very large memory allocation in 64-bit linux

I am trying to allocate a single very large piece of memory (>2.5gb) on a centos 64-bit linux. The hardware has more than 16gb physical memory. However, when I use malloc or calloc, they return null. The code run开发者_C百科s in root account, and as far as I checked with ulimit there is no limit on memory allocation. What can be the problem ?

Thanks.

Mete

Google the sbrk routine, it will help you.