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

NAME
       msgctl - System V 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 System V 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 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  pro-
	      cesses  (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 con-
	      sumed 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  main-
	      tains 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  inclusion  of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version
       of POSIX.  However, some old implementations required the inclusion of these header files,
       and  the  SVID  also  documented their inclusion.  Applications intended to be portable to
       such old systems may need to include these header files.

       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
       This  page  is  part of release 3.53 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					    2012-05-31					MSGCTL(2)
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