Char and strcpy in C

Question

I came across a part of question in which, I am getting an output, but I need a explanation why it is true and does work?

char arr[4]; 
strcpy(arr,\"This is a li         


        

Answer 1

It may work. It may not. It may fail immediately and obviously. It may fail at some arbitrary future time and in subtle ways that will drive you insane.

That is the often-insidious nature of undefined behaviour. Don't do it.

If it works at all, it's totally by accident and in no way guaranteed. It's possible that you're overwriting stuff on the stack or in other memory (depending on the implementation and how/where the actual variable str is defined^(a)) but that the memory being overwritten is not used after that point (given the simple nature of the code).

That possibility of it working accidentally in no way makes it a good idea.

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For the language lawyers among us, section J.2 (instances of undefined behaviour) of C11 clearly states:

An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]).

That informative section references 6.5.6, which is normative, and which states when discussing pointer/integer addition (of which a[b] is an example):

If both the pointer operand and the result point to elements of the same array object, or one past the last element of the array object, the evaluation shall not produce an overflow; otherwise, the behavior is undefined. If the result points one past the last element of the array object, it shall not be used as the operand of a unary * operator that is evaluated.

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^(a) For example, on my system, declaring the variable inside main causes the program to crash because the buffer overflow trashes the return address on the stack.

However, if I put the declaration at file level (outside of main), it seems to run just fine, printing the message then exiting the program.

But I assure you that's only because the memory you've trashed is not important for the continuation of the program in this case. It will almost certainly be important in anything more substantial than this example.