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

       msgctl - message control operations

       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/msg.h>&gt;

       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 &lt;sys/msg.h&gt; 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 &lt;sys/ipc.h&gt; 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,
              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.   Appro-
              priate  privilege  (Linux:  the  CAP_IPC_RESOURCE capability) is
              required to raise the msg_qbytes value beyond the system parame-
              ter 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
              &lt;sys/msg.h&gt; 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
       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  opera-
                  tion,  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  capa-

       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 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2008-05-20                         MSGCTL(2)