msgctl(2) — Linux manual page


msgctl(2)                  System Calls Manual                 msgctl(2)

NAME         top

       msgctl - System V message control operations

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/msg.h>

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

DESCRIPTION         top

       msgctl() performs the control operation specified by op 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 msqid_ds structure are as follows:

              This is an ipc_perm structure (see below) that specifies
              the access permissions on the message queue.

              Time of the last msgsnd(2) system call.

              Time of the last msgrcv(2) system call.

              Time of creation of queue or time of last msgctl() IPC_SET

              Number of bytes in all messages currently on the message
              queue.  This is a nonstandard Linux extension that is not
              specified in POSIX.

              Number of messages currently on the message queue.

              Maximum number of bytes of message text allowed on the
              message queue.

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

              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

       Valid values for op 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

              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.

              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

RETURN VALUE         top

       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 failure, -1 is returned and errno is set to indicate the

ERRORS         top

       EACCES The argument op 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

       EFAULT The argument op 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 op 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 op 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

       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

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr4.

       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 op.)

NOTES         top

       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

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                      msgctl(2)

Pages that refer to this page: ipcrm(1)ipcs(1)ipc(2)msgget(2)msgop(2)syscalls(2)capabilities(7)sysvipc(7)