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

       msgctl - System V 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 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:

              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_SYS_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.  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.

       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  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).

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

       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.)

       msgget(2),   msgrcv(2),   msgsnd(2),  capabilities(7),  mq_overview(7),

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Linux                             2017-09-15                         MSGCTL(2)
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