Помогите найти ошибку в коде на Си

Вот код

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#include
#include
#include
#include
#include
#include

void main(){
int j,i=0,n;
char temp=NULL,buffer=NULL;
char* kluch;
FILE *inp, *out;
int vybir;

while (vybir!=5){
clrscr();

printf("1.To code\n2.To decode\n3.Help\n4.About\n5.Exit\nYour choice->");
scanf("%d",&vybir);
clrscr();
if (vybir==1) {
inp = fopen("Input.txt" , "r");
out = fopen("Output.txt" , "w");
if(inp==NULL){
printf("Is not found Input.txt");
getch();
exit(1);
}
printf("Enter the key (without blanks)-> ");
scanf("%s", kluch);
n=strlen(kluch);
while(inp!= EOF)
{

fread((char*)&buffer,sizeof(buffer), 1, inp);
if(buffer==NULL) break;

j=fmod(i,n);
temp=buffer^kluch[j];

fwrite((char*)&temp,sizeof(temp), 1, out);
i++;
buffer=NULL;
}
fclose(inp);
fclose(out);
printf("The coded text saves in the file ");
getch();
exit(1);
}
if (vybir==2) {
inp = fopen("Output.txt" , "r");
out = fopen("Desifr.txt" , "w");
if(inp==NULL)
{
printf("Is not found Output.txt");
getch();
exit(1);
}
printf("Enter the key (without blanks)-> ");
scanf("%s", kluch);
n=strlen(kluch);
while(inp!= EOF)
{

fread((char*)&buffer,sizeof(buffer), 1, inp);
if(buffer==NULL) break;

j=fmod(i,n);
temp=buffer^kluch[j];
fwrite((char*)&temp,sizeof(temp), 1, out);
i++;
buffer=NULL;
}
fclose(inp);
fclose(out);
printf("The decoded text saves in the file ");
getch();
exit(1);}

if (vybir==3) {printf("\nFor coding:\n 1 - bring the text in the file .\n 2 - pass to the point of .\n 3 - enter the key.\n 4 - the coded text saved in the file ");
printf("\nFor decoding:\n 1 - bring the coded text in the file .\n 2 - pass to the point of .\n 3 - enter the key.\n 4 - the decoded text saved in the file "); getch(); }
if (vybir==4) {printf("\n\n\n\n\t\t\tThe program of coding and decoding is written by\n\n\t\t\t\t Students CS-1201: \n\n\t\t\t\t Kinash O.M\n\t\t\t\t Pryhodko V.M\n\t\t\t\t Pliska L.D\n\n\n\t\t\t\t\t 2013"); getch(); }
}

clrscr();
printf("\nGood Luck!!!\nPress any key...");
getch();}

не понимаю в чем ошибка, может в бибилотекках, раньше работало, сейчас ошибка, как изменить?