arrayscdoublestandardsmemset

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

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.)

Solution

  • 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.