# msgsnd

## 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



           msgsnd — XSI message send operation
    


## SYNOPSIS



           #include <sys/msg.h>
    
           int msgsnd(int _msqid_ , const void *_msgp_ , size_t _msgsz_ , int _msgflg_);
    


## DESCRIPTION



           The _msgsnd_() 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 _msgsnd_() function shall send a message to the queue associated
           with the message queue identifier specified by _msqid_.
    
           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 a non-zero positive type **long** that
           can be used by the receiving process for message selection.
    
           The structure member _mtext_ is any text of length _msgsz_ bytes. The
           argument _msgsz_ can range from 0 to a system-imposed maximum.
    
           The argument _msgflg_ specifies the action to be taken if one or
           more of the following is true:
    
            *  The number of bytes already on the queue is equal to
               **msg_qbytes** ; see _< sys/msg.h>_.
    
            *  The total number of messages on all queues system-wide is
               equal to the system-imposed limit.
    
           These actions are as follows:
    
            *  If (_msgflg_ & IPC_NOWAIT) is non-zero, the message shall not be
               sent and the calling thread shall return immediately.
    
            *  If (_msgflg_ & IPC_NOWAIT) is 0, the calling thread shall
               suspend execution until one of the following occurs:
    
               --  The condition responsible for the suspension no longer
                   exists, in which case the message is sent.
    
               --  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 the message is not sent 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_ ; see
           _< sys/msg.h>_:
    
            *  **msg_qnum** shall be incremented by 1.
    
            *  **msg_lspid** shall be set to the process ID of the calling
               process.
    
            *  **msg_stime** shall be set to the current time, as described in
               _Section 2.7.1_ , _IPC General Description_.
    


## RETURN VALUE



           Upon successful completion, _msgsnd_() shall return 0; otherwise, no
           message shall be sent, _msgsnd_() shall return -1, and _errno_ shall
           be set to indicate the error.
    


## ERRORS



           The _msgsnd_() function shall fail if:
    
           **EACCES** Operation permission is denied to the calling process; see
                  _Section 2.7_ , _XSI Interprocess Communication_.
    
           **EAGAIN** The message cannot be sent for one of the reasons cited
                  above and (_msgflg_ & IPC_NOWAIT) is non-zero.
    
           **EIDRM** The message queue identifier _msqid_ is removed from the
                  system.
    
           **EINTR** The _msgsnd_() function was interrupted by a signal.
    
           **EINVAL** The value of _msqid_ is not a valid message queue identifier,
                  or the value of _mtype_ is less than 1; or the value of _msgsz_
                  is greater than the system-imposed limit.
    
           _The following sections are informative._
    


## EXAMPLES



       **Sending a Message**
           The following example sends a message to the queue identified by
           the _msqid_ argument (assuming that value has previously been set).
           This call specifies that an error should be reported if no message
           is available.  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;
    
               msg.type = 1;
               strcpy(msg.text, "This is message 1");
               ...
               result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), 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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