrays c double standards memset
Edit
Report

1 Answer

37

The C99 standard Annex F says:

This annex specifies C language support for the IEC 60559 floating-point standard. The IEC 60559 floating-point standard is specifically Binary floating-point arithmetic for microprocessor systems, second edition (IEC 60559:1989), previously designated IEC 559:1989 and as IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE 754−1985). IEEE Standard for Radix-Independent Floating-Point Arithmetic (ANSI/IEEE 854−1987) generalizes the binary standard to remove dependencies on radix and word length. IEC 60559 generally refers to the floating-point standard, as in IEC 60559 operation, IEC 60559 format, etc. An implementation that defines __STDC_IEC_559__ shall conform to the specifications in this annex. Where a binding between the C language and IEC 60559 is indicated, the IEC 60559-specified behavior is adopted by reference, unless stated otherwise.

And, immediately after:

The C floating types match the IEC 60559 formats as follows:

  • The float type matches the IEC 60559 single format.
  • The double type matches the IEC 60559 double format.

Thus, since IEC 60559 is basically IEEE 754-1985, and since this specifies that 8 zero bytes mean 0.0 (as @David Heffernan said), it means that if you find __STDC_IEC_559__ defined, you can safely do a 0.0 initialization with memset.

answered 2011-01-07T20:33:11.443

Your Answer