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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:
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.)
From practical point of view: Is it OK on Intel platform? (Regardless of what the standard is saying.)
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It appears to be legal to memset an array of double s to 0 because 8 zeroes is defined to be floating point 0.0 .
memset allows you to fill individual bytes as memory and you are trying to set integer values (maybe 4 or more bytes.) Your approach will only work on the number 0 and -1 as these are both represented in binary as 00000000 or 11111111 . Show activity on this post. Because memset works on byte and set every byte to 1.
For one thing, sizeof(my_array) returns the total number of bytes in the data structure and not the number of elements.
Roughly speaking, the memset function is 15 times faster than std::fill in my test. This demonstrates that even if you are working with large volumes of data, you can be easily limited by your inefficient implementations. You are not automatically limited by your memory bandwidth.
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
floattype matches the IEC 60559 single format.- The
doubletype 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.
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If you are talking about IEEE754 then the standard defines +0.0 to double precision as 8 zero bytes. If you know that you are backed by IEEE754 floating point then this is well-defined.
As for Intel, I can't think of a compiler that doesn't use IEEE754 on Intel x86/x64.
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