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Date: Fri, 17 Sep 2021 16:31:13 -0700
From: Alan Coopersmith <>
Subject: freezero() and freezeroall()

OpenBSD 6.2 added a new interface to their libc:

               void freezero(void *ptr, size_t size);

The man page at defines it as:

              The freezero() function is similar to the free() function
              except it ensures memory is explicitly discarded. If ptr
              is NULL, no action occurs. If ptr is not NULL, the size
              argument must be equal to or smaller than the size of the
              earlier allocation that returned ptr. freezero()
              guarantees the memory range starting at ptr with length
              size is discarded while deallocating the whole object
              originally allocated.

          where "discarded" is defined as:

              pages of memory are disposed via munmap(2) and cached
              free objects are cleared with explicit_bzero(3).

The reliance on the caller to provide the size to clear allows the function
to be implemented independently of the underlying allocator library, and has
allowed this function to be implemented in libbsd, as well as fallback
implementations provided in portable software packages for systems without
their own implementation - including in OpenSSH, OpenNNTPd, OpenBGPd,
OpenIKEd, OpenSMTPd, sudo, tmux, libretls, and a few more I see in the search
results on  I've also seen it implemented in libc on
illumos and DragonflyBSD.

I'm working on adding it to the Solaris libc now, and since we can determine
the underlying allocation size, proposed also adding:

                 void freezeroall(void *ptr);

as basically doing: freezero(ptr, malloc_usable_size(ptr));

During review of this change I was asked if there's any existing equivalent
we should be following instead before we invent our own name - I couldn't
find one, but figured I'd ask here - are other libc implementations doing
anything like this?

	-Alan Coopersmith-     
	 Oracle Solaris Engineering -

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