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           #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);


           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 speci-
           fied 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 receiv-
           ing  process  for  message  selection  (see the description of msgrcv()
           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 imme-
           diately.  (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  modi-
           fied  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
           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  speci-
           fied,  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:
                  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 consid-
                  ered 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.
                  Used with msgtyp greater than 0 to read the first message in the
                  queue with message type that differs from msgtyp.
             the  setting  of  the SA_RESTART flag when establishing a signal han-
           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.


           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.


           When  msgsnd() fails, errno will be set to one among the following val-
           EACCES The calling process does not have write permission on  the  mes-
                  sage 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
           EINVAL Invalid  msqid  value,  or  nonpositive  mtype value, or invalid
                  msgsz value (less than 0 or greater than the system  value  MSG-
           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  val-
           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 speci-
                  fied in msgflg.
           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.


           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.


           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()
           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).


           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.

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