MSGCTL(2)                 (2020-11-01)                  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 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.

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          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 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 */
              };

          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: l l.
          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 associ-
               ated 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 struc-
               ture 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.

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               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 lim-
               its 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 informa-
               tion 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

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               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

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               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.  Appli-
          cations 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
          This page is part of release 5.10 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/.

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