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Specifying dimension of array in function and outside function does not give same result

This compiles fine under C99

const int DIM;
int main() {
 int tab[DIM];
}

while the following gives the error

int tab[DIM];
int main() {
}

error: variably modified tab at file scope

Why?

I know how to fix it :

#define DIM 10

But this does not provide the answer to my question Thanks for your reply

Answer*

Sizes of variable length arrays are calculated at run-time. 

Read for example the following quote from the C 2024 Standard (6.5.4.5 The sizeof and alignof operators):

> The `sizeof` operator yields the size (in bytes) of its operand, which
> can be an expression or the parenthesized name of a type. The size is
> determined from the type of the operand. The result is an integer.
> **If the type of the operand is a variable length array type, the operand is evaluated;** otherwise, the operand is not evaluated and
> the result is an integer constant.

or (6.7.7.3 Array declarators):

> If an identifier is declared as having a variably modified type, it
> shall be an ordinary identifier (as defined in 6.2.3), have no
> linkage, and have either block scope or function prototype scope. **If
> an identifier is declared to be an object with static or thread
> storage duration, it shall not have a variable length array type.**

Pay attention to that variables declared at file scope have static storage duration.

By the way in the first program you shall initialize the variable `DIM` because it has the qualifier const and in the second program you missed its definition.:)

In C variables declared with the qualifier const are not constant expressions that are evaluated at compile time.

Thus the first progam is compiled successfully because the declared array has automatic storage duration. Memory for the array is allocated at run time.

The second program is not compiled because the declared at file scope variable length array has static storage duration.

If you will include the directive

 #define DIM 10

then the declared array will not be a variable length array because its size is specified with a constant expression that is evaluated at compile time.

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Do not cite text from the C standard drafts as text from the official standard. Both what was said to be the "final working draft," N3054, and some later draft, N3096, differ considerably from the published standard, including the insertion of addition subclauses, making the clause numbering different between the drafts and the official standard. After a committee does its work, there is some additional balloting process that may result in changes to a standard before publication. Also, you had text cited from the standard that did not appear in the drafts or the standard.

Eric Postpischil
– Eric Postpischil

2024年11月27日 21:46:07 +00:00
Commented Nov 27, 2024 at 21:46
@EricPostpischil I cited N3088. In any case nothing was changed relative to variable length arrays in the provided quites.

Vlad from Moscow
– Vlad from Moscow

2024年11月27日 21:50:48 +00:00
Commented Nov 27, 2024 at 21:50
You may have quoted from draft N3088, but you cited the standard. That is the wrong thing to do. When quoting material, you should state the actual source it came from. That means, if it is from a draft, then saying it is from a draft, and which number, or, if it is from the official standard, then saying it is from the standard. The clause numbers changed, one word changed, and you had additional words in the quote that were apparently your own, not in the draft or the standard.

Eric Postpischil
– Eric Postpischil

2024年11月27日 21:58:47 +00:00
Commented Nov 27, 2024 at 21:58
@EricPostpischil TMaybe I made a typo when combined the quotes and my text.

Vlad from Moscow
– Vlad from Moscow

2024年11月28日 05:06:21 +00:00
Commented Nov 28, 2024 at 5:06

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Specifying dimension of array in function and outside function does not give same result

Answer*