msgop


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 caller-defined structure of the  fol-
       lowing 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()
       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 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
         the setting of the SA_RESTART flag when establishing  a  signal  han-

       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  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.

       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  sys-
         tem 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  han-
         dler.)

       actually copied into the mtext array.

ERRORS
       When msgsnd() fails, errno will be set to one among the following  val-
       ues:

       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
              signal.

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

       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-
       ues:

       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.

       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.

       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
       This  page  is  part of release 3.54 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/.



Linux                             2012-05-31                          MSGOP(2)
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