memcmp byte comparisons

memcmp byte comparisons

The memcmp function does a byte-wise comparison of a memory block. Its return value is like strcmp, returning 0 for equality, and a negative or positive value otherwise. However, note that memcmp is not like strcmp, and will not stop when it finds a zero byte.

memcmp return value. A pitfall with memcmp is that you cannot assume that it returns 1 or -1, but must compare the return result to zero (like the strcmp function).

    if (memcmp(&a, &b, sizeof(a)) == 1)  // Bug
    if (memcmp(&a, &b, sizeof(a)) > 0)   // Correct

memcmp object equality testing. Looking at the memcmp function, you might think of it as an opportunity to do a fast equality/inequality test on large objects by simply doing a byte-wise test. You would not be the first to think that.

Unfortunately, there are several obscure pitfalls with this approach. There are multiple ways in C++ that the data in two memory blocks might be identical, but the bytes different:

• Padding bytes (alignment)
• Two types of floating-point zero
• Multiple types of floating-point NaN
• Bitfield data members (unset padding bits)

You can only use memcmp for memory block comparisons (e.g. tensor equality) if you're sure that these situations cannot occur, such as a contiguous array of primitive data types. Not recommended otherwise.

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