Memory allocation overflow in c program

**bassam_fci** · 05-19-2009

In a large system I try to allocate about 1GB of memory on a 4GB

system and it blows up (returns a NULL pointer in C), Now I tried to

allocate 1GB from a small test program and it also returns null.

Also, I have implemented the /LARGEADDRESSLIMIT switch on the Linker,

to be able to address more than 2GB RAM. But the system does not want

to allocate 1GB, when the available memory is 1.8GB according to the

Task Manager!!

ex: BYTE *bb=(BYTE*) calloc(0.9, 0x40000000);
where BYTE is unsigned char

I can only allocate about 400MB, otherwise I get NULL

How much memory I can allocate from a C program ?

It looks like there is something I don't quite understand about the x86

memory model.

**matsp** · 05-19-2009

Code:

BYTE *bb=(BYTE*) calloc(0.9, 0x40000000);

0.9 bytes? That will convert to zero integer value. 40000000 records of 0 length may turn into a zero length, which will make calloc return NULL.

--
Mats

**Salem** · 05-19-2009

Well you also need to reboot your OS with the /3GB flag

Also bear in mind that your program code and loaded DLLs occupy some part of the user address space, as does the stack required by your program. These will also limit the amount of contiguous RAM you could allocate in a single block.

And what is "0.9" doing in your calloc call anyway?

**bassam_fci** · 05-19-2009

for matsp:

OK, that's true, so I then just allocated a specific number of bytes 400*1024*1024, for example
But I couldn's allocate more than that!
What is the use of having 3GB or so available, if I can't make use of them?

**bassam_fci** · 05-19-2009

for Salem:

OK, that looks as if it might be the case...contiguous RAM.
So there is no way of knowing what the max. CONTIGUOOUS RAM is that I can allocate.
It looks like I have to have a loop which starts by allocating, say 500MB and then gradually decreasing it until the allocation function does not return null.

**matsp** · 05-19-2009

Originally Posted by bassam_fci

for matsp:

OK, that's true, so I then just allocated a specific number of bytes 400*1024*1024, for example
But I couldn's allocate more than that!
What is the use of having 3GB or so available, if I can't make use of them?

Code:

#include <stdlib.h>
#include <stdio.h>

int main()
{
	char *p = calloc(1, 1500 * 1024 * 1024);
	printf("%p\n", (void *)p);
	if (!p)
	{
		printf("Errno = %d\n", errno);
	}
	free(p);
	return 0;
}

Works just fine.

As I don't have /3GB on the OS boot options (and I don't really want to reboot my machine and check if it works), I can't say for sure that it will work above 2GB, but I don't see why not.

--
Mats

**bassam_fci** · 05-19-2009

matsp & Salem; thanks a lot for your effort.
I'll try something to see if it works or not.
thanks in advance.

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