# msgrcv

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## 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 POSIX.1‐2017, _Section 3.226_ , _Message Queue_).
           It is unspecified whether this function interoperates with the
           realtime interprocess communication facilities defined in _Section_
           _2.8_ , _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 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(3p).
    
           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 to the process ID of the calling
               process.
    
            *  **msg_rtime** shall be set to the current time, as described in
               _Section 2.7.1_ , _IPC General Description_.
    


## 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 -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
                  _Section 2.7_ , _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 _Section 2.7_ , _XSI Interprocess_
           _Communication_ can be easily modified to use the alternative
           interfaces.
    


## RATIONALE



           None.
    


## FUTURE DIRECTIONS



           None.
    


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