char str[9] = "1234";
char *str2 = "5678";
strcpy(&str[strlen(str)], str2);

/* snprintf example */
#include <stdio.h>

int main ()
{
  char buffer [100];
  char strAdd[] = "neshto drugo";

   snprintf ( buffer, sizeof(buffer)-1, "alabala  %s", strAdd);
  return 0;
}

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

int main()
{
    char str[9] = "1234";
    char *str2 = "56789012345678901234567890";
    strcpy(&str[strlen(str)], str2);
    //printf("%s\n",str);
    return 0;
}
(sic)

void AppendString(char * mainstring, char * appended)
{
    while(* mainstring != 0x00){
        mainstring++;
    }
    while(* appended != 0x00){
        * mainstring = * appended;
        mainstring++;
        appended++;
    }
    * mainstring = 0x00;
}