Semaphore's

hi,
I've got a problem with that program. I wan't to make program which writes byte by byte to file, after written every sign is read and written to another file. How have i to organise semaphores? I gave to both semaphores value 0 in the beggining. thanks for help

#include<stdio.h>
#include<sys/types.h>
#include<sys/ipc.h>
#include<sys/sem.h>
#include <unistd.h>
#include<stdlib.h>
struct sembuf buf;
void down(int semid, short semnum)
{
	int sem;
	buf.sem_num=semnum;
	buf.sem_op=1;
	buf.sem_flg=0;
	if((sem=semop(semid, &buf, 1))==-1)
	{
		printf("Blad semop w podnies\n");
		return;
	}
}
void up(int semid, short semnum)
{
	int sem;
	buf.sem_num=semnum;
	buf.sem_op=-1;
	buf.sem_flg=0;
	if((sem=semop(semid, &buf, 1))==-1)
	{
		printf("Blad semop w opusc\n");
		return;
	}
}


////////////////////////

fk=fork();
if(fk>0)
{
	while(buf[i]!='\0')
	{	 	 	
		fp=fopen("plik", "aw");	
		fseek(fp, i, SEEK_SET);		
		fwrite(&buf[i], 1, 1, fp);
		fclose(fp);		
		printf("%d\n", i);						
		i++;	
		up(semid,1);		
		down(semid, 0);		
	}       
	
}               
else if(fk==0)  
{		
	char buf1[100];
	int j=0;	

	while(j<=a)
	{       
		down(semid, 1);
		fp=fopen("plik", "r");	
		fseek(fp, j, SEEK_SET);		
		fread(&buf1[j], 1, 1, fp);
		fclose(fp);								
		fr=fopen("plik1", "aw");			
		fseek(fr, j, SEEK_SET);	
		fwrite(&buf1[j], 1, 1, fr);		
		fclose(fr); 	
		j++;		
		up(semid, 0);		
	
		
	}