std::memset

Header: <cstring>

Copies the value static_cast(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.

# Declarations

void* memset( void* dest, int ch, std::size_t count );

# 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

#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';
}