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).

       ┌─────────────────┬───────────────┬─────────┐
       │Interface        │ Attribute     │ Value   │
       ├─────────────────┼───────────────┼─────────┤
       │bzero(),         │ Thread safety │ MT-Safe │
       │explicit_bzero() │               │         │
       └─────────────────┴───────────────┴─────────┘

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 location to 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 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  explicit_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 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 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
       explicit_bzero()   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.

SEE ALSO

       bstring(3), memset(3), swab(3)

COLOPHON

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       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.