How OpenSSL prevents memset() from being optimized out

mem_clr.c

/* Original: https://github.com/openssl/openssl/blob/2bcb232ebeb155c6f1241deb84a26ab23176f866/crypto/mem_clr.c */

/*
 * Copyright 2002-2016 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the OpenSSL license (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 */

#include <string.h>
#include <openssl/crypto.h>

/*
 * Pointer to memset is volatile so that compiler must de-reference
 * the pointer and can't assume that it points to any function in
 * particular (such as memset, which it then might further "optimize")
 */
typedef void *(*memset_t)(void *, int, size_t);

static volatile memset_t memset_func = memset;

void OPENSSL_cleanse(void *ptr, size_t len)
{
    memset_func(ptr, 0, len);
}