3) please explain the below c program for sorting

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

   #define MAXLINES 5000     /* max #lines to be sorted */

   char *lineptr[MAXLINES];  /* pointers to text lines */

   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   main()
   {
       int nlines;     /* number of input lines read */

       if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
           qsort(lineptr, 0, nlines-1);
           writelines(lineptr, nlines);
           return 0;
       } else {
           printf("error: input too big to sort\n");
           return 1;
       }
   }

   #define MAXLEN 1000  /* max length of any input line */
   int getline(char *, int);
   char *alloc(int);

   /* readlines:  read input lines */
   int readlines(char *lineptr[], int maxlines)
   {
       int len, nlines;
       char *p, line[MAXLEN];

       nlines = 0;
       while ((len = getline(line, MAXLEN)) > 0)
           if (nlines >= maxlines || p = alloc(len) == NULL)
               return -1;
           else {
               line[len-1] = '\0';  /* delete newline */
               strcpy(p, line);
               lineptr[nlines++] = p;
           }
       return nlines;
   }

   /* writelines:  write output lines */
   void writelines(char *lineptr[], int nlines)
   {
       int i;

       for (i = 0; i < nlines; i++)
           printf("%s\n", lineptr[i]);
   }

           if (nlines >= maxlines || ((p = alloc(len)) == NULL))

   #define MAXLINES 5000     /* max #lines to be sorted */
    #define MAXLEN 1000  /* max length of any input line */
   
   char *lineptr[MAXLINES];  /* pointers to text lines */
      int getline(char *, int);
   char *alloc(int);
   int readlines(char *lineptr[], int nlines);
   void writelines(char *lineptr[], int nlines);

   void qsort(char *lineptr[], int left, int right);

   /* sort input lines */
   main()
   {

}

int a = 5;
if (a = 0) // Always evaluated as false, no error.
{
    //...
}

int a = 5;
if (0 = a) // Generates compilation error, you cannot assign a to 0 (rvalue)
{
    //...
}

[sathish@localhost ~]$ gcc sort1.c
/tmp/ccZ1YzHU.o: In function `readlines':
sort1.c:(.text+0x96): undefined reference to `alloc'
sort1.c:(.text+0xf9): undefined reference to `gettline'
collect2: error: ld returned 1 exit status