MKBUF(3PVM)

NAME
pvm_mkbuf - Creates a new message buffer.

SYNOPSIS
C	int bufid = pvm_mkbuf( int encoding )
Fortran	call pvmfmkbuf( encoding, bufid )

PARAMETERS

encoding
    Integer specifying the buffer's encoding scheme.
    Options in C are:
    	Encoding value		MEANING
    	PvmDataDefault	0	XDR
    	PvmDataRaw	1	no encoding
    	PvmDataInPlace	2	data left in place
    Option names in Fortran are:
    	Encoding value		MEANING
    	PVMDEFAULT	0	XDR
    	PVMRAW	1	no encoding
    	PVMINPLACE	2	data left in place

bufid
    Integer message buffer identifier returned. Values less than zero indicate an error.

DESCRIPTION
The routine pvm_mkbuf creates a new message buffer and sets its encoding status to encoding. If pvm_mkbuf is successful, bufid will be the identifier for the new buffer, which can be used as a send buffer. If some error occurs then bufid will be < 0.

With the default setting XDR encoding is used when packing the message because PVM can not know if the user is going to add a heterogeneous machine before this message is sent. The other options to encoding allow the user to take advantage of knowledge about his virtual machine even when it is heterogeneous. For example, if the user knows that the next message will only be sent to a machine that understands the native format, then he can use PvmDataRaw encoding and save on encoding costs.

PvmDataInPlace encoding specifies that data be left in place during packing. The message buffer only contains the sizes and pointers to the items to be sent. When pvm_send is called the items are copied directly out of the user's memory. This option decreases the number of times a message is copied at the expense of requiring the user to not modify the items between the time they are packed and the time they are sent.

pvm_mkbuf is required if the user wishes to manage multiple message buffers and should be used in conjunction with pvm_freebuf. pvm_freebuf should be called for a send buffer after a message has been sent and is no longer needed.

Receive buffers are created automatically by the pvm_recv and pvm_nrecv routines and do not have to be freed unless they have been explicitly saved with pvm_setrbuf.

Typically multiple send and receive buffers are not needed and the user can simply use the pvm_initsend routine to reset the default send buffer.

There are several cases where multiple buffers are useful. One example where multiple message buffers are needed involves libraries or graphical interfaces that use PVM and interact with a running PVM application but do not want to interfere with the application's own communication.

When multiple buffers are used they generally are made and freed for each message that is packed.

RESTRICTIONS
PvmDataInPlace allows only dense (stride = 1) data in version 3.3. It cannot be used on shared memory (*MP) architectures; a PvmNotImpl error will occur at send time.

EXAMPLES
C:
	bufid = pvm_mkbuf( PvmDataRaw );
	/* send message */
	info = pvm_freebuf( bufid );

Fortran:
	CALL PVMFMKBUF(PVMDEFAULT, MBUF)
*	SEND MESSAGE HERE
	CALL PVMFFREEBUF( MBUF, INFO )

ERRORS
These error conditions can be returned by pvm_mkbuf
	PvmBadParam	giving an invalid encoding value.
	PvmNoMem	Malloc has failed. There is not enough memory
		to create the buffer

SEE ALSO
pvm_initsend(3PVM), pvm_freebuf(3PVM)