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

       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 (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	 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
	      <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 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 call‐
	      ing 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),

       This page is part of release 3.22 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2008-08-06			     MSGCTL(2)

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