Thread: Best way to declare an array size

Hello everyone,

What is the best way to give a size to an array ?

Either :

Code:

#include <stdio.h>

int main(int argc, char **argv)
{
const int size = 4;
char name[size];
return 0;
}

Or :

Code:

#include <stdio.h>
#define SIZE 4

int main(int argc, char **argv)
{
char name[SIZE];
return 0;
}

Thanks stahta01,

In my case I use the GCC compiler.

GCC is a good compiler to use, at least in my opinion. It is important to realize though that GCC is a collection of compilers, which are built to be versatile, and be very forgiving with code. In order to properly use it you have to be careful to run the correct program with the correct options. I will say that the reason that this is a problem for C is because it was only briefly made into a feature in C99. The compiler would interpret something like name[size] as a variable length array. Then when C11 came out this feature was made optional again for compilers to implement.

So it is best not to rely on VLAs if the code needs to be portable. At best, VLAs simplify what is offered by the malloc/realloc/free family of functions and do so in a way that is not as flexible. Once a VLA is declared, it will stay that length forever, whereas with realloc you can later adjust the size.

Here is an example of the code being compiled in various ways.

Code:

C:\Users\jk\Desktop>gcc --version
gcc (tdm64-1) 5.1.0
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

C:\Users\jk\Desktop>more foo.c
#include <stdio.h>

int main(int argc, char **argv)
{
    const int size = 4;
    char name[size];
    return 0;
}

C:\Users\jk\Desktop>g++ -c foo.c

C:\Users\jk\Desktop>del foo.o

C:\Users\jk\Desktop>gcc -ansi -pedantic -c foo.c
foo.c: In function 'main':
foo.c:6:5: warning: ISO C90 forbids variable length array 'name' [-Wvla]
     char name[size];
     ^

C:\Users\jk\Desktop>del foo.o

C:\Users\jk\Desktop>gcc -std=c99 -pedantic -c foo.c

C:\Users\jk\Desktop>del foo.o

C:\Users\jk\Desktop>gcc -std=c11 -pedantic -c foo.c

C:\Users\jk\Desktop>

As I said, GCC is very forgiving. First, I compiled as C++, which went fine, and then I tried various standards. I do think that GCC is a real C compiler. After all, it caught the problem when compiling under C90 rules.

Code:

const int size = 4;
char name[size];

Why is this a VLA when size is known at compile time ?

Prior to VLAs, this was a limitation of the language for a different reason: const variables were not considered the same as constant expressions, and thus they didn't work for fixed size arrays. VLAs are simply a way for this kind of code to work, and nothing more. Sorry if it feels like I'm not answering the question...

Thanks laserlight & whiteflags