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NAME

       msgctl - message control operations

SYNOPSIS

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

       int msgctl(int msqid, int cmd, struct msqid_ds *buf);

DESCRIPTION

       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 (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 in <sys/ipc.h> 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:

       IPC_STAT
              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.

       IPC_SET
              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,
              msg_perm.uid, msg_perm.gid, and (the least significant 9  bits  of)  msg_perm.mode.
              The  effective  UID  of  the calling process must match the owner (msg_perm.uid) or
              creator (msg_perm.cuid) of the message queue, or the  caller  must  be  privileged.
              Appropriate privilege (Linux: the CAP_IPC_RESOURCE capability) is required to raise
              the msg_qbytes value beyond the system parameter MSGMNB.

       IPC_RMID
              Immediately remove the message queue,  awakening  all  waiting  reader  and  writer
              processes  (with an error return and errno set to EIDRM).  The calling process must
              have appropriate privileges 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  currently  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.

RETURN VALUE

       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  message  queues.   (This  information can be used with
       repeated MSG_STAT operations to obtain information about all queues  on  the  system.)   A
       successful MSG_STAT operation returns the identifier of the queue whose index was given in
       msqid.

       On error, -1 is returned with errno indicating the error.

ERRORS

       On failure, errno is set to one of the following:

       EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the calling process does not
              have   read  permission  on  the  message  queue  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).

CONFORMING TO

       SVr4, POSIX.1-2001.

NOTES

       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
       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.)

SEE ALSO

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

COLOPHON

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