与sprintf(snprintf)重叠的内存 [英] Overlapping memory with sprintf(snprintf)

问题描述

如果我们有这个

char value_arr[8];    
// value_arr is set to some value
snprintf(value_arr, 8, "%d", *value_arr);

此行为是否已定义?

让我说说我出于某些不合理的原因

Let's say for some ungainly reason I have

char value_arr[8];
// value_arr is set to some value
int* value_i = reinterpret_cast<int*>(value_arr);

snprintf(value_arr, 8, "%d", *value_i); // the behaviour in question

是否可以保证，例如，如果 * value_i = 7，则 value_arr 将采用"7"的值.是否定义了此行为?这样，首先取消对 value_i 的引用，然后按值传递，然后对其进行格式化，然后将其存储到数组中.

Is there a guarantee that, for example, if *value_i = 7, then value_arr will take on the value of "7". Is this behavior defined? Such that value_i is first dereferenced, then passed by value, and then formatted, then stored into the array.

通常， * value_i 的值不会发生变化，但是将字符串存储到 value_arr 中会违反此规定.

Normally, the value of *value_i can be expected to not change, but storing the string into value_arr violates that.

当我测试它时，它似乎可以正常运行，但是我似乎在文档中找不到明确的答案.函数签名具有 ... ，据我所知与 va_list 有关，但是恐怕我对可变函数的工作不是很了解

It seems to function as expected when I test it, but I can't seem to find a definitive answer in the documentation. The function signature has ..., which to my knowledge has something to do with va_list, but I'm afraid I'm not very knowledgable on the workings of variadic functions.

int sprintf (char* str, const char* format, ... );

推荐答案

对于原始代码，评估表达式 * value_i 会违反严格的别名规则，从而导致未定义的行为.不允许将char数组别名为 int .

For the original code, evaluating the expression *value_i causes undefined behaviour by violating the strict aliasing rule. It is not permitted to alias a char array as int.

对于已编辑的代码， snprintf(value_arr，8，％d"，* value_arr); 很好，它将格式化数组中第一个字符的字符代码.在进入函数之前，对函数自变量的求值是有序的.(C ++ 17 intro.execution/11)

For the edited code, snprintf(value_arr, 8, "%d", *value_arr);, it is fine and will format the character code of the first character in the array. Evaluation of function arguments is sequenced-before entering the function. (C++17 intro.execution/11)

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