如何检查存储块中的所有字节是否都为零 [英] How to check whether all bytes in a memory block are zero

问题描述

我有一块内存，其中的元素大小固定，例如100字节，一个又一个地放入其中，所有元素都具有相同的固定长度，所以内存看起来像这样

I have a block of memory with elements of fixed size, say 100 bytes, put into it one after another, all with the same fixed length, so memory looks like this

<element1(100 bytes)><element2(100 bytes)><element3(100 bytes)>...

在某些情况下，我需要确定某个元素的所有字节是否都设置为0字节，因为这具有特殊含义(我并不是说这是一个好主意，但这就是我所处的情况.).

In some situations I need to determine whether all bytes of a certain element are set to the 0-byte because that has a special meaning (I didn't say it was a good idea, but that is the situation I am in).

问题是，我该如何有效地做到这一点.进一步:有一个简单的功能可以做到这一点.为了将字节设置为零，我可以使用memset或bzero，但是我不知道任何用于检查零的函数.

The question is, how do I do that efficiently. Further: is there a simple function to do it. For setting bytes to zero I can used memset or bzero, but I don't know of any function for checking for zero.

此刻我正在使用循环进行支票

At the moment I am using a loop for the check

char *elementStart = memoryBlock + elementNr*fixedElementSize;
bool special = true;
for ( size_t curByteNr=0; curByteNr<fixedElementSize; ++curByteNr )
{
  special &= (*(elementStart+curByteNr)) == 0;
}

当然，我可以使用更大的偏移量循环并使用mword或其他一些合适的更大类型一次检查几个字节.我想这会很有效率，但是我想知道是否有一个功能可以减轻我的负担.

Of course, I could loop with a bigger offset and check several bytes at once with a mword or some other suited bigger type. And I guess that would be rather efficient, but I would like to know whether there is a function to take that burden from me.

建议功能:

• ！memcmp(compareBlock，myBlock，fixedElementSize)

推荐答案

显而易见的可移植，高效的方法是:

The obvious portable, high efficiency method is:

char testblock [fixedElementSize];
memset (testblock, 0, sizeof testblock);

if (!memcmp (testblock, memoryBlock + elementNr*fixedElementSize, fixedElementSize)
   // block is all zero
else  // a byte is non-zero

在大多数实现中，库函数 memcmp()将使用大多数比较中可以使用的最大，最有效的单位大小.

The library function memcmp() in most implementations will use the largest, most efficient unit size it can for the majority of comparisons.

为了提高效率，请不要在运行时设置 testblock :

For more efficiency, don't set testblock at runtime:

static const char testblock [100];

根据定义，除非有初始化程序，否则静态变量会自动初始化为零.

By definition, static variables are automatically initialized to zero unless there is an initializer.

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