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           #include <sys/types.h>
           #include <sys/ipc.h>
           #include <sys/msg.h>
           int msgctl(int msqid, int cmd, struct msqid_ds *buf);


           msgctl()  performs the control operation specified by cmd on the System
           V message queue with identifier msqid.
           The msqid_ds data structure is defined in <sys/msg.h> as follows:
               struct msqid_ds {
                   struct ipc_perm msg_perm;     /* Ownership and permissions */
                   time_t          msg_stime;    /* Time of last msgsnd(2) */
                   time_t          msg_rtime;    /* Time of last msgrcv(2) */
                   time_t          msg_ctime;    /* Time of last change */
                   unsigned long   __msg_cbytes; /* Current number of bytes in
                                                    queue (nonstandard) */
                   msgqnum_t       msg_qnum;     /* Current number of messages
                                                    in queue */
                   msglen_t        msg_qbytes;   /* Maximum number of bytes
                                                    allowed in queue */
                   pid_t           msg_lspid;    /* PID of last msgsnd(2) */
                   pid_t           msg_lrpid;    /* PID of last msgrcv(2) */
           The ipc_perm structure is defined as follows  (the  highlighted  fields
           are settable using IPC_SET):
               struct ipc_perm {
                   key_t          __key;       /* Key supplied to msgget(2) */
                   uid_t          uid;         /* Effective UID of owner */
                   gid_t          gid;         /* Effective GID of owner */
                   uid_t          cuid;        /* Effective UID of creator */
                   gid_t          cgid;        /* Effective GID of creator */
                   unsigned short mode;        /* Permissions */
                   unsigned short __seq;       /* Sequence number */
           Valid values for cmd are:
                  Copy  information from the kernel data structure associated with
                  msqid into the msqid_ds structure pointed to by buf.  The caller
                  must have read permission on the message queue.
                  Write  the  values  of  some  members  of the msqid_ds structure
                  pointed to by buf to the kernel data structure  associated  with
                  this  message  queue,  updating  also its msg_ctime member.  The
                  owner  of  the message queue.  The third argument to msgctl() is
                  ignored in this case.
           IPC_INFO (Linux-specific)
                  Returns information about system-wide message queue  limits  and
                  parameters  in  the structure pointed to by buf.  This structure
                  is of type msginfo  (thus,  a  cast  is  required),  defined  in
                  <sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
                      struct msginfo {
                          int msgpool; /* Size in kibibytes of buffer pool
                                          used to hold message data;
                                          unused within kernel */
                          int msgmap;  /* Maximum number of entries in message
                                          map; unused within kernel */
                          int msgmax;  /* Maximum number of bytes that can be
                                          written in a single message */
                          int msgmnb;  /* Maximum number of bytes that can be
                                          written to queue; used to initialize
                                          msg_qbytes during queue creation
                                          (msgget(2)) */
                          int msgmni;  /* Maximum number of message queues */
                          int msgssz;  /* Message segment size;
                                          unused within kernel */
                          int msgtql;  /* Maximum number of messages on all queues
                                          in system; unused within kernel */
                          unsigned short int msgseg;
                                       /* Maximum number of segments;
                                          unused within kernel */
                  The msgmni, msgmax, and msgmnb settings can be changed via /proc
                  files of the same name; see proc(5) for details.
           MSG_INFO (Linux-specific)
                  Returns a msginfo structure containing the same  information  as
                  for IPC_INFO, except that the following fields are returned with
                  information about system resources consumed by  message  queues:
                  the msgpool field returns the number of message queues that cur-
                  rently exist on the system; the msgmap field returns  the  total
                  number  of  messages in all queues on the system; and the msgtql
                  field returns the total number of bytes in all messages  in  all
                  queues on the system.
           MSG_STAT (Linux-specific)
                  Returns  a  msqid_ds  structure  as  for IPC_STAT.  However, the
                  msqid argument is not a queue identifier, but instead  an  index
                  into  the  kernel's  internal  array  that maintains information
                  about all message queues on the system.


           On success, IPC_STAT, IPC_SET, and IPC_RMID  return  0.   A  successful
                  msqid, and does not have the CAP_IPC_OWNER capability.
           EFAULT The argument cmd has the value  IPC_SET  or  IPC_STAT,  but  the
                  address pointed to by buf isn't accessible.
           EIDRM  The message queue was removed.
           EINVAL Invalid  value  for cmd or msqid.  Or: for a MSG_STAT operation,
                  the index value specified in msqid referred  to  an  array  slot
                  that is currently unused.
           EPERM  The  argument  cmd  has  the  value IPC_SET or IPC_RMID, but the
                  effective user ID of the calling process is not the creator  (as
                  found  in msg_perm.cuid) or the owner (as found in msg_perm.uid)
                  of the message queue, and the caller is not  privileged  (Linux:
                  does not have the CAP_SYS_ADMIN capability).
           EPERM  An  attempt (IPC_SET) was made to increase msg_qbytes beyond the
                  system parameter  MSGMNB,  but  the  caller  is  not  privileged
                  (Linux: does not have the CAP_SYS_RESOURCE capability).


           SVr4, POSIX.1-2001.


           The  inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
           or by any version of POSIX.  However, some old implementations required
           the inclusion of these header files, and the SVID also documented their
           inclusion.  Applications intended to be portable to  such  old  systems
           may need to include these header files.
           The  IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(1)
           program to provide information on allocated resources.  In  the  future
           these may modified or moved to a /proc filesystem interface.
           Various  fields  in the struct msqid_ds were typed as short under Linux
           2.2 and have become long under Linux 2.4.  To take advantage of this, a
           recompilation  under glibc-2.1.91 or later should suffice.  (The kernel
           distinguishes old and new calls by an IPC_64 flag in cmd.)


           msgget(2),  msgrcv(2),  msgsnd(2),   capabilities(7),   mq_overview(7),

    Linux 2014-02-24 MSGCTL(2)


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