您是对的：它是不确定的行为，你不能指望它总是产生 0

You were right: it is undefined behavior and you cannot count it always producing 0.

至于为什么要在这种情况下，看到零：现代操作系统中称为页相对粗粒度块是比单个变量（在x86至少4KB）大得多的内存分配给进程。当你有一个全局变量，它将位于某处的网页上。假设 A 的类型为 INT [] [] 和 INT 是你的系统上的四个字节， A [27] ​​[27] ​​将从年初位于大约500字节 A 。所以只要 A 靠近页的开头，访问 A [27] ​​[27] ​​将被备份由实际存储器和读取也不会引起页面错误/访问冲突。

As for why you are seeing zero in this case: modern operating systems allocate memory to processes in relatively coarse-grained chunks called pages that are much larger than individual variables (at least 4KB on x86). When you have a single global variable, it will be located somewhere on a page. Assuming a is of type int[][] and ints are four bytes on your system, a[27][27] will be located about 500 bytes from the beginning of a. So as long as a is near the beginning of the page, accessing a[27][27] will be backed by actual memory and reading it won't cause a page fault / access violation.

当然，你不能指望这个。如果，例如， A 是近4KB其他的全局变量pceded $ P $然后 A [27] ​​[27] ​​将不会被存储备份，当你试着去读一下你的进程将会崩溃。

Of course, you cannot count on this. If, for example, a is preceded by nearly 4KB of other global variables then a[27][27] will not be backed by memory and your process will crash when you try to read it.

即使过程中不崩溃，你不能获得价值数 0 。如果你有一个现代化的多用户操作系统，它什么都不做一个非常简单的程序，但分配这个变量并打印该值时，你可能会看到 0 。操作系统设置存储器的内容时​​，交给了内存的过程，这样从一个进程或用户敏感数据不会泄露到其他一些良性的值（通常为全0）。

Even if the process does not crash, you cannot count on getting the value 0. If you have a very simple program on a modern multi-user operating system that does nothing but allocate this variable and print that value, you probably will see 0. Operating systems set memory contents to some benign value (usually all zeros) when handing over memory to a process so that sensitive data from one process or user cannot leak to another.

不过，有一个任意的存储器你读为零没有一般保证。你可以在这里存储上没有分配初始化的平台上运行您的程序，你会看到任何值发生在从上次使用是在那里。

However, there is no general guarantee that arbitrary memory you read will be zero. You could run your program on a platform where memory isn't initialized on allocation, and you would see whatever value happened to be there from its last use.

此外，如果 A 后面是被初始化为非零值，然后访问足够多的其他全局变量 A [27] ​​[27] 会告诉你任何值恰好在那里。

Also, if a is followed by enough other global variables that are initialized to non-zero values then accessing a[27][27] would show you whatever value happens to be there.