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NAME

       msgctl - 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 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() */
               time_t         msg_rtime;    /* Time of last msgrcv() */
               time_t         msg_ctime;    /* Time of last change */
               unsigned long  __msg_cbytes; /* Current number of bytes in
                                               queue (non-standard) */
               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() */
               pid_t          msg_lrpid;    /* PID of last msgrcv() */
           };

       The ipc_perm structure  is  defined  in  <sys/ipc.h>  as  follows  (the
       highlighted fields are settable using IPC_SET):

           struct ipc_perm {
               key_t key;            /* Key supplied to msgget() */
               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_IPC_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.

       IPC_INFO (Linux specific)
              Returns  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 bytes of buffer pool used
                                    to hold message data; unused */
                    int msgmap;  /* Max. # of entries in message
                                    map; unused */
                    int msgmax;  /* Max. # of bytes that can be
                                    written in a single message */
                    int msgmnb;  /* Max. # of bytes that can be written to
                                    queue; used to initialize msg_qbytes
                                    during queue creation (msgget()) */
                    int msgmni;  /* Max. # of message queues */
                    int msgssz;  /* Message segment size; unused */
                    int msgtql;  /* Max. # of messages on all queues
                                    in system; unused */
                    unsigned short int msgseg;
                                 /* Max. # of segments; unused */
                };

              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)
              Returns  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)
              Returns 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.

       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 process  is  not
                  privileged  (Linux:  it  does  not  have  the  CAP_SYS_ADMIN
                  capability).

NOTES

       The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the  ipcs(8)
       program  to  provide information on allocated resources.  In the future
       these may modified or moved to a /proc file system interface.

       Various fields in the struct msqid_ds were shorts under Linux  2.2  and
       have  become  longs  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.)

CONFORMING TO

       SVr4, SVID.  SVID does not document the EIDRM error condition.

SEE ALSO

       msgget(2), msgrcv(2), msgsnd(2), ipc(5), capabilities(7)