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harchar *secret;

const size_t SECRET_MAX = /* ... */
/* Initialize secret to a null-terminated byte string, 
   of less than SECRET_MAX chars */

size_t size = strlen(secret);
if (size >= SECRET_MAX) {
  /* Handle error */
}

char *new_secret;
/* Use calloc() to zero-out allocated space */
new_secret = (char *)calloc(size+1, sizeof(char));
if (!new_secret) {
  /* Handle error */
}
strcpy(new_secret, secret);

/* Process new_secret... */

/* Sanitize memory  */
memset_s(new_secret, '\0', size);
free(new_secret);
new_secret = NULL;