Question

I’m developing a program in C that uses an array of linked lists (a primitive hash table) as a data type to represent certain date information. The array has twelve elements corresponding to the months of the year, and each month has a linked list that contains nodes of data.

I developed the module that used this data type and it worked fine. I later discovered that I was accessing array elements that were out of bounds (for example accessing the 12th element by the index 12 instead of 11). But the program worked consistently without incident. I never received a segmentation fault. I have since corrected the coding error. Could anybody explain why accessing out-of-bounds elements would not result in a segfault?

This is not the first time it has happened. I have created a dynamically allocated multidimensional array, and for the sake of testing, I tried to access out-of-bounds elements. The program ran fine, produced accurate results, and did not seg fault in most circumstances. The only time I achieved one, I had to try accessing substantially out-of-bounds elements.

(These programs currently are windows console applications for testing. I am compiling with MinGW. I can include code, if it would be helpful.)

Answer 1

In C, accessing an array outside its bounds is undefined behavior.

That means that anything can happen, including the program behaving as you might expect it to.

The C language doesn’t require bounds checking on array accesses, and most C compilers don’t implement it.

For example, suppose you declare:

int before;
int array[10];
int after;

The order in which these are stored in memory is undefined, but suppose they’re stored contiguously in the order they’re declared. If you attempt to access array[-1], you might access before instead. If you attempt to access array[10], you might access after instead.

The burden is on the programmer to avoid accessing arrays outside their bounds.
Or there might not be anything allocated before and/or after your array.

An analogy: “The sign says I’m only allowed to cross the street when the light is green. I crossed on red, and nothing happened. Why didn’t a car hit me?” (There are languages that go out of their way to make the car hit you. C isn’t one of them.)