malloc -- Initializer element is not constant

[Zia Ur Rehman]

So here is a piece of code. It's in C but I don't think that matters (does it?).

#define SZ_GENERATOR_STATE 624

unsigned int * InitializeGenerator(unsigned int Seed){
    
    static unsigned int *GeneratorState = (unsigned int *)
                              malloc(SZ_GENERATOR_STATE * sizeof(unsigned int));
    if (GeneratorState==NULL) return GeneratorState;
    else {
        GeneratorState[0] = Seed;
        int i; for (i=1; i<SZ_GENERATOR_STATE; i++) {
            GeneratorState[i] = 1812433253 * (GeneratorState[i-1] ^ 
                                                     GeneratorState[i-1] >> 30);
        }
    }
    return GeneratorState;
}

When compiled the compiler (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)) returns the following error:

InitGenr.c:14:31: error: initializer element is not constant

The error is in the line where I use malloc to allocate SZ_GENERATOR_STATE * sizeof(unsigned int) memory to an unsigned integer array... What's wrong with this?

I Googled for similar problems and almost all of them have a scope issue. But I'm not allocating memory in global scope. Or maybe I don't completely understand the problem. Please guide.

Many thanks.

[vlad from moscow]

I think the compiler does not like that the variable declared as static. That is the compiler assumes that static variables shall be initialized by constant expressions.
In C++ this code is compiled successfully. So it is important whether C or C++ is used.:)

[zb0S216C]

In C, static variables/objects must be initialised with a constant expression or an aggregate initialiser-list with constant expressions. Since function calls are not constant expressions, even when they yield a constant variable/object, they cannot be used to initialise a static variable/object. In fact, in C, variables/objects declared constant are not constant expressions; only magic constants are constant expressions. Therefore, the initialisation of GeneratorState is invalid.

_Wazzak

[Zia Ur Rehman]

So what should I do?
If I remove the static keyword, I will not be able to return GeneratorState.
Should I not allocate memory dynamically?

[vlad from moscow]

You can rewrite your function for example the following way

#define SZ_GENERATOR_STATE 624

unsigned int * InitializeGenerator(unsigned int Seed)
{
    static unsigned int *GeneratorState = NULL;

    if ( !GeneratorState )
    {   
        GeneratorState = (unsigned int *)
                              malloc(SZ_GENERATOR_STATE * sizeof(unsigned int));
    }

    if (GeneratorState==NULL) return GeneratorState;
    else {
        GeneratorState[0] = Seed;
        int i; for (i=1; i<SZ_GENERATOR_STATE; i++) {
            GeneratorState[i] = 1812433253 * (GeneratorState[i-1] ^ 
                                                     GeneratorState[i-1] >> 30);
        }
    }
    return GeneratorState;
}

Though I do not understand why do you have to declare the pointer as static.

[Zia Ur Rehman]

@vlad from moscow: Thank you very much for the alternate implementation.

You see, I'm trying to write a relatively simple implementation of Mersenne Twister (http://en.wikipedia.org/wiki/Mersenne_twister) in C. The array or its pointer, GeneratorState gets passed around between functions a lot. I don't know why, but I had a hunch that defining it as static might come in handy. Should I get rid of that?