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

struct PS
{ char * uname;
  char * fullname; };

struct PS * newPS(char * a, char * b)
{ struct PS * n = (struct PS *)malloc(sizeof(struct PS));
  n->uname = a;
  n->fullname = b;
  return n; }

struct VecPS
{ struct PS * * data;
  int cap, num; };

struct VecPS * newVecPS(void)
{ struct VecPS * n = (struct VecPS *)malloc(sizeof(struct VecPS));
  n->data = NULL;
  n->cap = 0;
  n->num = 0; 
  return n; }

void delete(struct VecPS * v)
{ int i;
  for (i = 0; i < v->num; i += 1)
  { struct PS * p = v->data[i];
    free(p->uname);
    free(p->fullname);
    free(p); }
  free(v); }

void enlarge(struct VecPS * v)
{ int newcap = v->cap * 2;
  if (newcap == 0)
    newcap = 1;
  struct PS * * newdata = (struct PS * *)malloc(sizeof(struct PS *)*newcap);
  int i;
  for (i = 0; i < v->num; i += 1)
    newdata[i] = v->data[i];
  free(v->data);
  v->data = newdata;
  v->cap = newcap; }

void add(struct VecPS * v, struct PS * p)
{ if (v->num >= v->cap)
    enlarge(v);
  v->data[v->num] = p;
  v->num += 1; }

void sort(struct PS * * array, int size)
{ int begin, i;
  for (begin = 0; begin < size-1; begin += 1)
  { int smp = begin;
    for (i = begin+1; i < size; i += 1)
      if (strcmp(array[i]->uname, array[smp]->uname) < 0)
        smp = i; 
    struct PS * temp = array[begin];
    array[begin] = array[smp];
    array[smp] = temp; } }

int compa(const void *a, const void * b)
{ struct PS * pa = * (struct PS * *) a;
  struct PS * pb = * (struct PS * *) b;
  return strcmp(pa->uname, pb->uname); }

int main()
{ FILE * f = fopen("/etc/passwd", "r");
  if (f == NULL)
  { fprintf(stderr, "DCan't open /etc/passwd\n");
    exit(1); }
  char line[1000];
  struct VecPS * v = newVecPS();
  while (1)
  { char * s = fgets(line, sizeof(line), f);
    if (s == NULL) break;
    int len = strlen(line);
    if (line[len-1] != '\n')
    { fprintf(stderr, "line is too long\n");
      exit(1); }
    if (line[0] == '#') continue;
    char * p1 = strtok(line, ":\n");
    if (p1 == NULL) continue;
    char * p2 = strtok(NULL, ":\n");
    char * p3 = strtok(NULL, ":\n");
    char * p4 = strtok(NULL, ":\n");
    char * p5 = strtok(NULL, ":\n");
    if (p5 == NULL)
    { fprintf(stderr, "%s hasn't enough fields in /etc/passwd\n", p1);
      exit(1); }
    struct PS * p = newPS(strdup(p1), strdup(p5));
    add(v, p); }
//  sort(v->data, v->num);

  qsort(v->data, v->num, sizeof(struct PS *), compa);

  int i;
  for (i=0; i < v->num; i += 1)
  { struct PS * p = v->data[i];
    printf("%s is %s\n", p->uname, p->fullname); }   
  delete(v);
  fclose(f); }