Man Linux: Main Page and Category List

NAME

       msgrcv - XSI message receive operation

SYNOPSIS

       #include <sys/msg.h>

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

DESCRIPTION

       The  msgrcv()  function  operates  on  XSI message queues (see the Base
       Definitions volume  of  IEEE Std 1003.1-2001,  Section  3.224,  Message
       Queue).  It is unspecified whether this function interoperates with the
       realtime interprocess communication facilities defined in Realtime .

       The msgrcv() function shall read a message from  the  queue  associated
       with  the  message  queue identifier specified by msqid and place it in
       the user-defined buffer pointed to by msgp.

       The application shall ensure that the argument msgp points to  a  user-
       defined  buffer that contains first a field of type long specifying the
       type of the message, and then a data portion that holds the data  bytes
       of  the  message.  The structure below is an example of what this user-
       defined buffer might look like:

              struct mymsg {
                  long    mtype;     /* Message type. */
                  char    mtext[1];  /* Message text. */
              }

       The structure member mtype is the received message’s type as  specified
       by the sending process.

       The structure member mtext is the text of the message.

       The  argument msgsz specifies the size in bytes of mtext.  The received
       message shall be truncated to msgsz bytes if it is  larger  than  msgsz
       and  (msgflg  &  MSG_NOERROR)  is  non-zero.  The truncated part of the
       message shall be lost and no indication  of  the  truncation  shall  be
       given to the calling process.

       If  the  value  of  msgsz  is  greater  than {SSIZE_MAX}, the result is
       implementation-defined.

       The argument msgtyp specifies the type of message requested as follows:

        * If msgtyp is 0, the first message on the queue shall be received.

        * If  msgtyp is greater than 0, the first message of type msgtyp shall
          be received.

        * If msgtyp is less than 0, the first message of the lowest type  that
          is  less  than  or  equal  to  the absolute value of msgtyp shall be
          received.

       The argument msgflg specifies the action to be taken if  a  message  of
       the desired type is not on the queue. These are as follows:

        * If  (msgflg  &  IPC_NOWAIT)  is  non-zero,  the calling thread shall
          return immediately with a return  value  of  -1  and  errno  set  to
          [ENOMSG].

        * If  (msgflg  &  IPC_NOWAIT)  is  0, the calling thread shall suspend
          execution until one of the following occurs:

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

           * The message queue identifier msqid is removed  from  the  system;
             when  this  occurs,  errno  shall  be set equal to [EIDRM] and -1
             shall be returned.

           * The calling thread receives a signal that is  to  be  caught;  in
             this  case  a  message  is  not  received  and the calling thread
             resumes execution in the manner prescribed in sigaction() .

       Upon successful  completion,  the  following  actions  are  taken  with
       respect to the data structure associated with msqid:

        * msg_qnum shall be decremented by 1.

        * msg_lrpid  shall  be  set  equal  to  the  process ID of the calling
          process.

        * msg_rtime shall be set equal to the current time.

RETURN VALUE

       Upon successful completion, msgrcv() shall return a value equal to  the
       number  of  bytes  actually placed into the buffer mtext. Otherwise, no
       message shall be received, msgrcv() shall return (ssize_t)-1, and errno
       shall be set to indicate the error.

ERRORS

       The msgrcv() function shall fail if:

       E2BIG  The  value  of  mtext  is  greater  than  msgsz  and  (msgflg  &
              MSG_NOERROR) is 0.

       EACCES Operation permission is denied to the calling process;  see  XSI
              Interprocess Communication .

       EIDRM  The message queue identifier msqid is removed from the system.

       EINTR  The msgrcv() function was interrupted by a signal.

       EINVAL msqid is not a valid message queue identifier.

       ENOMSG The  queue  does  not  contain a message of the desired type and
              (msgflg & IPC_NOWAIT) is non-zero.

       The following sections are informative.

EXAMPLES

   Receiving a Message
       The following example receives the first message on the queue (based on
       the  value  of  the msgtyp argument, 0). The queue is identified by the
       msqid argument (assuming that the value has previously been set).  This
       call  specifies  that  an  error  should  be  reported if no message is
       available, but not if the message is too large.  The  message  size  is
       calculated directly using the sizeof operator.

              #include <sys/msg.h>
              ...
              int result;
              int msqid;
              struct message {
                  long type;
                  char text[20];
              } msg;
              long msgtyp = 0;
              ...
              result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
                       msgtyp, MSG_NOERROR | IPC_NOWAIT);

APPLICATION USAGE

       The   POSIX  Realtime  Extension  defines  alternative  interfaces  for
       interprocess communication (IPC). Application developers  who  need  to
       use  IPC should design their applications so that modules using the IPC
       routines described in XSI  Interprocess  Communication  can  be  easily
       modified to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       XSI Interprocess Communication , Realtime , mq_close() , mq_getattr() ,
       mq_notify() , mq_open() , mq_receive() ,  mq_send()  ,  mq_setattr()  ,
       mq_unlink()  ,  msgctl() , msgget() , msgsnd() , sigaction() , the Base
       Definitions volume of IEEE Std 1003.1-2001, <sys/msg.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the  referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .