Question

I am using msgget() syscall to get new msg queue. I have used the IPC_CREAT & IPC_EXCL flags in that. like
message_queue = msgget(ftok("/tmp", 100), (0666 | IPC_CREAT | IPC_EXCL));
Now, when my prog exists unexpectedly, the msg queue remains and I am failed to re-create the msg queue. So, my question is “How can I get back the existing msg queues’s ID ?”

and by the way, where does msg queue stores its id ?

Answer 1

Regd “How can I get back the existing msg queues’s ID ?”

From man msgget

   If  msgflg  specifies both IPC_CREAT and IPC_EXCL and a message queue already exists for key, then msgget() fails with errno set to EEX-
   IST.  (This is analogous to the effect of the combination O_CREAT | O_EXCL for open(2).)

Try without IPC_EXCL flag.

Regd. where does msg queue stores its id

from man proc

   /proc/sysvipc
          Subdirectory  containing  the  pseudo-files  msg,  sem  and  shm.  These files list the System V Interprocess Communication (IPC)
          objects (respectively: message queues, semaphores, and shared memory) that currently  exist  on  the  system,  providing  similar
          information  to that available via ipcs(1).  These files have headers and are formatted (one IPC object per line) for easy under-
          standing.  svipc(7) provides further background on the information shown by these files.