Does malloc lazily create the backing pages for an allocation on Linux (and other platforms)?

linux malloc
Aaron Maenpaa picture Aaron Maenpaa · May 26, 2009 · Viewed 18.2k times · Source

On Linux if I were to malloc(1024 * 1024 * 1024), what does malloc actually do?

I'm sure it assigns a virtual address to the allocation (by walking the free list and creating a new mapping if necessary), but does it actually create 1 GiB worth of swap pages? Or does it mprotect the address range and create the pages when you actually touch them like mmap does?

(I'm specifying Linux because the standard is silent on these kinds of details, but I'd be interested to know what other platforms do as well.)

Answer

Remus Rusanu picture Remus Rusanu · May 26, 2009

Linux does deferred page allocation, aka. 'optimistic memory allocation'. The memory you get back from malloc is not backed by anything and when you touch it you may actually get an OOM condition (if there is no swap space for the page you request), in which case a process is unceremoniously terminated.

See for example http://www.linuxdevcenter.com/pub/a/linux/2006/11/30/linux-out-of-memory.html

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