ADDMHF(3PVM)

NAME
pvm_addmhf, pvm_delmhf - Install or remove message-handler functions.

SYNOPSIS
C	int mhid = pvm_addmhf( int src, int tag, int ctx, int (*func)(int mid) )
	int info = pvm_delmhf( int mhid )

Fortran	Not available

PARAMETERS

src The tid of the sender.

tag The tag sent with the message.

ctx The context sent with the message.

func
    Function to call when message received.

mhid
    Message handler id.

info
    Result code.

mid Message buffer identifier for new active receive buffer.

DESCRIPTION
pvm_addmhf specifies a function that will be called whenever libpvm copies in a message whose header fields of src, tag, and ctx match those provided to pvm_addmhf().

The src and tag fields may be left unspecified (wildcard) by setting to -1.

The calling sequence of the message handler function is:

	int handler( int mid )

Where mid is the bufid of the received message. The handler function can be used to unpack and process the received message buffer. PVM automatically saves the current send and receive buffers, so the handler need not worry about interfering with message buffers in the regular program flow. PVM also sets the current receive buffer to the received message (using pvm_setrbuf()) before invoking the message handler, so the message can be unpacked directly. PVM will free this message buffer when the message handler returns, if the handler has not already done so. But, any other message buffers created by the handler routine should be freed using pvm_freebuf() before returning.

Note: Operation in the message handler context is somewhat restricted. The function may call some PVM functions, but not others. For example, it may compose and send a reply message as shown:

	pvm_packf( "%+ %s", PvmDataDefault, "got your message" );
	pvm_send( tid, tag );
	pvm_freebuf( pvm_setsbuf( 0 ) );

or equivalently:

	pvm_setsbuf( pvm_mkbuf( PvmDataDefault ) );
	pvm_pkstr( "got your message" );
	pvm_send( tid, tag );
	pvm_freebuf( pvm_setsbuf( 0 ) );

but is not allowed to call certain other PVM communication functions, such as multicast or receive.

pvm_addmhf returns the id number of the newly created message handler if successful; this number may be passed to pvm_delmhf to remove the entry. There is no guarantee to the ordering of id values returned by pvm_addmhf, or to the order in which message handlers will be invoked. PvmExists is returned if the handler already exists.

pvm_delmhf returns PvmOk if successful. PvmBadParam if pvm_delmhf is passed a negative id value. PvmNotFound if the id value is not found.

EXAMPLES
	/* Print a message when hosts are added to virtual machine */

	int
	hostAdded( int mid )
	{
		int n;
		pvm_unpackf( "%d", &n );
		printf( "*** %d new hosts just added ***\n", n );
	}

	void
	main()
	{
		int src, tag, ctx;

		. . .

		src = -1;
		tag = 99;
		ctx = -1;

		pvm_addmhf( src, tag, ctx, hostAdded );
		pvm_notify( PvmHostAdd, 99, -1, (int *) NULL );

		. . .
	}

ERRORS
The following error conditions can be returned by pvm_addmhf():

PvmExists
    Can't insert as handler already exists with same (tag, ctx, src) including "wild-cards" (those set to -1)

The following error conditions can be returned by pvm_delmhf():

PvmBadParam
    Invalid (negative) mhid passed in.

PvmNotFound
    Message handler mhid does not exist.

SEE ALSO
pvm_setrbuf(3PVM), pvm_setsbuf(3PVM), pvm_freebuf(3PVM)