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How to share semaphore between processes

Posted on 2006-10-29
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Last Modified: 2008-01-09
How to share semaphore between processes?
eg
static sem_t count_sem;
sem_init(&count_sem,1,3);

How to shared this  &count_sem between processes? So we can synchronise between processes.
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Question by:william007
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Expert Comment

by:Infinity08
ID: 17829577
Take a look at this wiki article about IPC (InterProcess Communication) :

http://en.wikipedia.org/wiki/Inter-process_communication

There's a few ways to achieve this. Shared memory could be a good option for you :

http://en.wikipedia.org/wiki/Shared_memory

What system/OS are you developing for ?
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by:william007
ID: 17829598
I am using Linux fedora core 5.
I just researching for semaphore.h, there is a system call called sem_open to create named semaphore, I think it is possible that we can use that.
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Expert Comment

by:Infinity08
ID: 17829669
If all you want to share between the processes is that semaphore, then that's indeed an option.

More info for that on the man page for sem_open :

http://www.penguintutor.com/man/man.php?topic=sem_open&section=3


    sem_t *sem = sem_open("/tmp/semname", O_CREAT, 0644, 0);
    if (sem == SEM_FAILED) {
      perror("sem_open() failed ");
    }

Chec kthe man pages for your system to get the implementation you'll use.
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by:william007
ID: 17829699
>sem_t *sem = sem_open("/tmp/semname", O_CREAT, 0644, 0);
This gives me segmentation fault
>sem_t *sem = sem_open("semname", O_CREAT, 0644, 0);
This Works ok.

What is the reason"?
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Author Comment

by:william007
ID: 17829727
Code
====
#include <semaphore.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <fcntl.h>
static sem_t *count_sem;
int main(int argc, char* argv[]){
char* semName=argv[1];
mode_t mode=0777;
unsigned int maxProcess=3;
unsigned int i=O_CREAT|O_EXCL;
int value=0;
count_sem=sem_open(semName,i,mode,maxProcess);
if (count_sem==(void *)-1){
printf("Error create named semaphore\n");
exit(1);
}
while(1){
      sem_getvalue(count_sem,&value);
      printf("value=%d\n",value);
      sem_wait(count_sem);
}


}

Several Test Results
================
[root@localhost Desktop]# ./sem "/tmp/name1"
Segmentation fault
[root@localhost Desktop]# ./sem "/tmp/name2"
Segmentation fault
[root@localhost Desktop]# ./sem "/tmp/name3"
Segmentation fault
[root@localhost Desktop]# ./sem "sem1"
Segmentation fault
[root@localhost Desktop]# ./sem "sem2"
value=3
value=2
value=1
value=0

[root@localhost Desktop]# ./sem "sem2"
Segmentation fault
[root@localhost Desktop]# ./sem "sem3"
value=3
value=2
value=1
value=0
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Accepted Solution

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Infinity08 earned 500 total points
ID: 17829728
I guess your implementation doesn't support slashes other than the first. Try :

    sem_t *sem = sem_open("/semname", O_CREAT, 0644, 0);

The leading slash is to make sure that other processes will refer to the same named semaphore when using "/semname".
Although your implementation might not need the leading slash either. For that you'll have to check the man pages on your system.
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Author Comment

by:william007
ID: 17829734
Now I have faced a big problem...How to clean up the named semaphore that I have created and forget to clear? What is the proper way to actually clean up with code?
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Author Comment

by:william007
ID: 17829735
*What is the proper way to actually clean up the semaphore in the  code?
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Assisted Solution

by:Infinity08
Infinity08 earned 500 total points
ID: 17829737
And about your last post (with the code)

From the man pages :

    "If O_EXCL and O_CREAT are set, sem_open() fails if the semaphore name exists."

Don't set both :)

Also, don't compare with (void *)-1 to check for errors, use SEM_FAILED for that as in my earlier example - it's a lot more portable.
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Assisted Solution

by:Infinity08
Infinity08 earned 500 total points
ID: 17829767
>> *What is the proper way to actually clean up the semaphore in the  code?
Use sem_close to close the semaphore in each process, and sem_unlink in the process that actually created it (to remove it from the file system).

    sem_close(sem);
    sem_unlink("/semname");

Don't forget to check the return codes !
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Author Comment

by:william007
ID: 17853551
Thanks=)
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