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 zeroes (bytes con-
       taining '') 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 specifica-
       tion 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 simi-
       lar 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

       This page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and  the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15								  BZERO(3)