gistfile1.c

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

#define MSG "and the random number is: "

size_t num_length(long num) {
  size_t result;

  if (num == 0) {
    return 1;
  } else if (num < 0) {
    result = 1;
    num = -num;
  } else {
    result = 0;
  }

  result += floor(1 + log(num)/log(10));
  return result;
}

char* append(char* msg, long num) {
  size_t old_length, new_length;

  old_length = strlen(msg);
  new_length = old_length + num_length(num) + 1;

  msg = realloc(msg, new_length);
  sprintf(msg + old_length, "%ld", num);

  return msg;
}

int main(void) {
  long num;
  char* msg;

  msg = malloc(sizeof(MSG));
  memcpy(msg, MSG, sizeof(MSG));

  srand(time(NULL));
  num = rand();

  msg = append(msg, num);

  printf("%s\n", msg);

  return 0;
}