Checking array size in C/C++ to avoid segmentation faults

There is no way in C(++) to get the length of an array from a pointer to its first element. (There are platform-specific functions like _msize in MSVCRT, but that only works on malloced pointers.)

What's typically done when passing arrays to functions is to pass the length along with the pointer so that bounds-checking can be done at runtime.

void myFunc(int* yourArray, int length)
{
    if (length < 8)
    {
        puts("Sorry, yourArray is too small for this function. We need 8 ints of space.");
        return;
    }

    // ...
}

void caller()
{
    int arr[LEN];
    myFunc(arr, LEN);
}

Use a custom wrapper for malloc (or write your own) that keeps additional information about the blocks it allocates. The one I use adds a few "guard bytes" to every allocation, embeds the length of the allocation as the a[-1], and checks the guard bytes and other things upon deallocation.

The answer is really fairly simple: if you want safety, use something that actually provides it--and that's not C, and not raw C-style arrays.

Without departing too far from the basic style of C and raw arrays, you can use C++ and an std::vector with [i] replaced by .at(i), and get bounds checking.

Using std::vector instead makes most of the problems with arrays easy. You can check the current size of the vector with its .size() member function. Most of the time, you don't need to do that though, because when you want to add something to it you just use its .push_back() member function.

At least in theory, you can sort of do most of the same sorts of things in C, but doing so gets relatively ugly. Although it's not terribly difficult to define a wrapper that (for example) puts a pointer and a current allocation size into a struct, you have to define functions to do all the manipulation on it, and even then you have to live with the fact that existing code won't know how to use it or deal with it. I've done this a few times, and if you need it badly enough you can make it work--but I long ago decided it just wasn't worth the pain.

A function that receives a pointer does not know of the length of the corresponding array. You must pass in as a parameter yourself explicitly:

void myFunc(int *yourArray, size_t yourArrayLen)

Once you've done that, throwing an error is trivial.

Of course, this still leaves the possibility that your caller might give you the wrong length. You can't really prevent that without either:

• implementing a custom data type to store arrays and then making sure the length stays in-sync with the true length at all times using encapsulation, or

• allowing static arrays only, e.g.

  void myFunc(int (*yourArray)[8]);