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Simple message relay using Fifo IPC's & file locking
Hi im new to C programming.
Im trying to create a simple program which relays messages between two users on a linux machine using IPC's.
At the moment a sent message may arrive at the source or destonation program. How could i fix this problem?
I want a message from 1st program to arrive at the 2nd program and vice versa.
I assume 1 way would be to lock/unlock the fifo file, so that only one user may send, while the other recives and vice versa.
How could i do this, and are the other solutions?
Here is some of the code
pid = fork()
for(;;)
{
if(pid == 0) // child
{
mkfifo(fifo, 0666); // create fifo if it already doesnt exist
if((fd = open(fifo, O_RDWR)) <0) // open fifo for reading and writing
printf("fifo failed");
for(;;) // recive and print msg
{
read(fd, msgbuf, MAXLEN+1)
printf("\n\n messgae rcvd: %s", msgbuf);
//sleep(1);
}
}
while(waitpid(pid, &status, WNOHANG) == 0)
{ // create and send msg
fd = open(fifo, O_WRONLY | O_NONBLOCK)
printf("\n\n please enter a message :");
load_string(msgbuf, MAXLEN);
nwrite = write(fd, msgbuf, MAXLEN+1)
//sleep(3);
}
}
Thanx = )
Im trying to create a simple program which relays messages between two users on a linux machine using IPC's.
At the moment a sent message may arrive at the source or destonation program. How could i fix this problem?
I want a message from 1st program to arrive at the 2nd program and vice versa.
I assume 1 way would be to lock/unlock the fifo file, so that only one user may send, while the other recives and vice versa.
How could i do this, and are the other solutions?
Here is some of the code
pid = fork()
for(;;)
{
if(pid == 0) // child
{
mkfifo(fifo, 0666); // create fifo if it already doesnt exist
if((fd = open(fifo, O_RDWR)) <0) // open fifo for reading and writing
printf("fifo failed");
for(;;) // recive and print msg
{
read(fd, msgbuf, MAXLEN+1)
printf("\n\n messgae rcvd: %s", msgbuf);
//sleep(1);
}
}
while(waitpid(pid, &status, WNOHANG) == 0)
{ // create and send msg
fd = open(fifo, O_WRONLY | O_NONBLOCK)
printf("\n\n please enter a message :");
load_string(msgbuf, MAXLEN);
nwrite = write(fd, msgbuf, MAXLEN+1)
//sleep(3);
}
}
Thanx = )
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ASKER
ok thanx i thought about the two pipes b4, Im gonna try implement that now.
But getting back to the lock issue, i added this code to sending program part
char * lck ="lck";
int lk;
extern int errno;
if((lk = open(lck, O_RDWR | O_CREAT | O_EXCL, 0600)) < 0)
{
if(errno == EEXIST)
{ printf("lck exists"); exit(1); }
else
{ printf("WTF"); exit(127); }
}
which works fine, creating the lock.
however i cannot remove it. i tried using close(lk); But it didnt work.
as when the program was run again 'lck exists" was displayed and lck file was still in directory
How can i remove the lock?
But getting back to the lock issue, i added this code to sending program part
char * lck ="lck";
int lk;
extern int errno;
if((lk = open(lck, O_RDWR | O_CREAT | O_EXCL, 0600)) < 0)
{
if(errno == EEXIST)
{ printf("lck exists"); exit(1); }
else
{ printf("WTF"); exit(127); }
}
which works fine, creating the lock.
however i cannot remove it. i tried using close(lk); But it didnt work.
as when the program was run again 'lck exists" was displayed and lck file was still in directory
How can i remove the lock?
For implementing two way messaging,
U can also think of Message Queues
U can also think of Message Queues
ASKER
Thanx i got it 2 work, how ever im wondering if i could ask you some easy, genral questions about linux.
1) how can i copy to floppy, i went mnt/floppy, but was unable to read/write to it.
The floppy drive works fine under windows.
1) how can i copy to floppy, i went mnt/floppy, but was unable to read/write to it.
The floppy drive works fine under windows.
Many linux implementations have a complete library of tools to read DOS formatted devices. You can get a pretty good list with:
man -k DOS
In particular, look at mtools(1) and mcopy(1).
Kent
From the looks of your code, the "while(waitpid(...))" block of code is executed by both the parent and child, but the upper block is execute only by the child.
The child task opens the fifo and waits for a message. The parent falls through to the while() block, opens the fifo, prints a message and writes a string to the fifo.
The child tries to continually reopen the fifo. First in O_RDWR, then in O_WRONLY, then looping on the sequence. The parent only opens the fifo O_WRONLY.
Kent