memset, memset_explicit, memset_s

Header: <string.h>

  1. Copies the value (unsigned char)ch into each of the first count characters of the object pointed to by dest.

# Declarations

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

void *memset_explicit( void *dest, int ch, size_t count );

(since C23)

errno_t memset_s( void *dest, rsize_t destsz, int ch, rsize_t count );

(since C11)

# Parameters

# Notes

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).

This optimization is prohibited for memset_explicit and memset_s: they are guaranteed to perform the memory write.

Third-party solutions for that include FreeBSD explicit_bzero or Microsoft SecureZeroMemory.

# Example

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
 
int main(void)
{
    char str[] = "ghghghghghghghghghghgh";
    puts(str);
    memset(str,'a',5);
    puts(str);
 
#ifdef __STDC_LIB_EXT1__
    set_constraint_handler_s(ignore_handler_s);
    int r = memset_s(str, sizeof str, 'b', 5);
    printf("str = \"%s\", r = %d\n", str, r);
    r = memset_s(str, 5, 'c', 10);   // count is greater than destsz  
    printf("str = \"%s\", r = %d\n", str, r);
#endif
}

# See also