So here is a piece of code. It's in C but I don't think that matters (does it?).
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#define SZ_GENERATOR_STATE 624
unsignedint * InitializeGenerator(unsignedint Seed){
staticunsignedint *GeneratorState = (unsignedint *)
malloc(SZ_GENERATOR_STATE * sizeof(unsignedint));
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.
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.:)
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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
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?
You can rewrite your function for example the following way
#define SZ_GENERATOR_STATE 624
unsignedint * InitializeGenerator(unsignedint Seed)
staticunsignedint *GeneratorState = NULL;
if ( !GeneratorState )
GeneratorState = (unsignedint *)
malloc(SZ_GENERATOR_STATE * sizeof(unsignedint));
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.
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@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?
