bzero, explicit_bzero − zero a byte string


     , size_t void bzero(void *s


     , size_t void explicit_bzero(void *s

The function erases the data in the n bytes of the memory
starting at the location pointed to by by writing zeros
(bytes containing '\0') to that area.

     The function performs the same task as It differs from
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

│Interface Attribute     Value   │
│          │ Thread safety MT‐Safe │

The function is deprecated (marked as LEGACY in
POSIX.1‐2001); use in new programs.  POSIX.1‐2008 removes
the specification of The function first appeared in 4.3BSD.

     The function is a nonstandard extension that is also
present on some of the BSDs.  Some other implementations
have a similar function, such as or

The function addresses a problem that security‐conscious
applications may run into when using if the compiler can
deduce that the location to zeroed will never again be
touched by a correct program, then it may remove the call
altogether.  This is a problem if the intent of the 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.

     The function does not solve all problems associated
with erasing sensitive data:


1. The function does not guarantee that sensitive data is
   completely erased from memory.  (The same is true of For
   example, there may be copies of the sensitive data in a
   register and in "scratch" stack areas.  The function is
   not aware of these copies, and can’t erase them.

2. In some circumstances, 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 call would need
   to take the address of the variable), then the 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
   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 qualifier does not 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 anticipate that future
compilers will recognize calls to and take steps to ensure
that all copies of the sensitive data are erased, including
copies in registers or in "scratch" stack areas.

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