msgctl(2) — Linux manual page


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

NAME         top

       msgctl - System V message control operations

SYNOPSIS         top

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

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

DESCRIPTION         top

       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;    /* Creation time/time of last
                                                modification via msgctl() */
               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 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_qnum   Number of messages currently on the message queue.

       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

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

       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

       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 struc‐
       ture define the access permissions for the message queue.  The per‐
       mission 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 sys‐

       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) 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 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 mes‐
              sages 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         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 error, -1 is returned with errno indicating the error.

ERRORS         top

       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

       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         top

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

NOTES         top

       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

       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         top

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

COLOPHON         top

       This page is part of release 5.08 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                            2020-04-11                        MSGCTL(2)

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