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



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 (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 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 struc-
              ture 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(8) program to provide information on  allo-
       cated 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  distin-
       guishes 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
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-06                         MSGCTL(2)