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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 creation or last
                                              modification by msgctl() */
               unsigned long   msg_cbytes; /* # of bytes in queue */
               msgqnum_t       msg_qnum;   /* # number of messages in queue */
               msglen_t        msg_qbytes; /* Maximum # of bytes in queue */
               pid_t           msg_lspid;  /* PID of last msgsnd(2) */
               pid_t           msg_lrpid;  /* PID of last msgrcv(2) */
           };

       The fields of the msgid_ds structure are as follows:

       msg_perm   This is an ipc_perm structure (see below) that specifies the access permissions
                  on the message queue.

       msg_stime  Time of the last msgsnd(2) system call.

       msg_rtime  Time of the last msgrcv(2) system call.

       msg_ctime  Time of creation of queue or time of last msgctl() IPC_SET operation.

       msg_cbytes Number  of  bytes  in  all  messages currently on the message queue.  This is a
                  nonstandard Linux extension that is not specified in POSIX.

       msg_qnum   Number of messages currently on the message queue.

       msg_qbytes Maximum number of bytes of message text allowed on the message queue.

       msg_lspid  ID of the process that performed the last msgsnd(2) system call.

       msg_lrpid  ID of the process that performed the last msgrcv(2) system call.

       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 */
           };

       The least significant 9 bits of the mode field of the ipc_perm structure define the access
       permissions for the message queue.  The permission bits are as follows:

       0400   Read by user
       0200   Write by user
       0040   Read by group
       0020   Write by group
       0004   Read by others
       0002   Write by others

       Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.

       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_SYS_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.  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 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 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)
              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.

       MSG_STAT_ANY (Linux-specific, since Linux 4.17)
              Return a msqid_ds structure as for MSG_STAT.  However, msg_perm.mode is not checked
              for  read access for msqid meaning that any user can employ this operation (just as
              any user may read /proc/sysvipc/msg to obtain the same information).

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 or MSG_STAT_ANY operations to obtain information about all queues on the
       system.)  A successful MSG_STAT or MSG_STAT_ANY 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 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), sysvipc(7)

COLOPHON

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