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Same SV key_t for sem. and shared mem. OK?

Posted on 1998-05-27
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Last Modified: 2013-12-26
Can I use the same key_t to identify a semaphore
and a shared memory region, or do they come out
of a single, global namespace?
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Question by:BillSeymour
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6 Comments
 
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Expert Comment

by:kellyjj
ID: 1293801
I don't think you can do that.  However, even if you could I would not recommend it.  
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seedy earned 50 total points
ID: 1293802
Yes, you can.  They come from different namespace.


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by:BillSeymour
ID: 1293803
Obviously, I can't judge the content of the answer.
(If I could, I wouldn't have needed to ask the question.)
Assuming that the answer is correct, it certainly gets
an A...it's exactly what I wanted to know.
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Expert Comment

by:seedy
ID: 1293804
Yes, you can judeg the content of the answer.  Here is the proof.
Put the following lines in a file, compile and run.
-- code start --
#include <stdio.h>
#include <sys/sem.h>
#include <sys/shm.h>

#define      MYKEY      0x1CD1

main()
{
      int shmid = -1, semid = -1;

      if ( (semid = semget(MYKEY, 2, IPC_CREAT|IPC_EXCL)) >= 0 )
            printf("Created Semaphore with KEY: %d\n", MYKEY);
      else
            perror("semaphore creation failed");

      if ( (shmid = shmget(MYKEY, 2, IPC_CREAT|IPC_EXCL)) >= 0 )
            printf("Created Shared-mem with key: %d\n", MYKEY);
      else
            perror("shared memory creation failed");

      if ( semctl(semid, 0, IPC_RMID) < 0)
            perror("semaphore - removal failed");
      if ( shmctl(shmid, IPC_RMID, (void *)NULL) < 0)
            perror("shared mem - removal failed");
}
-- code end--
0
 

Author Comment

by:BillSeymour
ID: 1293805
I've tested seedy's code on an Intel box running AIX 4.2,
and it works regardless of which object I create first.
That's not to say that it won't fail on SOME Unix box
(absence of evidence is not evidence of absence);
nevertheless, I'll add 50 points to the score if there's
a way for me to do that.  (The code implies a prediction,
and so the theory is falsifiable.  (Is there a smiley
for "egghead?")) --Bill

--Bill
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Expert Comment

by:seedy
ID: 1293806
Majority of the features under unix are common or standard.  Going by that experience, I would say that this is expected to work on ALL UNIXs.  If it won't work, I would assume it to be mentioned in the man pages!

For example the man pages on Digital OSF1 4.0, AIX 4.1, SunOS 5.5.1 and HP-UX 10.10 for semget states:

A semaphore identifier and associated data structure [...]
are created for key if one of the following is true:
       o key is equal to IPC_PRIVATE
       o key does not already have a ***semaphore identifier*** associated with it

You want to give me more points?  You must be a generous person!  You can do this by creating a new dummy question - asking me to answer it and then award the points.







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