---
layout: page
title: fi_msg(3)
tagline: Libfabric Programmer's Manual
---
{% include JB/setup %}

# NAME

fi_msg - Message data transfer operations

fi_recv / fi_recvv / fi_recvmsg
:   Post a buffer to receive an incoming message

fi_send / fi_sendv / fi_sendmsg  
fi_inject / fi_senddata
:   Initiate an operation to send a message

# SYNOPSIS

```c
#include <rdma/fi_endpoint.h>

ssize_t fi_recv(struct fid_ep *ep, void * buf, size_t len,
	void *desc, fi_addr_t src_addr, void *context);

ssize_t fi_recvv(struct fid_ep *ep, const struct iovec *iov, void **desc,
	size_t count, fi_addr_t src_addr, void *context);

ssize_t fi_recvmsg(struct fid_ep *ep, const struct fi_msg *msg,
	uint64_t flags);

ssize_t fi_send(struct fid_ep *ep, const void *buf, size_t len,
	void *desc, fi_addr_t dest_addr, void *context);

ssize_t fi_sendv(struct fid_ep *ep, const struct iovec *iov,
	void **desc, size_t count, fi_addr_t dest_addr, void *context);

ssize_t fi_sendmsg(struct fid_ep *ep, const struct fi_msg *msg,
	uint64_t flags);

ssize_t fi_inject(struct fid_ep *ep, const void *buf, size_t len,
	fi_addr_t dest_addr);

ssize_t fi_senddata(struct fid_ep *ep, const void *buf, size_t len,
	void *desc, uint64_t data, fi_addr_t dest_addr, void *context);

ssize_t fi_injectdata(struct fid_ep *ep, const void *buf, size_t len,
	uint64_t data, fi_addr_t dest_addr);
```

# ARGUMENTS

*ep*
: Fabric endpoint on which to initiate send or post receive buffer.

*buf*
: Data buffer to send or receive.

*len*
: Length of data buffer to send or receive, specified in bytes.  Valid
  transfers are from 0 bytes up to the endpoint's max_msg_size.

*iov*
: Vectored data buffer.

*count*
: Count of vectored data entries.

*desc*
: Descriptor associated with the data buffer.  See [`fi_mr`(3)](fi_mr.3.html).

*data*
: Remote CQ data to transfer with the sent message.

*dest_addr*
: Destination address for connectionless transfers.  Ignored for
  connected endpoints.

*src_addr*
: Source address to receive from for connectionless transfers.  Applies
  only to connectionless endpoints with the FI_DIRECTED_RECV capability
  enabled, otherwise this field is ignored.  If set to FI_ADDR_UNSPEC,
  any source address may match.

*msg*
: Message descriptor for send and receive operations.

*flags*
: Additional flags to apply for the send or receive operation.

*context*
: User specified pointer to associate with the operation.  This parameter is
  ignored if the operation will not generate a successful completion, unless
  an op flag specifies the context parameter be used for required input.

# DESCRIPTION

The send functions -- fi_send, fi_sendv, fi_sendmsg,
fi_inject, and fi_senddata -- are used to
transmit a message from one endpoint to another endpoint.  The main
difference between send functions are the number and type of
parameters that they accept as input.  Otherwise, they perform the
same general function.  Messages sent using fi_msg operations are
received by a remote endpoint into a buffer posted to receive such
messages.

The receive functions -- fi_recv, fi_recvv, fi_recvmsg --
post a data buffer to an endpoint to receive inbound messages.
Similar to the send operations, receive operations operate
asynchronously.  Users should not touch the posted data buffer(s)
until the receive operation has completed.

An endpoint must be enabled before an application can post send
or receive operations to it.  For connected endpoints, receive
buffers may be posted prior to connect or accept being called on
the endpoint.  This ensures that buffers are available to receive
incoming data immediately after the connection has been established.

Completed message operations are reported to the user through one or
more event collectors associated with the endpoint.  Users provide
context which are associated with each operation, and is returned to
the user as part of the event completion.  See fi_cq for completion
event details.

## fi_send

The call fi_send transfers the data contained in the user-specified
data buffer to a remote endpoint, with message boundaries being
maintained.  For connection based endpoints (FI_EP_MSG) the local
endpoint must be connected to a remote endpoint or destination before
fi_send is called.  Unless the endpoint has been configured
differently, the data buffer passed into fi_send must not be touched
by the application until the fi_send call completes asynchronously.

## fi_sendv

The fi_sendv call adds support for a scatter-gather list to fi_send.
The fi_sendv transfers the set of data buffers
referenced by the iov parameter to a remote endpoint as a single
message.

## fi_sendmsg

The fi_sendmsg call supports data transfers over both connected and
unconnected endpoints, with the ability to control the send operation
per call through the use of flags.  The fi_sendmsg function takes a
`struct fi_msg` as input.

```c
struct fi_msg {
	const struct iovec *msg_iov; /* scatter-gather array */
	void               **desc;   /* local request descriptors */
	size_t             iov_count;/* # elements in iov */
	fi_addr_t          addr;     /* optional endpoint address */
	void               *context; /* user-defined context */
	uint64_t           data;     /* optional message data */
};
```

## fi_inject

The send inject call is an optimized version of fi_send with the
following characteristics.  The data buffer is available for reuse
immediately on return from the call, and no CQ entry will be written
if the transfer completes successfully.

Conceptually, this means that the fi_inject function behaves as if
the FI_INJECT transfer flag were set, selective completions are enabled,
and the FI_COMPLETION flag is not specified.  Note that the CQ entry
will be suppressed even if the default behavior of the endpoint is
to write CQ entries for all successful completions.  See the flags
discussion below for more details. The requested message size that
can be used with fi_inject is limited by inject_size.

## fi_senddata

The send data call is similar to fi_send, but allows for the sending
of remote CQ data (see FI_REMOTE_CQ_DATA flag) as part of the
transfer.

## fi_injectdata

The inject data call is similar to fi_inject, but allows for the sending
of remote CQ data (see FI_REMOTE_CQ_DATA flag) as part of the
transfer.

## fi_recv

The fi_recv call posts a data buffer to the receive queue of the
corresponding endpoint.  Posted receives are searched in the order in
which they were posted in order to match sends.
Message boundaries are maintained.  The order in which
the receives complete is dependent on
the endpoint type and protocol.  For unconnected endpoints, the
src_addr parameter can be used to indicate that a buffer should be
posted to receive incoming data from a specific remote endpoint.

## fi_recvv

The fi_recvv call adds support for a scatter-gather list to fi_recv.
The fi_recvv posts the set of data buffers referenced by the iov
parameter to a receive incoming data.

## fi_recvmsg

The fi_recvmsg call supports posting buffers over both connected and
unconnected endpoints, with the ability to control the receive
operation per call through the use of flags.  The fi_recvmsg function
takes a struct fi_msg as input.

# FLAGS

The fi_recvmsg and fi_sendmsg calls allow the user to specify flags
which can change the default message handling of the endpoint.  Flags
specified with fi_recvmsg / fi_sendmsg override most flags previously
configured with the endpoint, except where noted (see fi_endpoint.3).
The following list of flags are usable with fi_recvmsg and/or
fi_sendmsg.

*FI_REMOTE_CQ_DATA*
: Applies to fi_sendmsg and fi_senddata.  Indicates
  that remote CQ data is available and should be sent as part of the
  request.  See fi_getinfo for additional details on
  FI_REMOTE_CQ_DATA.

*FI_CLAIM*
: Applies to posted receive operations for endpoints configured
  for FI_BUFFERED_RECV or FI_VARIABLE_MSG.  This flag is used to
  retrieve a message that was buffered by the provider.  See the
  Buffered Receives section for details.

*FI_COMPLETION*
: Indicates that a completion entry should be generated for the
  specified operation.  The endpoint must be bound to a completion
  queue with FI_SELECTIVE_COMPLETION that corresponds to the
  specified operation, or this flag is ignored.

*FI_DISCARD*
: Applies to posted receive operations for endpoints configured
  for FI_BUFFERED_RECV or FI_VARIABLE_MSG.  This flag is used to
  free a message that was buffered by the provider.  See the
  Buffered Receives section for details.

*FI_MORE*
: Indicates that the user has additional requests that will
  immediately be posted after the current call returns.  Use of this
  flag may improve performance by enabling the provider to optimize
  its access to the fabric hardware.

*FI_INJECT*
: Applies to fi_sendmsg.  Indicates that the outbound data buffer
  should be returned to user immediately after the send call returns,
  even if the operation is handled asynchronously.  This may require
  that the underlying provider implementation copy the data into a
  local buffer and transfer out of that buffer. This flag can only
  be used with messages smaller than inject_size.

*FI_MULTI_RECV*
: Applies to posted receive operations.  This flag allows the user to
  post a single buffer that will receive multiple incoming messages.
  Received messages will be packed into the receive buffer until the
  buffer has been consumed.  Use of this flag may cause a single
  posted receive operation to generate multiple events as messages are
  placed into the buffer.  The placement of received data into the
  buffer may be subjected to provider specific alignment restrictions.

  The buffer will be released by the provider when the available buffer
  space falls below the specified minimum (see FI_OPT_MIN_MULTI_RECV).
  Note that an entry to the associated receive completion queue will
  always be generated when the buffer has been consumed, even if other
  receive completions have been suppressed (i.e. the Rx context has been
  configured for FI_SELECTIVE_COMPLETION).  See the FI_MULTI_RECV
  completion flag [`fi_cq`(3)](fi_cq.3.html).

*FI_INJECT_COMPLETE*
: Applies to fi_sendmsg.  Indicates that a completion should be
  generated when the source buffer(s) may be reused.
  
*FI_TRANSMIT_COMPLETE*
: Applies to fi_sendmsg.  Indicates that a completion should not be
  generated until the operation has been successfully transmitted and
  is no longer being tracked by the provider.

*FI_DELIVERY_COMPLETE*
: Applies to fi_sendmsg.  Indicates that a completion should be
  generated when the operation has been processed by the destination.

*FI_FENCE*
: Applies to transmits.  Indicates that the requested operation, also
  known as the fenced operation, and any operation posted after the
  fenced operation will be deferred until all previous operations
  targeting the same peer endpoint have completed.  Operations posted
  after the fencing will see and/or replace the results of any
  operations initiated prior to the fenced operation.
  
  The ordering of operations starting at the posting of the fenced
  operation (inclusive) to the posting of a subsequent fenced operation
  (exclusive) is controlled by the endpoint's ordering semantics.

*FI_MULTICAST*
: Applies to transmits.  This flag indicates that the address specified
  as the data transfer destination is a multicast address.  This flag must
  be used in all multicast transfers, in conjunction with a multicast
  fi_addr_t.

# Buffered Receives

Buffered receives indicate that the networking layer allocates and
manages the data buffers used to receive network data transfers.  As
a result, received messages must be copied from the network buffers
into application buffers for processing.  However, applications can
avoid this copy if they are able to process the message in place
(directly from the networking buffers).

Handling buffered receives differs based on the size of the message
being sent.  In general, smaller messages are passed directly to the
application for processing.  However, for large messages, an application
will only receive the start of the message and must claim the rest.
The details for how small messages are reported and large messages may
be claimed are described below.

When a provider receives a message, it will write an entry to the completion
queue associated with the receiving endpoint.  For discussion purposes,
the completion queue is assumed to be configured for FI_CQ_FORMAT_DATA.
Since buffered receives are not associated with application posted buffers,
the CQ entry op_context will point to a struct fi_recv_context.

{% highlight c %}
struct fi_recv_context {
	struct fid_ep *ep;
	void *context;
};
{% endhighlight %}

The 'ep' field will point to the receiving endpoint or Rx context, and
'context' will be NULL.  The CQ entry's 'buf' will point to a provider
managed buffer where the start of the received message is located, and
'len' will be set to the total size of the message.

The maximum sized message that a provider can buffer is limited by
an FI_OPT_BUFFERED_LIMIT. This threshold can be obtained and may be adjusted
by the application using the fi_getopt and fi_setopt calls, respectively.
Any adjustments must be made prior to enabling the endpoint. The CQ entry 'buf'
will point to a buffer of received data. If the sent message is larger than the
buffered amount, the CQ entry 'flags' will have the FI_MORE bit set. When the
FI_MORE bit is set, 'buf' will reference at least FI_OPT_BUFFERED_MIN bytes
of data (see fi_endpoint.3 for more info).

After being notified that a buffered receive has arrived,
applications must either claim or discard the message.  Typically,
small messages are processed and discarded, while large messages
are claimed.  However, an application is free to claim or discard any
message regardless of message size.

To claim a message, an application must post a receive operation with the
FI_CLAIM flag set.  The struct fi_recv_context returned as part of the
notification must be provided as the receive operation's context.  The
struct fi_recv_context contains a 'context' field.  Applications may
modify this field prior to claiming the message.  When the claim
operation completes, a standard receive completion entry will be
generated on the completion queue.  The 'context' of the associated
CQ entry will be set to the 'context' value passed in through
the fi_recv_context structure, and the CQ entry flags will have the
FI_CLAIM bit set.

Buffered receives that are not claimed must be discarded by the application
when it is done processing the CQ entry data.  To discard a message, an
application must post a receive operation with the FI_DISCARD flag set.
The struct fi_recv_context returned as part of the notification must be
provided as the receive operation's context. When the FI_DISCARD flag is set
for a receive operation, the receive input buffer(s) and length parameters
are ignored.

IMPORTANT: Buffered receives must be claimed or discarded in a timely manner.
Failure to do so may result in increased memory usage for network buffering
or communication stalls.  Once a buffered receive has been claimed or
discarded, the original CQ entry 'buf' or struct fi_recv_context data may no
longer be accessed by the application.

The use of the FI_CLAIM and FI_DISCARD operation flags is also
described with respect to tagged message transfers in fi_tagged.3.
Buffered receives of tagged messages will include the message tag as part
of the CQ entry, if available.

The handling of buffered receives follows all message ordering
restrictions assigned to an endpoint.  For example, completions
may indicate the order in which received messages arrived at the
receiver based on the endpoint attributes.

# Variable Length Messages

Variable length messages, or simply variable messages, are transfers
where the size of the message is unknown to the receiver prior to the
message being sent.  It indicates that the recipient of a message does
not know the amount of data to expect prior to the message arriving.
It is most commonly used when the size of message transfers varies
greatly, with very large messages interspersed with much smaller
messages, making receive side message buffering difficult to manage.
Variable messages are not subject to max message length
restrictions (i.e. struct fi_ep_attr::max_msg_size limits), and may
be up to the maximum value of size_t (e.g. SIZE_MAX) in length.

Variable length messages support requests that the provider allocate and
manage the network message buffers.  As a result, the application
requirements and provider behavior is identical as those defined
for supporting the FI_BUFFERED_RECV mode bit.  See the Buffered
Receive section above for details.  The main difference is that buffered
receives are limited by the fi_ep_attr::max_msg_size threshold, whereas
variable length messages are not.

Support for variable messages is indicated through the FI_VARIABLE_MSG
capability bit.

# NOTES

If an endpoint has been configured with FI_MSG_PREFIX, the application
must include buffer space of size msg_prefix_size, as specified by the
endpoint attributes.  The prefix buffer must occur at the start of the
data referenced by the buf parameter, or be referenced by the first IO vector.
Message prefix space cannot be split between multiple IO vectors.  The size
of the prefix buffer should be included as part of the total buffer length.

# RETURN VALUE

Returns 0 on success. On error, a negative value corresponding to fabric
errno is returned. Fabric errno values are defined in
`rdma/fi_errno.h`.

See the discussion below for details handling FI_EAGAIN.

# ERRORS

*-FI_EAGAIN*
: Indicates that the underlying provider currently lacks the resources
  needed to initiate the requested operation.  The reasons for a provider
  returning FI_EAGAIN are varied.  However, common reasons include
  insufficient internal buffering or full processing queues.

  Insufficient internal buffering is often associated with operations that
  use FI_INJECT.  In such cases, additional buffering may become available as
  posted operations complete.

  Full processing queues may be a temporary state related to local
  processing (for example, a large message is being transferred), or may be
  the result of flow control.  In the latter case, the queues may remain
  blocked until additional resources are made available at the remote side
  of the transfer.

  In all cases, the operation may be retried after additional resources become
  available.  It is strongly recommended that applications check for transmit
  and receive completions after receiving FI_EAGAIN as a return value,
  independent of the operation which failed.  This is particularly important
  in cases where manual progress is employed, as acknowledgements or flow
  control messages may need to be processed in order to resume execution.

# SEE ALSO

[`fi_getinfo`(3)](fi_getinfo.3.html),
[`fi_endpoint`(3)](fi_endpoint.3.html),
[`fi_domain`(3)](fi_domain.3.html),
[`fi_cq`(3)](fi_cq.3.html)
