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This function is used in connectionless mode to receive a data unit from another transport user. The argument fd identifies the local transport endpoint through which data will be received, unitdata holds information associated with the received
data unit, iovcount contains the number of non-contiguous udata buffers which is limited to T_IOV_MAX, which is an implementation-defined value of at least 16, and flags is set on return to indicate that
the complete data unit was not received. If the limit on iovcount is exceeded, the function fails with TBADDATA. The argument unitdata points to a t_unitdata structure containing the following members:
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struct netbuf addr;
struct netbuf opt;
struct netbuf udata;
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The maxlen field of addr and opt must be set before calling this function to indicate the maximum size of the buffer for each. The udata field of t_unitdata
is not used. The iov_len and iov_base fields of "iov0" through iov [iovcount-1] must be set before calling t_rcvvudata() to define the buffer where the userdata
will be placed. If the maxlen field of addr or opt is set to zero then no information is returned in the buf field for this parameter.
On return from this call, addr specifies the protocol address of the sending user, opt identifies options that were associated with this data unit, and iov[0].iov_base
through iov [iovcount-1].iov_base contains the user data that was received. The return value of t_rcvvudata() is the number of bytes of user data given to the user.
Note that the limit on the total number of bytes available in all buffers passed:
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iov(0).iov_len + . . + iov(iovcount-1).iov_len)
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may be constrained by implementation limits. If no other constraint applies, it will be limited by INT_MAX. In practice, the availability of memory to an application is likely to impose a lower limit on the amount of data that can be sent or received using scatter/gather
functions.
By default, t_rcvvudata() operates in synchronous mode and waits for a data unit to arrive if none is currently available. However, if O_NONBLOCK is set by means of t_open(3NSL) or fcntl(2), t_rcvvudata() executes in asynchronous mode and fails if no data units are available.
If the buffers defined in the iov[] array are not large enough to hold the current data unit, the buffers will be filled and T_MORE will be set in flags on return to indicate that another t_rcvvudata() should
be called to retrieve the rest of the data unit. Subsequent calls to t_rcvvudata() will return zero for the length of the address and options, until the full data unit has been received.
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