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man:msgctl

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 4.16 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 2017-09-15 MSGCTL(2)

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