Initializing a dynamically-sized Variable-Length Array (VLA) to 0 [duplicate]

Question

The following line of code, which creates a variable-length array on the stack:

char name[length] = {'\0'};

Generates the following compiler diagnostics:

error: variable-sized object may not be initialized
warning: excess elements in array initializer
warning: (near initialization for ‘name’)

What options are available to me for initializing VLAs? Am I forced to use a line such as:

memset(name, 0, sizeof(name));

Instead?

Trematode

Answer 1

Yes, you must write code for the initialisation of VLAs (which could be a memset() like you have described, or any other way that you care to).

It is simply a constraint in the C standard (§6.7.8):

3. The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.

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The above was correct as written in 2010. As of C23, VLAs can now be initialised with an empty initialiser { }, which as with other compound types will initialise each element and sub-element recursively to zero of the appropriate type.

Orlop

Why can't I do it in the same line? – Trematode

@Matt Joiner: It's just a constraint of the language - I've added the relevant text from the standard to my answer. – Orlop

I think it is to avoid edge cases like this: int arr[foo] = {1,2,3}; where foo could be 1 or 2. What happens then? :p And to make it simple {0} is disallowed too, but I'm not sure. – Twiggy

Can't argue with the standard. Not what I wanted to hear, but you can't expect bells & whistles from a 36 year old language. – Trematode