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 Sys-
       tem 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.	Appro-
	      priate  privilege	 (Linux:  the  CAP_SYS_RESOURCE capability) is
	      required to raise the msg_qbytes value beyond the system parame-
	      ter 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.  The third argument to msgctl() is
	      ignored in this case.

       IPC_INFO (Linux-specific)
	      Return 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)
	      Return 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)
	      Return 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 call-
	      ing  process  does not have read permission on the message queue
	      msqid, and does not have the  CAP_IPC_OWNER  capability  in  the
	      user namespace that governs its IPC namespace.

       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 caller is not privileged (Linux:
	      does not have the CAP_SYS_ADMIN capability).

       EPERM  An attempt (IPC_SET) was made to increase msg_qbytes beyond  the
	      system  parameter	 MSGMNB,  but  the  caller  is	not privileged
	      (Linux: does not have the CAP_SYS_RESOURCE capability).

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

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 filesystem 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 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			     MSGCTL(2)