Reading File and enforcing order of operation between processes - c

Question

I am attempting to follow a tutorial which asks me to edit example code to get a program to run two processes which take turns to output the lyrics to a song ('There's a hole in the bucket').
My problem is that the file gets outputted as a whole and not alternativley like it should see screen shot for what i am talking about : http://imgur.com/NusvhVA
My code is below. Thanks.
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>
#define KEY 87654 //Unique semaphore key
int main()
{
int id; /* Number by which the semaphore is known within a program */
FILE *file;
file = fopen("207song.txt", "r" );
int c;
union semun {
int val;
struct semid_ds *buf;
ushort * array;
} argument;
argument.val = 1;
/* Create the semaphore with external key KEY if it doesn't already
exists. Give permissions to the world. */
id = semget(KEY, 1, 0666 | IPC_CREAT);
/* Always check system returns. */
if(id < 0) {
fprintf(stderr, "Unable to obtain semaphore.\n");
exit(0);
}
/* What we actually get is an array of semaphores. The second
argument to semget() was the array dimension - in our case
1. */
/* Set the value of the number 0 semaphore in semaphore array
# id to the value 0. */
if( semctl(id, 0, SETVAL, argument) < 0) {
fprintf( stderr, "Cannot set semaphore value.\n");
} else {
fprintf(stderr, "Semaphore %d initialized.\n", KEY);
}
int pid=fork();
const int HENRY_DONE = 0;
const int LIZA_DONE = 1;
volatile int flag = HENRY_DONE;
if(pid) {
struct sembuf operations[1];
int retval; /* Return value from semop() */
/* Get the index for the semaphore with external name KEY. */
id = semget(KEY, 1, 0666);
if(id < 0){
/* Semaphore does not exist. */
fprintf(stderr, "Program sema cannot find semaphore, exiting.\n");
exit(0);
}
operations[0].sem_num = 0;
/* Which operation? Subtract 1 from semaphore value : */
operations[0].sem_op = -1;
/* Set the flag so we will wait : */
operations[0].sem_flg = 0;
while(1){
//Process 1
//wait
operations[0].sem_op = -1;
retval = semop(id, operations, 1);
//critical section
printf("Liza's Part: \n");
fflush(stdout);
sleep(1);
while ((c = getc(file)) !=EOF)
if (c == "\n") {
putchar(c);
break;
}
else
putchar(c);
fflush(stdout);
operations[0].sem_op = 1;
//signal
retval = semop(id, operations, 1);
}
}else{
//Process 2
struct sembuf operations[1];
int retval; /* Return value from semop() */
/* Get the index for the semaphore with external name KEY. */
id = semget(KEY, 1, 0666);
if(id < 0){
/* Semaphore does not exist. */
fprintf(stderr, "Program sema cannot find semaphore, exiting.\n");
exit(0);
}
operations[0].sem_num = 0;
/* Which operation? Subtract 1 from semaphore value : */
operations[0].sem_op = -1;
/* Set the flag so we will wait : */
operations[0].sem_flg = 0;
while(1){
//wait
operations[0].sem_op = -1;
retval = semop(id, operations, 1);
//critical section
printf("Henry's Part: \n");
fflush(stdout);
sleep(1);
while ((c = getc(file)) !=EOF)
if (c == "\n") {
putchar(c);
break;
}
else
putchar(c);
fflush(stdout);
//signal
operations[0].sem_op = 1;
retval = semop(id, operations, 1);
}
}
}

Answer 1

If your while loop you have:
while ((c = getc(file)) !=EOF)
if (c == "\n") {
getc returns an integer, "\n" is a c-string of type char*. That
comparison will not match, leading the first comsumer to show the whole file.
You probably want
c == '\n'
note the single quotes 'rather than double " The single quote will be a char, which will compare reasonably with an int.