Sprintf to concat string

I have a string which I need to concat some data onto the end. My problem is that each time the loop goes round I get previous loop data on the end of my string. e.g data

1st loop iteration 1234@3 2nd loop iteration 3462@3@124 3rd loop iteration 3676@3@124@67 and so on..

what i need to do is receive the string and some data to the end, set the string for each loop iteration.

if(rank == 0)
{


    char *recv_str = malloc(PRECISION);
    int retVal = -10;
    int len;
    long exp;
    char *str_exp = malloc(sizeof(char) * 8);
    char *deli = "@";


    for(i=1; i<= size - 1 ; i++)
    {

       MPI_Recv(&exp,1,MPI_LONG,i,1,MPI_COMM_WORLD,&status);
       MPI_Recv(&len,1,MPI_INT,i,1,MPI_COMM_WORLD,&status);
       MPI_Recv(recv_str,len, MPI_CHAR, i, 1,MPI_COMM_WORLD,  &status); 

          sprintf(str_exp, "%lu", exp); 
          sprintf(recv_str + strlen(recv_str), deli);
          sprintf(recv_str + strlen(recv_str),str_exp);              


       retVal = mpf_set_str(pi_ret, recv_str, 10); 
       //printf("string = %s \n", recv_str);
       //printf("\nvalid? %d length %d\n", retVal, len);
       //printf("str_exp  = %s \n", str_exp);
       mpf_add(pi, pi, pi_ret);
       //printf("\nsize of %lu \n", strlen(recv_str));

    }

Do it this way:

char * recv_str = malloc(PRECISION * sizeof(*recv_str)); /* Make sure PRECISION is large enough!!! */
...
char str_exp[32] = ""; /* Make it's large enough to hold the decimal representaion of a long. */
char * deli = "@";

if (NULL == recv_str)
{
  perror("malloc() failed for recv_str");
  assert(1);
}

for(i=1; i<= size - 1 ; i++)
{
  strcpy(recv_str, ""); /* Reset the "string" to "empty" before each iteration! */

  ...

  sprintf(str_exp, "%ld", exp); 

  assert(sizeof(recv_str) <= (strlen(recv_str) + strlen(deli)));
  strcat(recv_str, deli);

  assert(sizeof(recv_str) <= (strlen(recv_str) + strlen(str_exp)));
  strcat(recv_str, str_exp); 

  ...
}

free(recv_str);

Update:

As it showed that initialsing recv_str to 0-length by doing

strcpy(recv_str, "");

did not solve the problem, as it most probably only sets the "string"'s 1^st byte to NUL/'\0', the more radical approach needed to be taken to clear out the whole "string" with 0s by doing:

memset(recv_str, 0, PRECISION * sizeof(*recv_str));