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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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bzero(3) Library Functions Manual bzero(3)
bzero, explicit_bzero - zero a byte string
Standard C library (libc, -lc)
#include <strings.h>
void bzero(size_t n;
void s[n], size_t n);
#include <string.h>
void explicit_bzero(size_t n;
void s[n], size_t n);
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".
None.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ bzero(), explicit_bzero() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
None.
explicit_bzero()
glibc 2.25.
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().
bzero()
4.3BSD.
Marked as LEGACY in POSIX.1-2001. Removed in POSIX.1-2008.
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 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
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:
• 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.
• 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.
bstring(3), memset(3), swab(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-06-28 bzero(3)
Pages that refer to this page: bstring(3), memset(3), string_copying(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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