Thread: Array can't be created (expression must have a constant value)

Hi! I've try to make a small program that shows the error, but I still don't understand why this doesn't work.

If I have the code:

Code:

#include <stdio.h>


int main() {


	char genKey[10];


	return 0;
}

It works fine. But if I try to create a variable and initialize it to 10. Then Visual Studio complains.

Code:

#include <stdio.h>


int main() {


	int textLen = 10;
	char genKey[textLen];


	return 0;
}

I thought this meant the same thing, but it doesn't? Because the code in the second example doesn't compile.

You are using a compiler that still adheres to the C89 C Standard, by default. Variable size arrays are valid only using the C99 Standard or higher.

It is possible that there is an option in your compiler to set the level to C99. In gcc, the option would be "-std=C99"

You are using a compiler that still adheres to the C89 C Standard, by default.

The Microsoft C compiler supports all of C90 any feature of the latest standards are only supported if they are required for C++ compliance. And since VLA are not supported by any of the current C++ standards they will probably not support this "feature" with their current compilers.

When declaring arrays you should either use a named constant (#define ARRAY_SIZE 10) or constant values.

By the way VLA are only truly supported by the C99 standard. Any standard before C99 this "feature" was not allowed, and with C11 the feature is optional and not required to be supported.

You can get the near equivalent of VLA using _alloca(). The difference is that a VLA will be freed when it goes out of scope (such as a local block created using left and right braces), while an _alloca() object will be freed when the function exits.

Code:

#include <stdlib.h>
int main()
{
    int textLen = 10;
    char * genKey = _alloca(textLen*sizeof(char));
    return 0;
}

Thanks. It helped

I am not sure if using alloca or _alloca is a good idea - see this discussion
c - Why is the use of alloca() not considered good practice? - Stack Overflow

If you don't like dynamic memory management it would be better to use C++.