bzero, explicit_bzero - zero a byte string
void bzero(void *s, size_t n);
void explicit_bzero(void *s, size_t n);
() function erases the data in the n
bytes of the memory
starting at the location pointed to by s
, by writing zeros (bytes
containing '\0') to that area.
() function performs the same task as bzero
It differs from bzero
() in that it guarantees that compiler
optimizations will not remove the erase operation if the compiler deduces that
the operation is "unnecessary".
() first appeared in glibc 2.25.
For an explanation of the terms used in this section, see attributes
|bzero (), explicit_bzero ()
() function is deprecated (marked as LEGACY in POSIX.1-2001);
(3) in new programs. POSIX.1-2008 removes the specification
(). The bzero
() function first appeared in 4.3BSD.
() function is a nonstandard extension that is also
present on some of the BSDs. Some other implementations have a similar
function, such as memset_explicit
() or memset_s
() function addresses a problem that security-conscious
applications may run into when using bzero
(): if the compiler can
deduce that the location to zeroed will never again be touched by a
program, then it may remove the bzero
() call altogether.
This is a problem if the intent of the bzero
() call was to erase
sensitive data (e.g., passwords) to prevent the possibility that the data was
leaked by an incorrect or compromised program. Calls to
() are never optimized away by the compiler.
() function does not solve all problems associated with
erasing sensitive data:
- The explicit_bzero() function does not guarantee that
sensitive data is completely erased from memory. (The same is true of
bzero().) For example, there may be copies of the sensitive data in
a register and in "scratch" stack areas. The
explicit_bzero() function is not aware of these copies, and can't
- In some circumstances, explicit_bzero() can decrease
security. If the compiler determined that the variable containing the
sensitive data could be optimized to be stored in a register (because it
is small enough to fit in a register, and no operation other than the
explicit_bzero() call would need to take the address of the
variable), then the explicit_bzero() call will force the data to be
copied from the register to a location in RAM that is then immediately
erased (while the copy in the register remains unaffected). The problem
here is that data in RAM is more likely to be exposed by a bug than data
in a register, and thus the explicit_bzero() call creates a brief
time window where the sensitive data is more vulnerable than it would
otherwise have been if no attempt had been made to erase the data.
Note that declaring the sensitive variable with the volatile
eliminate the above problems. Indeed, it will make them worse,
since, for example, it may force a variable that would otherwise have been
optimized into a register to instead be maintained in (more vulnerable) RAM
for its entire lifetime.
Notwithstanding the above details, for security-conscious applications, using
() is generally preferable to not using it. The
developers of explicit_bzero
() anticipate that future compilers will
recognize calls to explicit_bzero
() and take steps to ensure that all
copies of the sensitive data are erased, including copies in registers or in
"scratch" stack areas.