//// ------------- найденная функция -----------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h> /* To completely get rid of this,
                       implement your our strcpy as well */

static void my_strcat (char* dest, char* src)
{
  while (*dest) ++dest;
  while (*src) *(dest++) = *(src++);
  *dest = 0;
}

int main()
{
  char* a1 = malloc(100);
  char* b1 = malloc(100);

  strcpy (a1, "Vivek");
  strcpy (b1, " Ratnavel");

  my_strcat (a1, b1);
  printf ("%s\n", a1); /* => Vivek Ratnavel */

  free (a1);
  free (b1);
  return 0;
}


// источник:   https://coderoad.ru/4438541/%D0%9A%D0%BE%D0%BD%D0%BA%D0%B0%D1%82%D0%B5%D0%BD%D0%B0%D1%86%D0%B8%D1%8F-%D1%81%D1%82%D1%80%D0%BE%D0%BA-%D0%B1%D0%B5%D0%B7-strcat-%D0%B2-C