T_SND(3NSL) Networking Services Library Functions T_SND(3NSL)


t_snd - send data or expedited data over a connection


#include <xti.h>

int t_snd(int fd, void *buf, unsigned int nbytes, int flags);


This routine is part of the XTI interfaces which evolved from the TLI
interfaces. XTI represents the future evolution of these interfaces.
However, TLI interfaces are supported for compatibility. When using a TLI
routine that has the same name as an XTI routine, the tiuser.h header
file must be used. Refer to the TLI COMPATIBILITY section for a
description of differences between the two interfaces.

This function is used to send either normal or expedited data. The
argument fd identifies the local transport endpoint over which data
should be sent, buf points to the user data, nbytes specifies the number
of bytes of user data to be sent, and flags specifies any optional flags
described below:

If set in flags, the data will be sent as expedited data
and will be subject to the interpretations of the
transport provider.

If set in flags, this indicates to the transport provider
that the transport service data unit (TSDU) (or expedited
transport service data unit - ETSDU) is being sent
through multiple t_snd() calls. Each t_snd() with the
T_MORE flag set indicates that another t_snd() will
follow with more data for the current TSDU (or ETSDU).

The end of the TSDU (or ETSDU) is identified by a t_snd()
call with the T_MORE flag not set. Use of T_MORE enables
a user to break up large logical data units without
losing the boundaries of those units at the other end of
the connection. The flag implies nothing about how the
data is packaged for transfer below the transport
interface. If the transport provider does not support the
concept of a TSDU as indicated in the info argument on
return from t_open(3NSL) or t_getinfo(3NSL), the T_MORE
flag is not meaningful and will be ignored if set.

The sending of a zero-length fragment of a TSDU or ETSDU
is only permitted where this is used to indicate the end
of a TSDU or ETSDU; that is, when the T_MORE flag is not
set. Some transport providers also forbid zero-length

If set in flags, requests that the provider transmit all
data that it has accumulated but not sent. The request
is a local action on the provider and does not affect any
similarly named protocol flag (for example, the TCP PUSH
flag). This effect of setting this flag is
protocol-dependent, and it may be ignored entirely by
transport providers which do not support the use of this

Note that the communications provider is free to collect data in a send
buffer until it accumulates a sufficient amount for transmission.

By default, t_snd() operates in synchronous mode and may wait if flow
control restrictions prevent the data from being accepted by the local
transport provider at the time the call is made. However, if O_NONBLOCK
is set by means of t_open(3NSL) or fcntl(2), t_snd() will execute in
asynchronous mode, and will fail immediately if there are flow control
restrictions. The process can arrange to be informed when the flow
control restrictions are cleared by means of either t_look(3NSL) or the
EM interface.

On successful completion, t_snd() returns the number of bytes (octets)
accepted by the communications provider. Normally this will equal the
number of octets specified in nbytes. However, if O_NONBLOCK is set or
the function is interrupted by a signal, it is possible that only part of
the data has actually been accepted by the communications provider. In
this case, t_snd() returns a value that is less than the value of nbytes.
If t_snd() is interrupted by a signal before it could transfer data to
the communications provider, it returns -1 with t_errno set to TSYSERR
and errno set to EINTR.

If nbytes is zero and sending of zero bytes is not supported by the
underlying communications service, t_snd() returns -1 with t_errno set

The size of each TSDU or ETSDU must not exceed the limits of the
transport provider as specified by the current values in the TSDU or
ETSDU fields in the info argument returned by t_getinfo(3NSL).

The error TLOOK is returned for asynchronous events. It is required only
for an incoming disconnect event but may be returned for other events.


On successful completion, t_snd() returns the number of bytes accepted by
the transport provider. Otherwise, -1 is returned on failure and t_errno
is set to indicate the error.

Note that if the number of bytes accepted by the communications provider
is less than the number of bytes requested, this may either indicate that
O_NONBLOCK is set and the communications provider is blocked due to flow
control, or that O_NONBLOCK is clear and the function was interrupted by
a signal.


On failure, t_errno is set to one of the following:

Illegal amount of data:

o A single send was attempted specifying a TSDU
(ETSDU) or fragment TSDU (ETSDU) greater than
that specified by the current values of the
TSDU or ETSDU fields in the info argument.

o A send of a zero byte TSDU (ETSDU) or zero byte
fragment of a TSDU (ETSDU) is not supported by
the provider.

o Multiple sends were attempted resulting in a
TSDU (ETSDU) larger than that specified by the
current value of the TSDU or ETSDU fields in
the info argument - the ability of an XTI
implementation to detect such an error case is
implementation-dependent. See WARNINGS, below.

The specified file descriptor does not refer to a
transport endpoint.

An invalid flag was specified.

O_NONBLOCK was set, but the flow control mechanism
prevented the transport provider from accepting any data
at this time.

An asynchronous event has occurred on this transport

This function is not supported by the underlying transport

The communications endpoint referenced by fd is not in
one of the states in which a call to this function is

This error indicates that a communication problem has been
detected between XTI and the transport provider for which
there is no other suitable XTI error (t_errno).

A system error has occurred during execution of this


The XTI and TLI interface definitions have common names but use different
header files. This, and other semantic differences between the two
interfaces are described in the subsections below.

Interface Header

The XTI interfaces use the header file, xti.h. TLI interfaces should not
use this header. They should use the header:
#include <tiuser.h>

Error Description Values

The t_errno values that can be set by the XTI interface and cannot be set
by the TLI interface are:

The t_errno values that this routine can return under different
circumstances than its XTI counterpart are:

In the TBADDATA error cases described above, TBADDATA is returned, only
for illegal zero byte TSDU ( ETSDU) send attempts.


See attributes(7) for descriptions of the following attributes:

|MT Level | Safe |


fcntl(2), t_getinfo(3NSL), t_look(3NSL), t_open(3NSL), t_rcv(3NSL),


It is important to remember that the transport provider treats all users
of a transport endpoint as a single user. Therefore if several processes
issue concurrent t_snd() calls then the different data may be intermixed.

Multiple sends which exceed the maximum TSDU or ETSDU size may not be
discovered by XTI. In this case an implementation-dependent error will
result, generated by the transport provider, perhaps on a subsequent XTI
call. This error may take the form of a connection abort, a TSYSERR, a

If multiple sends which exceed the maximum TSDU or ETSDU size are
detected by XTI, t_snd() fails with TBADDATA.

May 7, 1998 T_SND(3NSL)