how to use malloc to reserve space for 1 million floats

how to use malloc to reserve space for 1 million floats

[Anna Sidera]

Hello,

I want to use the following command:

int myvariable = 1000;
float *myarray = malloc(pow(myvariable,3)*sizeof(float));

but I don't know if it will work because if sizeof(float) is equal to 4 then pow(myvariable,3)*sizeof(float) is equal to 4 billion which is larger than the maximum integer which is about 2 billion.

Can you tell me what is the right way to create an array of pow(myvariable,3) floats?

Many Thanks,
Anna

Re: how to use malloc to reserve space for 1 million floats

[Jonathan Wakely]

On 27 August 2012 10:40, Anna Sidera wrote:
> Hello,
>
> I want to use the following command:
>
> int myvariable = 1000;
> float *myarray = malloc(pow(myvariable,3)*sizeof(float));
>
> but I don't know if it will work because if sizeof(float) is equal to 4 then pow(myvariable,3)*sizeof(float) is equal to 4 billion which is larger than the maximum integer which is about 2 billion.

malloc takes a size_t parameter, not an int, so it's an unsigned value
and can represent larger positive numbers than int.

Whether your system will allow an allocation that size is another
matter. It might be a problem on a 32-bit platform.

Re: how to use malloc to reserve space for 1 million floats

[Ángel González]

On 27/08/12 11:40, Anna Sidera wrote:
> Hello,
>
> I want to use the following command:
>
> int myvariable = 1000;
> float *myarray = malloc(pow(myvariable,3)*sizeof(float));
>
> but I don't know if it will work because if sizeof(float) is equal to 4 then pow(myvariable,3)*sizeof(float) is equal to 4 billion which is larger than the maximum integer which is about 2 billion.
>
> Can you tell me what is the right way to create an array of pow(myvariable,3) floats?
>
> Many Thanks,
> Anna

How much memory do you have available?
The parameter to malloc is a size_t, you should have no problems
providing a size of 4000000000 in a 64 bit system.
If you're using a 32 bit system, then you will have problems stating
that size. But the address space is also smaller than that,
so you couldn't reserve so much memory*, even if you were able to
provide that number to malloc().

* using conventional methods. But seems silly not to be using a flat
address space nowadays...

Re: how to use malloc to reserve space for 1 billion floats

[Anna Sidera]

The system has 10 giga byte RAM.

The command:
uname -a
gives the following output:
Linux olympus 2.6.26-2-amd64 #1 SMP Wed Aug 19 22:33:18 UTC 2009 x86_64 GNU/Linux
I think this means that the system is 64 bit.

So you think that the command:
int myvariable = 1000;
float *myarray = malloc(pow(myvariable,3)*sizeof(float));
will work if pow(myvariable,3) is smaller than 2 billion?

Many Thanks,
Anna

----- Original Message -----
From: Ángel González <keisial@gmail.com>
Date: Monday, August 27, 2012 2:14 pm
Subject: Re: how to use malloc to reserve space for 1 million floats

> On 27/08/12 11:40, Anna Sidera wrote:
> > Hello,
> >
> > I want to use the following command:
> >
> > int myvariable = 1000;
> > float *myarray = malloc(pow(myvariable,3)*sizeof(float));
> >
> > but I don't know if it will work because if sizeof(float) is 
> equal to 4 then pow(myvariable,3)*sizeof(float) is equal to 4 
> billion which is larger than the maximum integer which is about 2 
> billion.>
> > Can you tell me what is the right way to create an array of 
> pow(myvariable,3) floats?
> >
> > Many Thanks,
> > Anna
> 
> How much memory do you have available?
> The parameter to malloc is a size_t, you should have no problems
> providing a size of 4000000000 in a 64 bit system.
> If you're using a 32 bit system, then you will have problems stating
> that size. But the address space is also smaller than that,
> so you couldn't reserve so much memory*, even if you were able to
> provide that number to malloc().
> 
> * using conventional methods. But seems silly not to be using a flat
> address space nowadays...
> 
>

Re: how to use malloc to reserve space for 1 billion floats

[Ángel González]

On 28/08/12 05:58, Anna Sidera wrote:
> The system has 10 giga byte RAM.
>
> The command:
> uname -a
> gives the following output:
> Linux olympus 2.6.26-2-amd64 #1 SMP Wed Aug 19 22:33:18 UTC 2009 x86_64 GNU/Linux
> I think this means that the system is 64 bit.
It does.

> So you think that the command:
> int myvariable = 1000;
> float *myarray = malloc(pow(myvariable,3)*sizeof(float));
> will work if pow(myvariable,3) is smaller than 2 billion?
>
> Many Thanks,
> Anna
Yes. (the 2+e9 limit being roughly for the 7,45GB needed, not due to
malloc, to which you
could ask for even bigger amounts )