Declaring an array with the size of a global

Question

may i declare an array with a global element in C?
Can i declare with a const type? It runs on Xcode, however I fear it isn't correct, because glob is not a const type (same thought on static type).

#include <stdio.h>
#include <stdilib.h>

int glob;
void main (){
    int const con;
    static int stat;
    int arr[glob];
    int arr2[con];
    int arr3[stat];
}

In addition, I'm in need of practicing finding mistakes in C code and correcting them for a test (CS student) and could not find a resource for it.

thank you in advance.

`int const con;` is useless, as the variable(!) is not initialised. C does not have symbolic constants like C++ or Pascal. They are all different languages. And writing `const` after the type is an obsolescence feature; write `const int i = ...;` instead. — too honest for this site, Feb 22 '16 at 22:16
Just to make clear: if you expect the array to resize when the variables change, you are wrong. — too honest for this site, Feb 22 '16 at 22:32
I've changed my mind. This is the real duplicate: http://stackoverflow.com/q/18848537/3745896 — River, Feb 22 '16 at 22:38
`int glob; ...int arr[glob]` is invalid [ref](http://stackoverflow.com/q/17559407/2410359) as `glob` is 0. — chux - Reinstate Monica, Feb 22 '16 at 22:41

BingsF · Answer 1 · 2016-02-22T22:29:29.830

0

Globals and statics are automatically initialized to 0 if an initializer isn't provided. The variable con local to main however isn't, so its value is undefined.

edited Feb 22 '16 at 22:29

answered Feb 22 '16 at 22:05

BingsF

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What variable has undefined value in this example? – River Feb 22 '16 at 22:10
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there is another problem here. You can't declare arrays of size zero. But this is not the question. – Eugene Sh. Feb 22 '16 at 22:13
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@River - `int const con;`, does it not? – BingsF Feb 22 '16 at 22:13
@EugeneSh. by the C standard this is true, however both GCC and Clang provide extensions allowing zero-length arrays. – BingsF Feb 22 '16 at 22:15
@BingsF GCC just cursed me yesterday for that. So it's fresh on me. – Eugene Sh. Feb 22 '16 at 22:19
@BingsF: The question is about C, not some extensions. Note these extensions were to allow some kind of flexible array member in `struct`s and have been superseded by a standard construct since C99. Zero-length VLAs are allowed, though. – too honest for this site Feb 22 '16 at 22:19
@BingsF I was just clarifying, since you didn't use the actual variable names. – River Feb 22 '16 at 22:24
@River - I see, thanks :) – BingsF Feb 22 '16 at 22:25
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Actually `con` is uninitialised, resulting in UB. Even worse, it cannot get a value assigned. `stat` is **not**. – too honest for this site Feb 22 '16 at 22:26
@Olaf are not `static` variables auto-initialized to `0`? – River Feb 22 '16 at 22:28
@River Yes they are. – Michi Feb 22 '16 at 22:28
Why should be [This](http://pastebin.com/raw/kGm1N0zv) not legal? – Michi Feb 22 '16 at 22:29
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@River - Oh jeez, brain fart. Fixed. – BingsF Feb 22 '16 at 22:29
Why did he got `-2` Down votes? – Michi Feb 22 '16 at 22:30
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@Michi: Because he fell into the same trap as I did? And because he did not really answer the actual question. – too honest for this site Feb 22 '16 at 22:31
@Michi - Because you're trying to put 1 thing in a container that holds 0 things. – BingsF Feb 22 '16 at 22:32
@Olaf It could be, but from my side, this Answer , I don't see whats wrong with it :| – Michi Feb 22 '16 at 22:32
@BingsF Please exlplain me[What's wrong with this](http://pastebin.com/raw/kGm1N0zv) ? – Michi Feb 22 '16 at 22:33
@Michi: You access an array outside its boundaries. And array of length `0` has **zero** elements. – too honest for this site Feb 22 '16 at 22:35
@Olaf Strange that `GCC-5.3` + `Valgrind` [doesn't seems to complain](http://pastebin.com/raw/Dry0RJgu) about this. – Michi Feb 22 '16 at 22:38
Note: VLA length must be > 0 http://stackoverflow.com/q/17559407/2410359 – chux - Reinstate Monica Feb 22 '16 at 22:38
@chux: Hmm, I was looking for that, but apparently searched the wrong places. On second thought it sounds logical and (surprisingly:-) consistent. – too honest for this site Feb 22 '16 at 22:48

score 0 · Answer 2 · edited May 23 '17 at 12:15

may i declare an array with a global element in C?

With C99 and optionally in C11, variable length arrays (VLA) are supported.
Array length must be more than 0.

int glob;
void main (){
  int arr[glob];     // bad array length 0
  int arrX[glob+1];  // OK array length > 0

Can i declare with a const type?

With VLA, yes, it is irrelevant if the type of the array size is const or not.

// Assume VLAs are allowed
int a[5];         // OK 5 is a constant
const int b = 4;
int c[b];         // OK
int d = 7;
int e[d];         // OK

With non-VLA, the array size must be a constant. A variable, const or not, is no good.

// Assume VLAs not allowed
int a[5];         // OK 5 is a constant
const int b = 4;
int c[b];         // No good, b is not a constant.

In addition, I'm in need of practicing finding mistakes in C code and correcting them for a test

Enable all warnings of your compiler is step 1.

Declaring an array with the size of a global

2 Answers2