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NAME

       msgrcv, msgsnd - message 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 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  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.

       The argument msgtyp 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_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.

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

CONFORMING TO

       SVr4, POSIX.1-2001.

NOTES

       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).

SEE ALSO

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

COLOPHON

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       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.