Segmentation fault in porgram

Below is the program for sorting the file in ascending and reverse order
this program is working properly under window but it gives segmentation fault under unix
Can u find where i m mistaking....
u can give command line arguments like this

./myprogram -a filename.txt for sortin filename in ascending order
./myprogram -r filename.txt for sorting filename in reverse order

Program

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFER_LEN 100  /* Length of input buffer    */
#define NUM_P 100       /* maximum number of strings */
#define TRUE  1
#define FALSE 0

void main(int argc, char *argv[])
{
   FILE *fp;
   char buffer[BUFFER_LEN];    /* space to store an input string    */
   char *pS[NUM_P] = { NULL }; /* Array of string pointers          */
   char *pTemp = NULL;         /* Temporary pointer                 */
   int i = 0, a=0;             /* Loop counter                      */
   int sorted = FALSE;         /* Indicated when strings are sorted */
   int last_string = 0;        /* Index of last string entered      */
   
   if(strcmp(argv[1],"-a")==0)
   a=1;
   if(strcmp(argv[1],"-r")==0)
   a=2;

   fp =fopen(argv[2],"r"); /* Opening the file in read mode     */

   while((*fgets(buffer,(BUFFER_LEN-1),fp) != '\0') && (i < NUM_P))
   {
     pS[i] = (char*)malloc(strlen(buffer) + 1);
     if(pS[i]==NULL)           /* Check for no memory allocated     */
     {
       printf(" Memory allocation failed. Program terminated.\n");
       return;
     }
     strcpy(pS[i++], buffer);
   }
   last_string = i;            /* Save last string index            */

   switch(a)
   {
   case 1:
    {
     /* Sorting in ascending order */
      while(!sorted)
      {
	sorted = TRUE;
	for (i = 0 ; i < last_string - 1 ; i++)
	  if(strcmp(pS[i], pS[i + 1]) > 0)
	  {
	    sorted = FALSE;     /* We were out of order */
	    pTemp = pS[i];      /* Swap pointers pS[i]  */
	    pS[i] = pS[i + 1];  /*       and            */
	    pS[i + 1] = pTemp;  /*     pS[i + 1]        */
	  }
      }
      break;
    }
   case 2:
    {
     /* Sorting in decending order */
      while(!sorted)
      {
	sorted = TRUE;
	for (i = 0 ; i < last_string - 1 ; i++)
	  if (strcmp(pS[i], pS[i + 1]) < 0)
	  {
	    sorted = FALSE;     /* We were out of order */
	    pTemp = pS[i];      /* Swap pointers pS[i]  */
	    pS[i] = pS[i + 1];  /*       and            */
	    pS[i + 1] = pTemp;  /*     pS[i + 1]        */
	  }
      }
      break;
    }
   default:
    {
     printf("wrong argument passed");
     exit(1);
    }
   }
   /* Displayed the sorted strings */
   printf("\nYour input sorted in order is:\n\n");
   for (i = 0 ; i < last_string ; i++)
   {
     printf("%s\n", pS[i] );
     free( pS[i] );
     pS[i] = NULL;
   }
}

 

This is the Introduce yourself section....

 

1. main returns int, not void

2. (*fgets(buffer,(BUFFER_LEN-1),fp) != '\0')
When fgets() reaches the end, it returns NULL.
Then when you try to dereference that result, you segfault.
The correct loop condition is
fgets(buffer,(BUFFER_LEN-1),fp) != NULL

Also, you don't have to account for the \0 in your count, so you can just do
fgets(buffer,BUFFER_LEN,fp) != NULL