c - Zero a large memory mapping with `madvise` -

i have following problem:

i allocate large chunk of memory (multiple gib) via mmap map_anonymous. chunk holds large hash map needs zeroed every , then. not entire mapping may used in each round (not every page faulted in), memset not idea - takes long.

what best strategy quickly?

will

madvise(ptr, length, madv_dontneed); 

guarantee me subsequent accesses provide new empty pages?

from linux man madvise page:

this call not influence semantics of application (except in case of madv_dontneed), may influence performance. kernel free ignore advice.

...

madv_dontneed

subsequent accesses of pages in range succeed, result either in reloading of memory contents underlying mapped file (see mmap(2)) or zero-fill-on-demand pages mappings without underlying file.

...

the current linux implementation (2.4.0) views system call more command advice ...

or have munmap , remap region anew?

it has work on linux , ideally have same behaviour on os x.

there easier solution problem portable:

mmap(ptr, length, prot_read|prot_write, map_fixed|map_private|map_anonymous, -1, 0); 

since map_fixed permitted fail arbitrary implementation-specific reasons, falling memset if returns map_failed advisable.