MSGGET(2)                  Linux Programmer's Manual                 MSGGET(2)

       msgget - get a message queue identifier

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

       int msgget(key_t key, int msgflg);

       The  function  returns  the  message queue identifier associated to the
       value of the key argument.  A new message queue is created if  key  has
       value  IPC_PRIVATE  or key isn't IPC_PRIVATE, no existing message queue
       is associated to  key,  and  IPC_CREAT  is  asserted  in  msgflg  (i.e.
       msgflg&IPC_CREAT  is  nonzero).   The  presence in msgflg of the fields
       IPC_CREAT and IPC_EXCL plays the same role, with respect to  the  exis-
       tence  of  the  message queue, as the presence of O_CREAT and O_EXCL in
       the mode argument of the open(2) system call: i.e. the msgget  function
       fails if msgflg asserts both IPC_CREAT and IPC_EXCL and a message queue
       already exists for key.

       Upon creation, the lower 9 bits  of  the  argument  msgflg  define  the
       access  permissions  of  the message queue.  These permission bits have
       the same format and semantics as the access  permissions  parameter  in
       open(2)  or  creat(2)  system  calls.  (The execute permissions are not

       Furthermore, while creating, the system  call  initializes  the  system
       message queue data structure msqid_ds as follows:

              msg_perm.cuid  and msg_perm.uid are set to the effective user-ID
              of the calling process.

              msg_perm.cgid and msg_perm.gid are set to the effective group-ID
              of the calling process.

              The  lowest  order 9 bits of msg_perm.mode are set to the lowest
              order 9 bit of msgflg.

              msg_qnum, msg_lspid, msg_lrpid, msg_stime and msg_rtime are  set
              to 0.

              msg_ctime is set to the current time.

              msg_qbytes is set to the system limit MSGMNB.

       If  the  message  queue already exists the access permissions are veri-
       fied, and a check is made to see if it is marked for destruction.

       If successful, the return value will be the message queue identifier (a
       nonnegative integer), otherwise -1 with errno indicating the error.

       For a failing return, errno will be set to one among the following val-

       EACCES     A message queue exists for key, but the calling process  has
                  no access permissions to the queue.

       EEXIST     A message queue exists for key and msgflg was asserting both
                  IPC_CREAT and IPC_EXCL.

       EIDRM      The message queue is marked for removal.

       ENOENT     No message queue exists for key and msgflg wasn't  asserting

       ENOMEM     A  message  queue  has  to be created but the system has not
                  enough memory for the new data structure.

       ENOSPC     A message queue has to be created but the system  limit  for
                  the  maximum  number  of  message  queues  (MSGMNI) would be

       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
       is  used  for  key,  the  system call ignores everything but the lowest
       order 9 bits of msgflg and creates a new message queue (on success).

       The following is a system limit on message queue resources affecting  a
       msgget call:

       MSGMNI     System  wide maximum number of message queues: policy depen-

       Use of IPC_PRIVATE does not actually prohibit other processes from get-
       ting access to the allocated message queue.

       There  is  currently no intrinsic way for a process to ensure exclusive
       access to a message queue.  Asserting both IPC_CREAT  and  IPC_EXCL  in
       msgflg  only ensures (on success) that a new message queue will be cre-
       ated, it doesn't imply exclusive access to the message queue.

       SVr4, SVID.  SVr4 does not document the EIDRM error code.

       ftok(3), ipc(5), msgctl(2), msgsnd(2), msgrcv(2)

Linux 0.99.13                     1993-11-01                         MSGGET(2)