为什么printf中的格式被标记为restrict? [英] Why is the format in printf marked as restrict?

问题描述

刚好看到了的原型printf(和其他 fprintf 类函数) -

I just happened to look at the prototype of the printf (and other fprintf class of functions) -

int printf(const char * restrict format, ...);

关键字restrict(如果我理解正确的话)不允许通过两个指针访问同一个对象，如果其中一个被标记为restrict.

The keyword restrict if I understand correctly disallows access to the same object through two pointers if one of them is marked restrict.

引用 C 标准中相同内容的示例是 此处.

An example that cites the same from the C standard is here.

我认为将格式标记为 restrict 的一个好处是可以避免在执行过程中格式字符串被修改的可能性(比如因为 %n 格式说明符).

One benefit of marking the format as restrict I think is saving the function from the chance that the format string might get modified during the execution (say because of the %n format specifier).

但这会带来更大的限制吗?这是否会使以下函数调用无效?

But does this impose a bigger constraint? Does this make the following function call invalid?

char format[] = "%s";
printf(format, format);

因为这里显然存在别名.为什么要在 printf 的 format 参数中添加 restrict 关键字?

Because there is clearly an aliasing here. Why was the restrict keyword added to the format argument of printf?

推荐答案

cppreference

在声明了受限指针 P 的块的每次执行期间(通常是每次执行一个函数体，其中 P 是一个函数参数)，如果某个可通过P(直接或间接)访问的对象被修改，以任何方式，则对该对象的所有访问strong>(读取和写入)在该块中必须通过 P(直接或间接)发生，否则行为未定义.

cppreference

During each execution of a block in which a restricted pointer P is declared (typically each execution of a function body in which P is a function parameter), if some object that is accessible through P (directly or indirectly) is modified, by any means, then all accesses to that object (both reads and writes) in that block must occur through P (directly or indirectly), otherwise the behavior is undefined.

^{(强调我的)}

这意味着:

char format[] = "%s";
printf(format, format);

定义良好，因为printf不会尝试修改format.

Is well-defined because printf won't attempt to modify format.

restrict 唯一使未定义的事情是在 printf 运行时使用 %…n 写入格式字符串"(例如 char f[] = "%hhn"; printf(f, (signed char *)f);).

The only thing that restrict makes undefined is 'writing to the format string using %…n while printf is running' (e.g. char f[] = "%hhn"; printf(f, (signed char *)f);).

为什么要在 printf 的格式参数中添加 restrict 关键字?

Why was the restrict keyword added to the format argument of printf?

restrict 本质上是编译器可能用来更好地优化代码的提示.

restrict is essentially a hint the compiler might use to optimize your code better.

因为 restrict 可能会也可能不会使代码运行得更快，但它永远不会使它变慢(假设编译器是健全的)，所以应该总是使用它，除非:

Since restrict may or may not make code run faster, but it can never make it slower (assuming the compiler is sane), it should be used always, unless:

• 使用它会导致 UB
• 在这种特定情况下没有显着的性能提升

这篇关于为什么printf中的格式被标记为restrict?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！