ptr = sodium_malloc(size) returns a pointer from which exactly "size" bytes
can be accessed.
ptr = sodium_allocarray(count, size) allocates enough storage space for
"count" pointers or scalars of unit size "size".
In both cases, the region is immediately followed by a guard page.
As a result, any attempt to access a memory location after ptr[size - 1] will
immediately trigger a segmentation fault.
The allocated region is mlock()ed and filled with 0xd0 bytes.
A read-only page with the size, a guard page, as well as a canary are
placed before the returned pointer.
The canary is checked by sodium_free(); as a result, altering data right
before ptr is likely to cause sodium_free() to kill the process.
sodium_free() munlock()s the region and fills it with zeros before
actually calling free().
sodium_mprotect_noaccess(), sodium_mprotect_readonly() and
sodium_mprotect_readwrite() can be used to change the protection on the set
of allocated pages.
Reverting the protection to read+write is not required before calling
sodium_free().
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