shared memory father and child in c - memory

Question

David Shwartz helped me alot and now it kinda works...
do you have any idea for more elgant way to parse the input, if the input consists more than 2 numbers to add which need to be processed by the child ? I want the child to get only two integers so that's why I created the shared memory so the father will send the child the result(shared memory) + another integer.
Thank you all.
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
volatile int *shared=0;
int shmid;
int main()
{
char line[256];
int readByte;
int fd[2]; //pipe to son, who processes addition
int pid;
shmid=shmget ( IPC_PRIVATE, sizeof(int) , 0600 );
shared=shmat ( shmid, 0 , 0);
if ( pipe(fd) )
{
perror("pipe");
exit(-1);
}
pid=fork();
if (pid!=0) // father
{
close (fd[0]);
readByte=read(0, line, 256);
line[readByte-1]='\0';
printf("%d",readByte);
int arr[2];
int i=0;
int j=0;
int flag=0;
char num[10];
while (i<readByte)
{
if (line[i]=='+' )
{
i++;
j=0;
flag=1;
}
while (line[i]!='+' && line[i]!='\0')
{
num[j]=line[i];
i++;
j++;
}
num[j]='\0';
if (flag==0)
arr[0]=atoi(num);
else
{
arr[1]=atoi(num);
i++;
}
}
printf("first %d\n",arr[0]);
printf("sec %d\n",arr[1]);
write(fd[1], &arr, sizeof(arr));
wait(NULL);
printf ( "%d\n" , *shared );
}
else
// son
{
int arr[2];
int sum;
readByte = read(fd[0], &arr, sizeof(arr));
printf("son printing: %d\n",arr[0]);
printf("son printing: %d\n",arr[1]);
sum =arr[0]+arr[1];
*shared=sum;
close (fd[0]);
shmdt ( (const void *) shared );
}
shmdt ( (const void *) shared );
shmctl ( shmid , IPC_RMID , 0 );
close(fd[1]);
return 0;
}

Answer 1

You throw away the return value of shmat. And you expect shared to be shared, but it's just a regular variable. Also, you need to prevent the compiler from optimizing away accesses to the shared memory. Here it is with all the fatal bugs fixed:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
volatile int *shared;
int shmid;
int main()
{
int s,i;
shmid=shmget ( IPC_PRIVATE, sizeof(int), 0600 );
shared=shmat ( shmid, 0 , 0);
*shared=100;
printf ( "%d\n" , *shared);
if ( fork()==0 ) // son
{
*shared=1000;
shmdt ( (const void *) shared );
}
else // father
{
wait ( &s );
printf ( "%d\n" , *shared);
shmdt ( (const void *) shared );
shmctl ( shmid , IPC_RMID , 0 );
}
return 0;
}