NAME
SP_receive, SP_scat_receive - Receive message from Spread
SYNOPSIS
#include <sp.h>
int SP_receive( mailbox mbox, service *service_type, char
sender[MAX_GROUP_NAME], int max_groups, int *num_groups, char
groups[][MAX_GROUP_NAME], int16 *mess_type, int *endian_mismatch, int
max_mess_len, char *mess);
int SP_scat_receive( mailbox mbox, service *service_type, char
sender[MAX_GROUP_NAME], int max_groups, int *num_groups, char
groups[][MAX_GROUP_NAME], int16 *mess_type, int *endian_mismatch,
scatter *scat_mess);
DESCRIPTION
SP_receive is the general purpose receive function for the toolkit.
This receives not only data messages but also membership messages for
the connection. Messages for all groups joined on this connection will
arrive to the same mailbox so a call to SP_receive will get a single
’message’ from any one of the groups. After the receive completes a
number of fields are set to values indicating meta information about
the message (such as groups, mess_type, endianness, type, etc).
This function is the most complex used in Spread because it is the only
way for the system to return information to the application. The
meaning of many of the fields changes depending on whether the message
is a data message or a membership message.
The SP_receive function will block if no messages are available.
The mbox gives which connection to receive a message on. Service_type
is a pointer to a variable of type ’service’ which will be set to the
message type of the message just received. This will be either a
REG_MESSAGE or MEMBERSHIP_MESS, and the specific type. The variable
that service_type points to is also an input parameter and should be
set to either 0, for normal semantics, or DROP_RECV (a defined
constant) if you want to receive the ’non-reliable’ semantics that will
truncate messages if the provided buffers for message data and group
lists are too small. More information about DROP_RECV semantics are
provided later in this man page.
The rest of the parameters differ in meaning depending on the
service_type. If the service_type is a REG_MESSAGE (i.e. data message)
then:
The sender is a pointer to an array of characters of at least
MAX_GROUP_NAME size. This will be set to the name of the sending
connection(its private group name).
The max_groups is the maximum number of groups you have allocated space
for in the ’groups’ array passed to the receive function.
Num_groups is a pointer to an int which will be set to the number of
groups set in the ’groups’ array.
The groups array holds upto max_groups group names, each of which is a
string of at most MAX_GROUP_NAME characters. All of the groups which
are receiving this message will be listed here, unless the array is too
small and then as many as can fit will be listed and the num_groups
value will be set to be negative. For example, if your groups array
could store 5 group names, but a message for 7 groups arrived, the
first five group names would appear in the groups array and num_groups
would be set to -7.
The mess_type field will be set to the message type field the
application sent with the original message, this is only a short int
(16bits). This value is already endian corrected before the application
receives it.
Endian_mismatch will be set to true (1) if the endianness of the
sending machine differs from that of this receiving machine. If the
BUFFER_TOO_SHORT error is returned then the endian_mismatch field will
be set to the size of the incoming message as a negative value. So if
the message requires 300 bytes of buffer space and only 200 bytes was
provided in the mess buffer, endian_mismatch will be set to -300.
The actual message body being received will be passed into the buffer
given by mess which is at least max_mess_len bytes in size. If the
message being received is larger then this buffer the default behaivor
will be to return a BUFFER_TOO_SHORT error and provide the required
length in the endian_mismatch field. If the DROP_RECV flag was passed
in the service_type field, then as much data as possible will be
returned and the extra data will be discarded by the system and the
return value of SP_receive will indictate an error. If the
SP_scat_receive form is used then instead of the mess buffer and length
fields, a single scat_mess scatter structure should be passed to
receive filled in with whatever buffers you wish to receive into and
their lengths. These buffers must be valid memory areas. They will be
filled in by the receive call in the order they are listed.
If this is a MEMB_MESSAGE (i.e. membership message) and it specifically
is a TRANS_MESS, then:
The sender char array will be set to the name of the group for which
the membership change is occuring.
The max_groups and max_mess_len fields are not used and the num_groups
is 0 and groups since there are no normal groups for a transitional
membership, the sender field is used instead. The mess_type field is
set to -1. The endian_mismatch field will be zero since the
transitional does not have any endian issues. The mess field will be
left empty. So in essence the only information you get is the sender
field is set to the group name which received a transitional membership
change. The importance of the TRANS_MEMB_MESS is that it tells the
application that all messages received after it and before the
REG_MEMB_MESS for the same group are ’clean up’ messages to put the
messages in a consistant state before actually changing memberships.
For more explanations of this please see other documentation and
research papers.
If This is a MEMB_MESSAGE (i.e. membership message) and it specifically
is a REG_MEMB_MESS, then:
The sender char array will be set to the name of the group for which
the membership change is occuring.
The max_groups and max_mess_len fields have the same meaning as before,
and the mess_type field will be set to the index of this process in the
array of group members. The endian_mismatch field will again be set to
0 since there are no endian issues with regular memberships.
The groups array and mess body are used to provide two kinds of
membership information about the change that just occured in this
group. The num_groups field will be set to the number of members in
the group in the new membership (i.e. after the change occured).
Correspondingly the groups array will be set to the private group names
of all members of this group in the new membership. This list of names
is always in the same order at all receipients and thus can be used to
deterministically pick a group representative if one is needed by the
application.
The second set of information is stored in the message body and
provides a list of all the private group names of those processes which
came with your process from the old group membership into this new
membership. The data buffer will include the following fields:
group_id;
int num_vs_members;
char vs_set[][MAX_GROUP_NAME];
The location of the beginning of each field is provided by the accessor
functions SP_get_gid_offset_memb_mess, SP_get_num_vs_offset_memb_mess,
and SP_get_vs_set_offset_memb_mess. Each accessor function gives the
byte offset in the message body of the corresponding field.
The vs_set array will have num_vs_members group names, each of which is
a fixed length string. The content of the vs_set array is dependent
upon the type of the membership change:
CAUSED_BY_JOIN:
Vs_set contains the private group of the joining
process.
CAUSED_BY_LEAVE:
Vs_set contains the private group of the leaving
process.
CAUSED_BY_DISCONNECT:
Vs_set contains the private group of the disconnecting
process.
CAUSED_BY_NETWORK:
Vs_set contains the group names of the members of the
new membership who came with me (the current process) to
the new membership. Of course, all new members can be
determined by comparing it with the groups parameter of
the SP_receive call.
If this is a MEMB_MESSAGE (i.e. membership message) and it is neither a
REG_MEMB_MESS or a TRANS_MEMB_MESS, then it represents exactly the
situtation where the member receiving this message has left a group and
this is notification that the leave has occured, thus it is sometimes
called a self-leave message. The simplest test for this is if a
message is CAUSED_BY_LEAVE and REG_MEMB_MESS is FALSE then it is a
self-leave message. TRANS_MEMB_MESS never have a CAUSED_BY_ type
because they only serve to signal upto where SAFE delivery and AGREED
delivery (with no holes) is guaranteed in the complete old group
membership.
The other members of the group this member just left will receive a
normal TRANS_MEMB_MESS, REG_MEMB_MESS pair as described above showing
the membership change.
The fields of SP_receive in this case will be as follows:
The sender char array will be set to the name of the group for which
the membership change is occuring.
The max_groups and max_mess_len fields have the same meaning as before,
and the mess_type and endian_mismatch fields will again be set to 0.
The groups array and mess body are used to provide two kinds of
membership information about the change that just occured in this
group. The num_groups field will be set to 0 and the groups array will
be empty, since this member is no longer part of the group and thus has
no knowledge of it. The mess body will also be empty.
RETURN VALUES
Returns the size of the message received on success or one of the
following errors ( < 0 ):
ILLEGAL_SESSION
The mbox given to receive on was illegal.
ILLEGAL_MESSAGE
The message had an illegal structure, like a scatter not filled
out correctly.
CONNECTION_CLOSED
During communication to receive the message communication
errors occured and the receive could not be completed.
GROUPS_TOO_SHORT
If the groups array is too short to hold the entire list of
groups this message was sent to then this error is returned and
the num_groups field will be set to the negative of the number
of groups needed.
BUFFER_TOO_SHORT
If the message body buffer mess is too short to hold the
message being received then this error is returned and the
endian_mismatch field is set to the negative value of the
required buffer length.
BUGS
None.
AUTHOR
Yair Amir <yairamir@cnds.jhu.edu>
Jonathan Stanton <jonathan@cnds.jhu.edu>
Commedia Project <commedia@cnds.jhu.edu>
SEE ALSO
libsp(3)