Can I malloc a space larger than 3G?

**meili100** · 05-18-2008

We know that a process memory space is limited by 4G (1G for kernel).
I am thinking if I can malloc a space larger than 3G. So I tried the following:

Code:

int main()
{
	char* x = (char*)malloc(3*1024*1024*1024);
	memset(x,'m',3*1024*1024*1024);
	printf("&#37;s",x);
	return 0;
}

AND It works! But the print out result isn't that nice.....
Anybody knows why?

I also tried

Code:

char x[0x7fffffff];

This time compiler complains for its size. So I guess the implementations of malloc and char[] are different, right?

**whiteflags** · 05-18-2008

>> why?

printf( "%s", x ); prints x as a C string. A C string must be zero terminated.

>> This time compiler complains for its size. So I guess the implementations of malloc and char[]
>> are different, right?

Stack memory is much smaller than the heap.

**meili100** · 05-18-2008

Thanks.
Put aside the stack and printf. let's focus on

Code:

char* x = (char*)malloc(3*1024*1024*1024);

How come I can allocate a 3G space for a process?!

Originally Posted by citizen

>> why?

printf( "%s", x ); prints x as a C string. A C string must be zero terminated.

>> This time compiler complains for its size. So I guess the implementations of malloc and char[]
>> are different, right?

Stack memory is much smaller than the heap.

**whiteflags** · 05-18-2008

Because malloc returns memory from a much larger pool maintained by the operating system. As you are aware though, a ceiling (though a generous one) for your process is imposed.

**CornedBee** · 05-19-2008

I'm still surprised you can do it. Is this a 64-bit sytem?

**MacGyver** · 05-19-2008

I believe someone wrote a simple program on this forum awhile back to find out how much memory one could get out of the *alloc() functions. I think it was about 512 MB for me on a 32-bit install of Windows XP with MinGW.

There are obviously other ways of getting more memory, but that was just what occurred at the time. If you really wanted to use massive amounts of memory, I don't think the *alloc() family of functions are the proper way to go about it... at least not on Windows.

**robwhit** · 05-19-2008

Why wouldn't you be able to malloc 3G? It would just allocate some swap if you didn't have the memory, right?

**matsp** · 05-19-2008

Originally Posted by robwhit

Why wouldn't you be able to malloc 3G? It would just allocate some swap if you didn't have the memory, right?

Yes, but there is a limit within the OS that is arbitrarily set to be able to tell the difference between user-mode memory and kernel memory. On Linux, it's either at 1GB or 3GB, on Windows it's either 2GB or 3GB (both applies to 32-bit OS's).

This limit is HARD - you can not allocate past it, no matter how much RAM you have, or any other parameters (swap-space). From that hard limit, you can only go down by other restricting factors such as amount of actual RAM, swap space, space already used in process space (for example, Linux "uses" the first 128MB as "no touch area" to capture use of NULL-pointers, and both OS's put a stack area of a few megabytes somewhere, and of course, you need at least a few kilobytes of code for your application and probably some system library code to interface to the OS, etc).

I'm not aware of any "common" 32-bit OS that gives you more than 3GB of memory-space for an application.

--
Mats

**swoopy** · 05-19-2008

>So I tried the following
>AND It works!
Probably not, since you never checked for success.

**Elysia** · 05-19-2008

Not to mention even if 3 GB was the limit, there's probably a ton of other stuff in your virtual memory, as well, taking up space. At least your main stack would take up 1 MB place, which would leave you with 2.99 GB of memory left.
And another big factor is memory fragmentation. There's just unlikely to be 3 GB of contiguous space free. There's probably some bytes here and there that are allocated for other things, which would instantly make the request fail.
So even if the OS did allow you to allocate that much, it's unlikely it would succeed.

**robwhit** · 05-19-2008

Thanks Mats.

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