CODEWITHSUNDEEP
carrays
forty_tw0
forty_tw0

Reputation: 35

"Indexed" array function argument in C

I recently saw a function written in the following form void func(int size, int a[size]){ ... } and I was curious about the argument a[size]. At first I thought maybe it had something to do with indexing the array, but running the code a was in fact an array. Then I thought maybe it had to do with the size of the array, so I tried passing in different value and seeing if it would affect the array length, but it seemed not to (unless maybe it had something to do with writing past the array, but this feels unlikely). So my question is essentially, what does the "index" do in the function arguments?

Upvotes: 2

Views: 779

Answers (1)

Barmar
Barmar

Reputation: 782130

It's the array size, but it's ignored when it's in a function parameter declaration. A function parameter of the form int a[size] or int a[] is treated identically to int *a, because arrays decay to pointers when used as function arguments.

So it's essentially just a form of self-documentation. The caller is supposed to pass an array that contains size elements.

Note that this only applies to the first dimension in a multidimensional array parameter. If you have something like

void func(int width, int height, int a[height][width]) { ... }

The width part of the array declaration is used, but the height is ignored. a is a pointer to an array of int[width] rows; the width is needed when indexing the first dimension.

Upvotes: 4

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