#include <string.h>
#include <stdlib.h>
#include <stdio.h>

int main()
{
   char *a = "12 34 56";
   int a_l = strlen(a);
   printf("str1: \"%s\" (%d)\n", a, a_l);

   char *b = "___";
   int b_l = strlen(b);
   printf("str2: \"%s\" (%d)\n", b, b_l);

   for (int i = 0; i < a_l; i++) {
      if (a[i] == ' ') {
         char *o = malloc(a_l + b_l);

         strncpy(o, a, i);
         strncpy(o + i, b, a_l);
         //strncpy help

         printf("out:  \"%s\"\n", o);
      }
   }

   return 0;
}