Open System Services System Calls Reference Manual (G06.29+, H06.08+, J06.03+)

System Functions (s and S) spt_writevz(2)
as noted later).
Upon successful completion, the spt_writevz() function returns the number of bytes actually
written to the file associated with filedes.
If the O_APPEND status flag of the file is set, the file offset is set to the end of the file prior to
each write.
Write requests to a pipe or FIFO file are handled the same as writes to a regular file with these
exceptions:
• No file offset is associated with a pipe; therfore, each spt_writevz() request appends to
the end of the pipe.
• If the size of the spt_writevz() request is less than or equal to the value of the
PIPE_BUF system variable, the spt_writevz() function is guaranteed to be atomic. The
data is not interleaved with data from other processes doing writes on the same pipe.
• If the size of the spt_writevz() request is greater than the value of the PIPE_BUF sys-
tem variable, the file system attempts to resize the pipe buffer from 2 * PIPE_BUF to
65,536 bytes. If the resizing is successful, the file system performs atomic writes of up to
32,768 bytes and can transfer up to 52 kilobytes of data from the pipe buffer on subse-
quent spt_readz() or spt_readvz() calls by the client.
If the file system cannot resize the buffer, it continues to use the existing buffer. A
second attempt at resizing occurs after approximately a minute elapses.
Writes of greater than PIPE_BUF bytes can have data interleaved, on arbitrary boun-
daries, with writes by other processes, whether or not the O_NONBLOCK flag is set.
• If the O_NONBLOCK flag is not set, a spt_writevz() request to a full pipe causes the
process to block until enough space becomes available to handle the entire request.
• If the O_NONBLOCK flag is set, spt_writevz() requests are handled differently in
these ways:
— The spt_writevz() function does block the process.
— spt_writevz() requests for PIPE_BUF or fewer bytes either succeed completely
and return the number of bytes written, or return the value -1 and set errno to
[EAGAIN].
—Aspt_writevz() request for greater than PIPE_BUF bytes either transfers what
it can and returns the number of bytes written, or transfers no data and returns the
value -1 with errno set to [EAGAIN]. Also, if a request is greater than
PIPE_BUF bytes and all data previously written to the pipe has been read,
spt_writevz() transfers at least PIPE_BUF bytes.
When attempting to write to a file descriptor for a special character device (a terminal) that can-
not accept data immediately:
• If the O_NONBLOCK flag is clear, the spt_writevz() function blocks until the data can
be accepted or an error occurs.
•
If the O_NONBLOCK fl
ag is set, the spt_writevz() function returns the value -1 and
errno is set to [EAGAIN].
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