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msgrcv

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



NAME
       msgop - message operations

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

       int msgsnd(int msqid, struct msgbuf *msgp, size_t msgsz, int msgflg);

       ssize_t  msgrcv(int msqid, struct msgbuf *msgp, size_t msgsz, long msg-
       typ, int msgflg);

DESCRIPTION
       To send or receive a message, the calling process allocates a 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 non-negative integer value.  Messages of  zero  length
       (i.e.,  no  mtext  field)  are  permitted.  The mtype field must have a
       strictly positive integer value that can be used by the receiving  pro-
       cess for message selection (see the section about msgrcv).

       The calling process must have write permission to send and read permis-
       sion to receive a message on the queue.

       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 on the queue, msgsnd succeeds immedi-
       ately.  (The queue capacity is defined by the msg_bytes  field  in  the
       associated data structure for the message queue.  During queue creation
       this field is initialised to MSGMNB bytes, but this limit can be  modi-
       fied  using  msgctl.)  If insufficient space is available on the queue,
       then the default behaviour of msgsnd is to block  until  space  becomes
       available.   If  IPC_NOWAIT is asserted 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).
       (msgsnd and msgrcv are never automatically restarted after being inter-
       rupted 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.

       The system call msgrcv reads a message from the message queue specified
       by  msqid into the msgbuf pointed to by the msgp argument, removing the
       read message from the queue.

       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 if the msgflg argument asserts
       MSG_NOERROR, the message text will be truncated (and the truncated part
       will be lost), otherwise the message isn't removed from the  queue  and
       the system call fails returning 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 on the queue
              of  type  msgtyp  is  read,  unless  MSG_EXCEPT  was asserted in
              msgflg, in which case the first message on the queue of type not
              equal to msgtyp will be read.

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

       The msgflg argument asserts none, one or more (or-ing them) of the fol-
       lowing flags:

              IPC_NOWAIT For immediate return if no message of  the  requested
              type  is  on the queue.  The system call fails with errno set to
              ENOMSG.

              MSG_EXCEPT Used with msgtyp greater than 0  to  read  the  first
              message on 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
       asserted in msgflg, the calling process is blocked  until  one  of  the
       following conditions occurs:

              A message of the desired type is placed on 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.

       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 a failure both functions return -1 with errno indicating the  error,
       otherwise msgsnd returns 0 and msgrvc returns the number of bytes actu-
       ally copied into the mtext array.

ERRORS
       When msgsnd fails, at return errno will be set to one among the follow-
       ing values:

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

       EACCES     The calling process has no write permission on  the  message
                  queue.

       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 has not enough memory to make a copy of the  sup-
                  plied msgbuf.

       When msgrcv fails, at return errno will be set to one among the follow-
       ing values:

       E2BIG      The message text length is greater than msgsz and  MSG_NOER-
                  ROR isn't asserted in msgflg.

       EACCES     The  calling  process  does  not have read permission on the
                  message queue.

       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 received a signal that had to be caught.

       EINVAL     Illegal msgqid value, or msgsz less than 0.

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

NOTES
       The followings are system limits affecting a msgsnd system call:

       MSGMAX     Maximum size for a message text: the implementation set this
                  value to 8192 bytes.

       MSGMNB     Default maximum size in bytes  of  a  message  queue:  16384
                  bytes.   The  super-user  can increase the size of a message
                  queue beyond MSGMNB by a msgctl 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).

CONFORMING TO
       SVr4, SVID.

NOTE
       The pointer argument is declared as struct msgbuf * with libc4,  libc5,
       glibc  2.0,  glibc  2.1.  It  is  declared  as void * (const void * for
       msgsnd()) with glibc 2.2, following the SUSv2.

SEE ALSO
       ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2)



Linux 2.4.18                      2002-06-01                          MSGOP(2)