BZERO(3) BSD Library Functions Manual BZERO(3)NAME
bzero, explicit_bzero -- write zeroes to a byte string
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <strings.h>
void
bzero(void *b, size_t len);
void
explicit_bzero(void *b, size_t len);
DESCRIPTION
The bzero() function writes len zero bytes to the string b. If len is zero, bzero() does nothing.
The explicit_bzero() variant behaves the same, but will not be removed by a compiler's dead store optimization pass, making it useful for
clearing sensitive memory such as a password.
SEE ALSO memset(3), swab(3)HISTORY
A bzero() function appeared in 4.3BSD. Its prototype existed previously in <string.h> before it was moved to <strings.h> for IEEE Std
1003.1-2001 (``POSIX.1'') compliance.
The explicit_bzero() function first appeared in OpenBSD 5.5 and FreeBSD 11.0.
BSD October 6, 2014 BSD
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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 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
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Linux 2017-09-15 BZERO(3)