我在哪里可以买到有效的SSL证书？(Where could I buy a valid SSL certificate? [closed])

 你是对的：它是未定义的行为，你不能算它总是生成0 。 
 
 至于为什么在这种情况下您看到零：现代操作系统将内存分配给相对粗糙的块（称为页面）的进程，这些块大于单个变量（x86上至少为4KB）。 当您有一个单一的全局变量时，它将位于页面上的某个位置。 假设a类型为int[][] ， int s是系统上的四个字节， a[27][27]将从a开始位于大约500个字节。 所以只要a接近页面的开头，访问a[27][27]将被实际的内存支持，读取它不会导致页面错误/访问冲突。 
 
 当然，你不能指望这一点。 例如，如果a之前有其他全局变量的近4KB，则a[27][27]不会被内存支持，并且当您尝试读取它时，您的进程将崩溃。 
 
 即使进程没有崩溃，也不能指望获取值0 。 如果您在现代多用户操作系统上有一个非常简单的程序，除了分配此变量并打印该值之外，您可能会看到0 。 操作系统将存储器内容设置为一些良性值（通常为全零），将内存移交给进程，以便一个进程或用户的敏感数据不能泄漏到另一个进程。 
 
 但是，没有一般的保证，您读取的任意内存将为零。 你可以在内存没有初始化分配的平台上运行你的程序，你会看到最后一次使用中发生的任何值。 
 
 此外，如果a后面有足够的其他全局变量被初始化为非零值，那么访问a[27][27]会显示出任何值发生在那里。 

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