MSGCTL(2)                   Linux Programmer's Manual                  MSGCTL(2)

       msgctl - System V message control operations

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/msg.h>

       int msgctl(int msqid, int cmd, struct msqid_ds *buf);

       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

       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

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

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

       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:

              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.

              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)

              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.

              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

       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).

       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.

       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).

       POSIX.1-2001, POSIX.1-2008, SVr4.

       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.)

       msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),

       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

Linux                              2020-11-01                          MSGCTL(2)