C Arrays – Why Can’t C Arrays Have Zero Length?

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The C11 standard says the arrays, both sized and variable length "shall have a value greater than zero." What is the justification for not allowing a length of 0?

Especially for variable length arrays it makes perfect sense to have a size of zero every once and a while. It is also useful for static arrays when their size is from a macro or build configuration option.

Interestingly GCC (and clang) provide extensions that allow zero length arrays. Java also allows arrays of length zero.

Best Answer

The issue I would wager is that C arrays are just pointers to the beginning of an allocated chunk of memory. Having a 0 size would mean that you have a pointer to... nothing? You can't have nothing, so there would have had to be some arbitrary thing chosen. You can't use null, because then your 0 length arrays would look like null pointers. And at that point every different implementation is going to pick different arbitrary behaviors, leading to chaos.

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Why are zero-based arrays the norm

I don't think any of us can provide a stronger argument than Edsger W. Dijkstra's article "Why numbering should start at zero".

C Programming – Why C Arrays Do Not Track Their Length

C arrays do keep track of their length, as the array length is a static property:

int xs[42];  /* a 42-element array */

You can't usually query this length, but you don't need to because it's static anyway – just declare a macro XS_LENGTH for the length, and you're done.

The more important issue is that C arrays implicitly degrade into pointers, e.g. when passed to a function. This does make some sense, and allows for some nice low-level tricks, but it loses the information about the length of the array. So a better question would be why C was designed with this implicit degradation to pointers.

Another matter is that pointers need no storage except the memory address itself. C allows us to cast integers to pointers, pointers to other pointers, and to treat pointers as if they were arrays. While doing this, C is not insane enough to fabricate some array length into existence, but seems to trust in the Spiderman motto: with great power the programmer will hopefully fulfill the great responsibility of keeping track of lengths and overflows.

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Why are zero-based arrays the norm

C Programming – Why C Arrays Do Not Track Their Length

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