C++ 我可以在“字节数”设置为零的情况下调用 memcpy() 和 memmove() 吗？
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Can I call memcpy() and memmove() with "number of bytes" set to zero?
提问by sharptooth
Do I need to treat cases when I actully have nothing to move/copy with memmove()/memcpy()as edge cases
我是否需要治疗的情况下，当我actully什么都没有移动/复制用memmove()/memcpy()作为边缘情况
int numberOfBytes = ...
if( numberOfBytes != 0 ) {
memmove( dest, source, numberOfBytes );
}
or should I just call the function without checking
或者我应该只调用函数而不检查
int numberOfBytes = ...
memmove( dest, source, numberOfBytes );
Is the check in the former snippet necessary?
是否需要检查前一个片段？
回答by Mike Seymour
From the C99 standard (7.21.1/2):
来自 C99 标准 (7.21.1/2)：
Where an argument declared as
size_t nspecifies the length of the array for a function,ncan have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.
如果参数声明为
size_t n指定函数数组的长度，则n在调用该函数时可以具有零值。除非在本小节中对特定函数的描述中另有明确说明，否则此类调用的指针参数仍应具有有效值，如 7.1.4 中所述。在这样的调用中，定位字符的函数没有找到，比较两个字符序列的函数返回零，复制字符的函数复制零个字符。
So the answer is no; the check is not necessary (or yes; you can pass zero).
所以答案是否定的；不需要检查（或者是；您可以通过零）。
回答by Matteo Italia
As said by @You, the standard specifies that the memcpy and memmove should handle this case without problem; since they are usually implemented somehow like
正如@You 所说，标准规定 memcpy 和 memmove 应该毫无问题地处理这种情况；因为它们通常以某种方式实现
void *memcpy(void *_dst, const void *_src, size_t len)
{
unsigned char *dst = _dst;
const unsigned char *src = _src;
while(len-- > 0)
*dst++ = *src++;
return _dst;
}
you should not even have any performance penality other than the function call; if the compiler supports intrinsics/inlining for such functions, the additional check may even make the code a micro-little-bit slower, since the check is already done at the while.
除了函数调用之外，您甚至不应该有任何性能损失；如果编译器支持此类函数的内在/内联，则额外的检查甚至可能会使代码稍微慢一点，因为检查已经在 while 中完成了。
