Difference between array type and array allocated with malloc

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There are several different pieces at play here.

The first is the difference between declaring an array as

int array[n];

and

int* array = malloc(n * sizeof(int));

In the first version, you are declaring an object with automatic storage duration. This means that the array lives only as long as the function that calls it exists. In the second version, you are getting memory with dynamic storage duration, which means that it will exist until it is explicitly deallocated with free.

The reason that the second version works here is an implementation detail of how C is usually compiled. Typically, C memory is split into several regions, including the stack (for function calls and local variables) and the heap (for malloced objects). The stack typically has a much smaller size than the heap; usually it’s something like 8MB. As a result, if you try to allocate a huge array with

int array[n];

Then you might exceed the stack’s storage space, causing the segfault. On the other hand, the heap usually has a huge size (say, as much space as is free on the system), and so mallocing a large object won’t cause an out-of-memory error.

In general, be careful with variable-length arrays in C. They can easily exceed stack size. Prefer malloc unless you know the size is small or that you really only do want the array for a short period of time.

Hope this helps!

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