Problem in c program coding and illetracy with C programming language

.#include<stdio.h>
  #include<sys/types.h>
  #include<unistd.h>
  typedef struct decrypt
  {
  int p;
  int k;
 int c;
 c=p^k;
  }decrypt;
  int main()
  {
  decrypt s[10];
  int n,fd,i;
  fd=open("p.c",O_CREAT,777);//creates a new file
  if(fd<0)
  {
  printf("\nFailed to create file\n\n");exit(-1) 
  }
  fd=open("p.c",O_RDWR,777)//open the file for read and write
  printf("Enter the number to decrypt:");
  scanf("%d",&n);
  printf("Enter the key\n");
  for(i=0;i<n;i++0
  { 
  
  scanf("%d",&s[i].p);
  
  scanff("%d",&s[i].n);
  write( fd,s[i],sizeof(s) );//write to the file which the file discriptor is pointing to
  }
  
 return 0;//return success to kernel
  }

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

void CallArgError(char *appname)
{
    printf("Usage: %s MODE\n\n", appname);
    puts("MODE: e = encrypt/d = decrypt");
    exit(1);
}

int ReadVar(char *Var)
{
    fgets(Var, 258, stdin);
    fflush(stdin);

    int length = strlen(Var);

    int i;
    for(i = 0; i < length; i++)
        Var[i] = tolower(Var[i]);

    return length - 1;
}

void EnDecrypt(char *PlainText, int PlainTextLength, char *Key, int KeyLength, char option)
{
    int i, a = 0;
    char buf = 0;
    for(i = 0; i < PlainTextLength; i++)
    {
        if(option == 'e')
            buf = ((PlainText[i] - 97) + (Key[a] - 97)) % 26;
        else if(option == 'd')
            buf = ((PlainText[i] - 97) - (Key[a] - 97)) % 26;

        buf += 97;
        if(isalpha(buf))
            printf("%c", tolower(buf));

        a++;
        if(a > KeyLength)
            a = 0;
    }
}

int main(int argc, char **argv)
{
    char PlainText[258];
    printf("Enter a 256-length Plaintext:");
    int PlainTextLength = ReadVar(PlainText);

    puts("");

    char Key[258];
    printf("Enter a 256-length key:");
    int KeyLength = ReadVar(Key);

    puts("");

    if((strcmp("e", argv[1]) == 0) || (strcmp("d", argv[1]) == 0))
        EnDecrypt(PlainText, PlainTextLength, Key, KeyLength, argv[1][0]);
    else
        CallArgError(argv[0]);

    puts("");

    return 0;
}