#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 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 (non-standard) */
               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 in <sys/ipc.h> as follows (the  high-
       lighted 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
              following members of  the  structure  are  updated:  msg_qbytes,
              or its effective user ID must be either that of the  creator  or
              owner of the message queue.

       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
       IPC_INFO or MSG_INFO operation returns the index of  the  highest  used
       entry  in  the  kernel's internal array recording information about all
              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 process is not privileged (Linux:
              it does not have the CAP_SYS_ADMIN capability).

       SVr4, POSIX.1-2001.

       The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the  ipcs(8)
       program  to  provide information on allocated resources.  In the future
       these may modified or moved to a /proc file system 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),

       This page is part of release 3.23 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

Linux                             2008-08-06                         MSGCTL(2)
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