是memset的（安培; MYSTRUCT，0，sizeof的MYSTRUCT）一样MYSTRUCT = {0};？ [英] Is memset(&mystruct, 0, sizeof mystruct) same as mystruct = { 0 };?

问题描述

我被一个数组/结构的默认阅读有关初始化值，并有这样的疑问：

是 memset的（安培; MYSTRUCT，0，sizeof的MYSTRUCT）相同 MYSTRUCT = {0}; ？

如果不是，有什么区别呢？

解决方案

  

是memset的（安培; MYSTRUCT，0，sizeof的MYSTRUCT）一样MYSTRUCT = {0}; ？

没有。

  memset的（安培; MYSTRUCT，0，sizeof的MYSTRUCT）;
 

...会告诉编译器调用，我们执行过程中预计将设置函数在MYSTRUCT为零的数据。

  MYSTRUCT = {0};
 

...将设置告诉编译器通过自身的数据设定为零，这意味着它将：

• 如果可能的话，在编译（例如，用于静态变量，在MYSTRUCT数据设置为零的为 tristopia 和奥利奇查尔斯沃思在评论评论）


• 或如果没有的（如自动变量），生成装配code当变量初始化，将数据设置为零（这是比调用一个函数来做到更好）。

请注意，也许的编译器可以优化到memset的编译时指令（想与第二个版本替换第一个版本），但我不会依赖上为 memset的是运行时库中的函数，而不是一些语言内在的（我不是一个编译器作者/语言的律师，虽然）

这是C ++的到来，我自己的观点是，更可以在编译做，更多的编译器知道在编译的时候，执行甚至开始之前，更好的：它使编译器可能优化code和/或生成警告/错误。

在目前的情况下，使用 MYSTRUCT = {0}; 符号来初始化结构始终是安全的比使用memset的，因为它是非常的非常容易编写C说错话了 memset的没有编译器抱怨。

下面的例子表明，它很容易比它似乎做code做不同的事情：

  //只有1个字节将被设置为0
memset的（安培; MYSTRUCT，0，sizeof的（炭））;//可能会溢出数据，可能损坏
//它周围的数据，并且希望崩溃的过程。
memset的（安培; MYSTRUCT，0，sizeof的（myLARGEstruct））;//不会将数据257.相反，它会截断
//整数，并设置每个字节为1
memset的（安培; MYSTRUCT，257的sizeof（MYSTRUCT））;//将每个字节设置的sizeof（MYSTRUCT）模256的值
memset的（安培; MYSTRUCT，sizeof的（MYSTRUCT），0）;// 将工作。总是。
MYSTRUCT = {0};
 

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

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

if it's not, what's difference?

解决方案

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

No.

memset(&mystruct, 0, sizeof mystruct) ;

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

mystruct = { 0 };

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

• 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).

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).

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.

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.

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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