t_snd -- send normal or expedited data over a connection


cc [options] file -lnsl
#include <xti.h>

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


This function is a TLI/XTI data transfer routine used to send either normal or expedited data over a connection.

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 (via t_open or fcntl), t_snd will execute in asynchronous mode, and will fail immediately if there are flow control restrictions.

Even when there are no flow control restrictions, t_snd will wait if STREAMS internal resources are not available, regardless of the state of O_NONBLOCK.

On successful completion, t_snd returns the number of bytes accepted by the transport provider. Normally this will equal the number of bytes specified in nbytes. However, if O_NONBLOCK is set, it is possible that only part of the data will be accepted by the transport provider. In this case, t_snd will set T_MORE for the data that was sent (see below) and will return a value less than nbytes. If nbytes is 0 and the sending of 0 bytes is not supported by the underlying transport provider, t_snd will return -1 with t_errno set to TBADDATA. A return value of 0 indicates that the request to send a 0-length data message was sent to the provider.

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

If T_MORE is set in flags, or is set as described above, an indication is sent 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. 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 or t_getinfo, the T_MORE flag is not meaningful and will be ignored.

The size of each TSDU or ETSDU must not exceed the limits of the transport provider as returned by t_open or t_getinfo. If the size is exceeded, a TSYSERR with system error EPROTO will occur. However, the t_snd may not fail because EPROTO errors may not be reported immediately. In this case, a subsequent call that accesses the transport endpoint will fail with the associated TSYSERR.

If t_snd is issued from the T_IDLE state, the provider may silently discard the data. If t_snd is issued from any state other than T_DATAXFER, T_INREL or T_IDLE, the provider will generate a TSYSERR with system error EPROTO (which may be reported in the manner described above).


the file descriptor for the transport endpoint over which data will be sent.

points to the user data.

specifies the number of bytes of user data to be sent.

specifies optional flags on return.

State transitions

On entry, T_DATAXFER or T_INREL; unchanged on exit.


X/Open® Transport Interface Library (shared object)

Network Services Library (shared object)


t_snd is applicable only for connection-mode transport services that return a service type of T_COTS or T_COTS_ORD in response to t_open or t_getinfo.

Return values

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


On failure, t_errno may be set to one of the following:

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

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

This function is not supported by the underlying transport provider.

A system error (see intro(S)) has been detected during execution of this function.

nbytes is 0 and sending 0 bytes is not supported by the transport provider; or, the number of bytes on a single send was greater than the number specified for nbytes by the ``info'' argument on the t_open or fcntl; or, the maximum size was exceeded during multiple sends.

An asynchronous event has occurred on the transport endpoint specified by fd and requires immediate attention.

An invalid flag was specified.

The function was issued in the wrong sequence on the transport endpoint referenced by fd.

A communication problem has been detected with the transport provider and there is no other value of t_errno to describe the error condition.


The t_snd routine does not look for a disconnect indication (showing that the connection was broken) before passing data to the provider.

In asynchronous mode, if the number of bytes accepted exceeds the number requested by the transport provider, the provider may be blocked because of flow control.

If several processes issue concurrent calls to t_snd (multiple sends), the data from those processes may be intermixed (since several users of the same endpoint are treated as a single user by the transport provider).

If the maximum size of a TSDU or ETSDU is exceeded as a result of multiple sends, XTI may not detect the error. If the error is detected, t_snd fails with TBADDATA. If the error is not detected, t_snd or a subsequent call fails on an error indicating that the connection has been aborted.


fcntl(S), intro(S), t_getinfo(NET), t_look(NET), t_open(NET), t_rcv(NET)
© 2005 The SCO Group, Inc. All rights reserved.
SCO OpenServer Release 6.0.0 - 01 June 2005