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.Dd November 1, 2024
.Dt EXPLICIT_MEMSET 3
.Os
.Sh NAME
.Nm explicit_memset
.Nm memset_explicit
.Nd guarantee writing a byte to a byte string
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In string.h
.Ft void *
.Fn explicit_memset "void *b" "int c" "size_t len"
.Ft void *
.Fn memset_explicit "void *b" "int c" "size_t len"
.Sh DESCRIPTION
The
.Fn explicit_memset
and
.Fn memset_explicit
functions write
.Fa len
bytes of value
.Fa c
(converted to an unsigned char) to the string
.Fa b .
They are guaranteed not to be optimized away by the compiler even if
.Fa b
is no longer used and is about to be freed or go out of scope.
.Sh RETURN VALUES
The
.Fn explicit_memset
function returns the original value of
.Fa b .
.Sh EXAMPLES
Create a buffer on the stack for a secret key, use it, and then zero it
in memory before throwing it away.
.Bd -literal -offset indent
void
f(void)
{
	uint8_t key[32];

	crypto_random(key, sizeof(key));
	do_crypto_stuff(key, sizeof(key));
	\&...

	explicit_memset(key, 0, sizeof(key));
}
.Ed
.Sh SEE ALSO
.Xr consttime_memequal 3 ,
.Xr memset 3
.Sh STANDARDS
The
.Fn memset_explicit
function conforms to
.St -isoC-2023 .
.Sh HISTORY
The
.Fn explicit_memset
function appeared in
.Nx 7.0 .
The
.Fn memset_explicit
alias was added in
.Nx 11.0 .