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       msgctl - message control operations


       #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 (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:

              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,
              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.

              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.


       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.


       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).


       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),


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