NAME¶

std::memset - std::memset

Synopsis¶

Defined in header <cstring>
void* memset( void* dest, int ch, std::size_t count );

Copies the value static_cast<unsigned char>(ch) into each of the first count
characters of the object pointed to by dest. If the object is a
potentially-overlapping subobject or is not TriviallyCopyable (e.g., scalar,
C-compatible struct, or an array of trivially copyable type), the behavior is
undefined. If count is greater than the size of the object pointed to by dest, the
behavior is undefined.

Parameters¶

dest - pointer to the object to fill
ch - fill byte
count - number of bytes to fill

Return value¶

dest

Notes¶

std::memset may be optimized away (under the as-if rules) if the object modified by
this function is not accessed again for the rest of its lifetime (e.g., gcc bug
8537). For that reason, this function cannot be used to scrub memory (e.g., to fill
an array that stored a password with zeroes).

Solutions for that include std::fill with volatile pointers, (C23)
memset_explicit(), (C11) memset_s, FreeBSD explicit_bzero or Microsoft
SecureZeroMemory.

Example¶

// Run this code

#include <bitset>
#include <climits>
#include <cstring>
#include <iostream>

int main()
{
int a[4];
using bits = std::bitset<sizeof(int) * CHAR_BIT>;
std::memset(a, 0b1111'0000'0011, sizeof a);
for (int ai : a)
std::cout << bits(ai) << '\n';
}

Output:¶

00000011000000110000001100000011
00000011000000110000001100000011
00000011000000110000001100000011
00000011000000110000001100000011

See also¶

memcpy copies one buffer to another
(function)
memmove moves one buffer to another
(function)
copies the given wide character to every position in a wide
wmemset character array
(function)
fill copy-assigns the given value to every element in a range
(function template)
fill_n copy-assigns the given value to N elements in a range
(function template)
is_trivially_copyable checks if a type is trivially copyable
(C++11) (class template)
C documentation for
memset