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NAME

       msgrcv, msgsnd - System V message queue operations

SYNOPSIS

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

       int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

       ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
                      int msgflg);

DESCRIPTION

       The  msgsnd() and msgrcv() system calls are used, respectively, to send
       messages to, and receive messages from, a System V message queue.   The
       calling  process  must  have  write  permission on the message queue in
       order to send a message, and read permission to receive a message.

       The msgp argument is a pointer to a  caller-defined  structure  of  the
       following general form:

           struct msgbuf {
               long mtype;       /* message type, must be > 0 */
               char mtext[1];    /* message data */
           };

       The  mtext  field  is  an  array  (or  other  structure)  whose size is
       specified by msgsz, a nonnegative  integer  value.   Messages  of  zero
       length (i.e., no mtext field) are permitted.  The mtype field must have
       a strictly positive integer value.  This  value  can  be  used  by  the
       receiving  process  for  message  selection  (see  the  description  of
       msgrcv() below).

   msgsnd()
       The msgsnd() system call appends a copy of the message  pointed  to  by
       msgp to the message queue whose identifier is specified by msqid.

       If  sufficient  space  is  available  in  the  queue, msgsnd() succeeds
       immediately.  (The queue capacity is defined by the msg_qbytes field in
       the  associated  data  structure  for  the message queue.  During queue
       creation this field is initialized to MSGMNB bytes, but this limit  can
       be  modified  using  msgctl(2).)  If insufficient space is available in
       the queue, then the default behavior of  msgsnd()  is  to  block  until
       space  becomes  available.   If IPC_NOWAIT is specified in msgflg, then
       the call instead fails with the error EAGAIN.

       A blocked msgsnd() call may also fail if:

       * the queue is removed, in which case the system call fails with  errno
         set to EIDRM; or

       * a  signal  is  caught, in which case the system call fails with errno
         set  to  EINTR;see  signal(7).   (msgsnd()  is  never   automatically
         restarted  after being interrupted by a signal handler, regardless of
         the setting  of  the  SA_RESTART  flag  when  establishing  a  signal
         handler.)

       Upon  successful completion the message queue data structure is updated
       as follows:

              msg_lspid is set to the process ID of the calling process.

              msg_qnum is incremented by 1.

              msg_stime is set to the current time.

   msgrcv()
       The msgrcv() system call removes a message from the queue specified  by
       msqid and places it in the buffer pointed to by msgp.

       The  argument  msgsz specifies the maximum size in bytes for the member
       mtext of the structure pointed to by the msgp argument.  If the message
       text  has  length  greater  than  msgsz,  then  the behavior depends on
       whether  MSG_NOERROR  is  specified  in  msgflg.   If  MSG_NOERROR   is
       specified,  then  the message text will be truncated (and the truncated
       part will be lost); if MSG_NOERROR is not specified, then  the  message
       isn't  removed  from  the  queue and the system call fails returning -1
       with errno set to E2BIG.

       Unless MSG_COPY is specified in msgflg (see below), the msgtyp argument
       specifies the type of message requested, as follows:

       * If msgtyp is 0, then the first message in the queue is read.

       * If  msgtyp  is greater than 0, then the first message in the queue of
         type msgtyp is read, unless MSG_EXCEPT was specified  in  msgflg,  in
         which case the first message in the queue of type not equal to msgtyp
         will be read.

       * If msgtyp is less than 0, then the first message in  the  queue  with
         the  lowest  type  less than or equal to the absolute value of msgtyp
         will be read.

       The msgflg argument is a bit mask constructed by ORing together zero or
       more of the following flags:

       IPC_NOWAIT
              Return immediately if no message of the requested type is in the
              queue.  The system call fails with errno set to ENOMSG.

       MSG_COPY (since Linux 3.8)
              Nondestructively fetch a copy of  the  message  at  the  ordinal
              position   in  the  queue  specified  by  msgtyp  (messages  are
              considered to be numbered starting at 0).

              This flag must be specified in conjunction with IPC_NOWAIT, with
              the  result  that, if there is no message available at the given
              position, the call fails  immediately  with  the  error  ENOMSG.
              Because  they  alter  the  meaning of msgtyp in orthogonal ways,
              MSG_COPY and MSG_EXCEPT may not both be specified in msgflg.

              The MSG_COPY flag was added for the implementation of the kernel
              checkpoint-restore  facility and is available only if the kernel
              was built with the CONFIG_CHECKPOINT_RESTORE option.

       MSG_EXCEPT
              Used with msgtyp greater than 0 to read the first message in the
              queue with message type that differs from msgtyp.

       MSG_NOERROR
              To truncate the message text if longer than msgsz bytes.

       If  no  message of the requested type is available and IPC_NOWAIT isn't
       specified in msgflg, the calling process is blocked until  one  of  the
       following conditions occurs:

       * A message of the desired type is placed in the queue.

       * The  message  queue  is  removed  from the system.  In this case, the
         system call fails with errno set to EIDRM.

       * The calling process catches a signal.  In this case, the system  call
         fails  with  errno  set  to  EINTR.  (msgrcv() is never automatically
         restarted after being interrupted by a signal handler, regardless  of
         the  setting  of  the  SA_RESTART  flag  when  establishing  a signal
         handler.)

       Upon successful completion the message queue data structure is  updated
       as follows:

              msg_lrpid is set to the process ID of the calling process.

              msg_qnum is decremented by 1.

              msg_rtime is set to the current time.

RETURN VALUE

       On  failure  both  functions return -1 with errno indicating the error,
       otherwise msgsnd() returns 0 and msgrcv() returns the number  of  bytes
       actually copied into the mtext array.

ERRORS

       When  msgsnd()  fails,  errno  will  be  set to one among the following
       values:

       EACCES The calling process  does  not  have  write  permission  on  the
              message queue, and does not have the CAP_IPC_OWNER capability.

       EAGAIN The  message  can't  be sent due to the msg_qbytes limit for the
              queue and IPC_NOWAIT was specified in msgflg.

       EFAULT The address pointed to by msgp isn't accessible.

       EIDRM  The message queue was removed.

       EINTR  Sleeping on a full message queue condition, the process caught a
              signal.

       EINVAL Invalid  msqid  value,  or  nonpositive  mtype value, or invalid
              msgsz value (less than  0  or  greater  than  the  system  value
              MSGMAX).

       ENOMEM The  system  does  not  have enough memory to make a copy of the
              message pointed to by msgp.

       When msgrcv() fails, errno will be  set  to  one  among  the  following
       values:

       E2BIG  The  message  text  length is greater than msgsz and MSG_NOERROR
              isn't specified in msgflg.

       EACCES The calling process does not have read permission on the message
              queue, and does not have the CAP_IPC_OWNER capability.

       EAGAIN No  message  was  available  in  the  queue  and  IPC_NOWAIT was
              specified in msgflg.

       EFAULT The address pointed to by msgp isn't accessible.

       EIDRM  While the process was sleeping to receive a message, the message
              queue was removed.

       EINTR  While the process was sleeping to receive a message, the process
              caught a signal; see signal(7).

       EINVAL msgqid was invalid, or msgsz was less than 0.

       EINVAL (since Linux 3.14)
              msgflg specified MSG_COPY, but not IPC_NOWAIT.

       EINVAL (since Linux 3.14)
              msgflg specified both MSG_COPY and MSG_EXCEPT.

       ENOMSG IPC_NOWAIT was  specified  in  msgflg  and  no  message  of  the
              requested type existed on the message queue.

       ENOMSG IPC_NOWAIT  and  MSG_COPY were specified in msgflg and the queue
              contains less than msgtyp messages.

       ENOSYS (since Linux 3.8)
              MSG_COPY was specified in msgflg, and this kernel was configured
              without CONFIG_CHECKPOINT_RESTORE.

CONFORMING TO

       SVr4, POSIX.1-2001.

       The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their definitions
       can be obtained by defining the _GNU_SOURCE feature test macro.

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 msgp argument is declared as struct msgbuf  *  with  libc4,  libc5,
       glibc  2.0,  glibc  2.1.   It  is declared as void * with glibc 2.2 and
       later, as required by SUSv2 and SUSv3.

       The following limits on message queue  resources  affect  the  msgsnd()
       call:

       MSGMAX Maximum  size  for  a  message  text: 8192 bytes (on Linux, this
              limit can be read and modified via /proc/sys/kernel/msgmax).

       MSGMNB Default maximum size in bytes of a message  queue:  16384  bytes
              (on   Linux,   this   limit   can   be  read  and  modified  via
              /proc/sys/kernel/msgmnb).  The superuser can increase  the  size
              of a message queue beyond MSGMNB by a msgctl(2) system call.

       The  implementation has no intrinsic limits for the system wide maximum
       number of message headers (MSGTQL) and for the system wide maximum size
       in bytes of the message pool (MSGPOOL).

BUGS

       In  Linux  3.13  and  earlier, if msgrcv() was called with the MSG_COPY
       flag, but without IPC_NOWAIT, and the message queue contained less than
       msgtyp  messages,  then  the call would block until the next message is
       written to the queue.  At that point, the call would return a  copy  of
       the  message,  regardless  of  whether  that message was at the ordinal
       position msgtyp.  This bug is fixed in Linux 3.14.

       Specifying both MSG_COPY and MSC_EXCEPT in msgflg is  a  logical  error
       (since  these  flags  impose  different interpretations on msgtyp).  In
       Linux 3.13 and earlier, this error was not diagnosed by msgrcv().  This
       bug is fixed in Linux 3.14.

SEE ALSO

       msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)

COLOPHON

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



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