长度为0的可变数组?

In C, an array normally isn't allowed to have size 0 (unless I use the one or other compiler-side extension).

在C中，通常不允许数组具有0大小(除非使用一个或其他编译器端扩展)。

OTOH, there are VLAs whose length might turn out to be 0.

OTOH，有VLAs其长度可能是0。

Are they allowed?

他们允许吗?

I am talking about the following code:

我说的是以下代码:

void send_stuff()
{
    char data[4 * !!flag1 + 2 * !!flag2];
    uint8_t cursor = 0;
    if (flag1) {
        // fill 4 bytes of data into &data[cursor]
        cursor += 4;
    }
    if (flag2) {
        // fill 2 bytes of data into &data[cursor]
        cursor += 2;
    }
}

The result is a data array with a length of 0, 2, 4 or 6, depending on the combination of the flags.

结果是一个长度为0、2、4或6的数据数组，具体取决于标记的组合。

The question is now: Is this valid code for the case the array turns out to have length 0?

现在的问题是:对于数组的长度为0的情况，这个有效代码是否有效?

1 个解决方案

#1

This is not valid, if we go to the draft C99 standard section 6.7.5.2 Array declarators paragraph 5 says (emphasis mine):

这是无效的，如果我们看C99标准章节6.7.5.2数组声明者第5段说(强调我的):

if the size is an expression that is not an integer constant expression: if it occurs in a declaration at function prototype scope, it is treated as if it were replaced by *; otherwise, each time it is evaluated it shall have a value greater than zero.[...]

如果size是一个非整数常量表达式的表达式:如果它出现在函数原型作用域中的声明中，则将其视为被*替换;否则，每次评估时，它的值都应该大于0。

In fact with clang enabling the sanitizer for undefined behavior using the -fsanitize=undefined flag can generate a run-time warning for this case see it live:

事实上，通过使用-fsanitize=未定义的标志，clang可以为未定义的行为启用消毒剂，可以为这个案例生成一个运行时警告:

runtime error: variable length array bound evaluates to non-positive value 0

运行时错误:变量长度数组绑定的值为0

#1