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BZERO(3)                                                                Linux Programmer's Manual                                                               BZERO(3)

NAME
       bzero, explicit_bzero - zero a byte string

SYNOPSIS
       #include <strings.h>

       void bzero(void *s, size_t n);

       #include <string.h>

       void explicit_bzero(void *s, size_t n);

DESCRIPTION
       The  bzero()  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.

       The explicit_bzero() 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".

RETURN VALUE
       None.

VERSIONS
       explicit_bzero() first appeared in glibc 2.25.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                             β”‚ Attribute     β”‚ Value   β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚bzero(), explicit_bzero()                                                                                                             β”‚ Thread safety β”‚ MT-Safe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

CONFORMING TO
       The  bzero()  function  is deprecated (marked as LEGACY in POSIX.1-2001); use memset(3) in new programs.  POSIX.1-2008 removes the specification of bzero().  The
       bzero() function first appeared in 4.3BSD.

       The explicit_bzero() 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().

NOTES
       The explicit_bzero() function addresses a problem that security-conscious applications may run into when using bzero(): if the compiler can deduce that the loca‐
       tion to be zeroed will never again be touched by a correct program, then it may remove the bzero() call altogether.  This is a problem if the intent of the  bze‐
       ro() 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 ex‐
       plicit_bzero() are never optimized away by the compiler.

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

       1. 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 erase them.

       2. 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  ad‐
          dress 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 qualifier does not eliminate the above problems.  Indeed, it will make them worse, since, for  exam‐
       ple, 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 explicit_bzero() is generally preferable to not using it.  The developers of ex‐
       plicit_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.

SEE ALSO
       bstring(3), memset(3), swab(3)

Linux                                                                          2021-03-22                                                                       BZERO(3)