The write() function shall attempt to write nbyte bytes from the
buffer pointed to by buf to the file associated with the open
file descriptor, fildes.
Before any action described below is taken, and if nbyte is zero
and the file is a regular file, the write() function may detect
and return errors as described below. In the absence of errors,
or if error detection is not performed, the write() function
shall return zero and have no other results. If nbyte is zero and
the file is not a regular file, the results are unspecified.
On a regular file or other file capable of seeking, the actual
writing of data shall proceed from the position in the file
indicated by the file offset associated with fildes. Before
successful return from write(), the file offset shall be
incremented by the number of bytes actually written. On a regular
file, if the position of the last byte written is greater than or
equal to the length of the file, the length of the file shall be
set to this position plus one.
On a file not capable of seeking, writing shall always take place
starting at the current position. The value of a file offset
associated with such a device is undefined.
If the O_APPEND flag of the file status flags is set, the file
offset shall be set to the end of the file prior to each write
and no intervening file modification operation shall occur
between changing the file offset and the write operation.
If a write() requests that more bytes be written than there is
room for (for example, the file size limit of the process or the
physical end of a medium), only as many bytes as there is room
for shall be written. For example, suppose there is space for 20
bytes more in a file before reaching a limit. A write of 512
bytes will return 20. The next write of a non-zero number of
bytes would give a failure return (except as noted below).
If the request would cause the file size to exceed the soft file
size limit for the process and there is no room for any bytes to
be written, the request shall fail and the implementation shall
generate the SIGXFSZ signal for the thread.
If write() is interrupted by a signal before it writes any data,
it shall return -1 with errno set to [EINTR].
If write() is interrupted by a signal after it successfully
writes some data, it shall return the number of bytes written.
If the value of nbyte is greater than {SSIZE_MAX}, the result is
implementation-defined.
After a write() to a regular file has successfully returned:
* Any successful read() from each byte position in the file
that was modified by that write shall return the data
specified by the write() for that position until such byte
positions are again modified.
* Any subsequent successful write() to the same byte position
in the file shall overwrite that file data.
Write requests to a pipe or FIFO shall be handled in the same way
as a regular file with the following exceptions:
* There is no file offset associated with a pipe, hence each
write request shall append to the end of the pipe.
* Write requests of {PIPE_BUF} bytes or less shall not be
interleaved with data from other processes doing writes on
the same pipe. Writes of greater than {PIPE_BUF} bytes may
have data interleaved, on arbitrary boundaries, with writes
by other processes, whether or not the O_NONBLOCK flag of the
file status flags is set.
* If the O_NONBLOCK flag is clear, a write request may cause
the thread to block, but on normal completion it shall return
nbyte.
* If the O_NONBLOCK flag is set, write() requests shall be
handled differently, in the following ways:
-- The write() function shall not block the thread.
-- A write request for {PIPE_BUF} or fewer bytes shall have
the following effect: if there is sufficient space
available in the pipe, write() shall transfer all the
data and return the number of bytes requested.
Otherwise, write() shall transfer no data and return -1
with errno set to [EAGAIN].
-- A write request for more than {PIPE_BUF} bytes shall
cause one of the following:
-- When at least one byte can be written, transfer what
it can and return the number of bytes written. When
all data previously written to the pipe is read, it
shall transfer at least {PIPE_BUF} bytes.
-- When no data can be written, transfer no data, and
return -1 with errno set to [EAGAIN].
When attempting to write to a file descriptor (other than a pipe
or FIFO) that supports non-blocking writes and cannot accept the
data immediately:
* If the O_NONBLOCK flag is clear, write() shall block the
calling thread until the data can be accepted.
* If the O_NONBLOCK flag is set, write() shall not block the
thread. If some data can be written without blocking the
thread, write() shall write what it can and return the number
of bytes written. Otherwise, it shall return -1 and set
errno to [EAGAIN].
Upon successful completion, where nbyte is greater than 0,
write() shall mark for update the last data modification and last
file status change timestamps of the file, and if the file is a
regular file, the S_ISUID and S_ISGID bits of the file mode may
be cleared.
For regular files, no data transfer shall occur past the offset
maximum established in the open file description associated with
fildes.
If fildes refers to a socket, write() shall be equivalent to
send() with no flags set.
If the O_DSYNC bit has been set, write I/O operations on the file
descriptor shall complete as defined by synchronized I/O data
integrity completion.
If the O_SYNC bit has been set, write I/O operations on the file
descriptor shall complete as defined by synchronized I/O file
integrity completion.
If fildes refers to a shared memory object, the result of the
write() function is unspecified.
If fildes refers to a typed memory object, the result of the
write() function is unspecified.
If fildes refers to a STREAM, the operation of write() shall be
determined by the values of the minimum and maximum nbyte range
(packet size) accepted by the STREAM. These values are determined
by the topmost STREAM module. If nbyte falls within the packet
size range, nbyte bytes shall be written. If nbyte does not fall
within the range and the minimum packet size value is 0, write()
shall break the buffer into maximum packet size segments prior to
sending the data downstream (the last segment may contain less
than the maximum packet size). If nbyte does not fall within the
range and the minimum value is non-zero, write() shall fail with
errno set to [ERANGE]. Writing a zero-length buffer (nbyte is 0)
to a STREAMS device sends 0 bytes with 0 returned. However,
writing a zero-length buffer to a STREAMS-based pipe or FIFO
sends no message and 0 is returned. The process may issue
I_SWROPT ioctl() to enable zero-length messages to be sent across
the pipe or FIFO.
When writing to a STREAM, data messages are created with a
priority band of 0. When writing to a STREAM that is not a pipe
or FIFO:
* If O_NONBLOCK is clear, and the STREAM cannot accept data
(the STREAM write queue is full due to internal flow control
conditions), write() shall block until data can be accepted.
* If O_NONBLOCK is set and the STREAM cannot accept data,
write() shall return -1 and set errno to [EAGAIN].
* If O_NONBLOCK is set and part of the buffer has been written
while a condition in which the STREAM cannot accept
additional data occurs, write() shall terminate and return
the number of bytes written.
In addition, write() shall fail if the STREAM head has processed
an asynchronous error before the call. In this case, the value of
errno does not reflect the result of write(), but reflects the
prior error.
The pwrite() function shall be equivalent to write(), except that
it writes into a given position and does not change the file
offset (regardless of whether O_APPEND is set). The first three
arguments to pwrite() are the same as write() with the addition
of a fourth argument offset for the desired position inside the
file. An attempt to perform a pwrite() on a file that is
incapable of seeking shall result in an error.
