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 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 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.  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 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 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.

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 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),
       svipc(7)

COLOPHON
       This page is part of release 4.14 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)