memset(&mystruct, 0, sizeof mystruct) 和 mystruct = { 0 }; 一样吗? [英] Is memset(&mystruct, 0, sizeof mystruct) same as mystruct = { 0 };?

问题描述

我正在阅读数组/结构的默认初始化值，并有以下问题:

I'm reading about the initialized values by default of an array/struct and have this question:

是 memset(&mystruct, 0, sizeof mystruct) 和 mystruct = { 0 }; 一样吗?

如果不是，那有什么区别?

if it's not, what's difference?

推荐答案

是 memset(&mystruct, 0, sizeof mystruct) 与 mystruct = { 0 }; 相同?

is memset(&mystruct, 0, sizeof mystruct) same as mystruct = { 0 }; ?

没有

memset(&mystruct, 0, sizeof mystruct) ;

... 将告诉编译器调用我们期望将执行期间 mystruct 中的数据设置为零的函数.

... will tell the compiler to call a function that we expect will set during execution the data in mystruct to zero.

mystruct = { 0 };

... 将设置告诉编译器自己将数据设置为零，这意味着它将:

... will set tell the compiler set by itself the data to zero, which means it will:

• 如果可能，编译时将 mystruct 中的数据设置为零(例如对于静态变量，如 tristopia 和 Oli Charlesworth 在评论中评论)
• 或如果不是(例如自动变量)，以生成在变量初始化时将数据设置为零的汇编代码(这比调用函数来执行此操作要好).

• if possible, set the data in mystruct to zero at compilation (e.g. for static variables, as tristopia and Oli Charlesworth remarked in the comments)
• or if not (e.g. auto variables), to generate the assembly code that will set the data to zero when the variable is initialized (which is better than calling a function to do that).

请注意，也许编译器可以将 memset 优化为编译时指令(就像用第二个版本替换第一个版本)，但我不会依赖因为 memset 是来自运行时库的函数，而不是某种语言内在的(我不是编译器作者/语言律师，虽然).

Note that perhaps the compiler could optimize the memset into a compile-time instruction (like replacing the first version with the second version), but I wouldn't rely on that as memset is a function from the runtime library, not some language intrinsic (I'm not a compiler writer/language lawyer, though).

来自 C++，我自己的观点是，您在编译时可以做的越多，编译器在编译时知道的越多，甚至在执行开始之前就越好:它使编译器能够优化代码和/或生成警告/错误.

Coming from C++, my own viewpoint is that the more you can do at compilation and the more the compiler knows at compile time, before the execution even starts, the better: It enables the compiler to possibly optimize the code and/or generate warning/errors.

在当前情况下，使用 mystruct = { 0 }; 符号来初始化 struct 总是比使用 memset 更安全，因为它非常非常 使用 memset 很容易在 C 中写出错误的东西，而编译器不会抱怨.

In the current case, using the mystruct = { 0 }; notation to initialize a struct is always safer than using the memset because it is very very easy write the wrong thing in C with a memset without the compiler complaining.

以下示例表明代码很容易执行与看起来不同的操作:

The following examples show that it is easy for the code to do something different than it appears to do:

// only the 1st byte will be set to 0
memset(&mystruct, 0, sizeof(char)) ;          

// will probably overrun the data, possibly corrupting
// the data around it, and you hope, crashing the process.
memset(&mystruct, 0, sizeof(myLARGEstruct)) ; 

// will NOT set the data to 257. Instead it will truncate the
// integer and set each byte to 1
memset(&mystruct, 257, sizeof(mystruct)) ;    

// will set each byte to the value of sizeof(mystruct) modulo 256
memset(&mystruct, sizeof(mystruct), 0) ;      

// will work. Always.
mystruct = { 0 } ;

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