Is accessing a global array outside its bound undefined behavior?

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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.

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