The read() function shall attempt to read nbyte bytes from the
file associated with the open file descriptor, fildes, into the
buffer pointed to by buf. The behavior of multiple concurrent
reads on the same pipe, FIFO, or terminal device is unspecified.
Before any action described below is taken, and if nbyte is zero,
the read() function may detect and return errors as described
below. In the absence of errors, or if error detection is not
performed, the read() function shall return zero and have no
other results.
On files that support seeking (for example, a regular file), the
read() shall start at a position in the file given by the file
offset associated with fildes. The file offset shall be
incremented by the number of bytes actually read.
Files that do not support seeking—for example, terminals—always
read from the current position. The value of a file offset
associated with such a file is undefined.
No data transfer shall occur past the current end-of-file. If the
starting position is at or after the end-of-file, 0 shall be
returned. If the file refers to a device special file, the
result of subsequent read() requests is implementation-defined.
If the value of nbyte is greater than {SSIZE_MAX}, the result is
implementation-defined.
When attempting to read from an empty pipe or FIFO:
* If no process has the pipe open for writing, read() shall
return 0 to indicate end-of-file.
* If some process has the pipe open for writing and O_NONBLOCK
is set, read() shall return -1 and set errno to [EAGAIN].
* If some process has the pipe open for writing and O_NONBLOCK
is clear, read() shall block the calling thread until some
data is written or the pipe is closed by all processes that
had the pipe open for writing.
When attempting to read a file (other than a pipe or FIFO) that
supports non-blocking reads and has no data currently available:
* If O_NONBLOCK is set, read() shall return -1 and set errno to
[EAGAIN].
* If O_NONBLOCK is clear, read() shall block the calling thread
until some data becomes available.
* The use of the O_NONBLOCK flag has no effect if there is some
data available.
The read() function reads data previously written to a file. If
any portion of a regular file prior to the end-of-file has not
been written, read() shall return bytes with value 0. For
example, lseek() allows the file offset to be set beyond the end
of existing data in the file. If data is later written at this
point, subsequent reads in the gap between the previous end of
data and the newly written data shall return bytes with value 0
until data is written into the gap.
Upon successful completion, where nbyte is greater than 0, read()
shall mark for update the last data access timestamp of the file,
and shall return the number of bytes read. This number shall
never be greater than nbyte. The value returned may be less than
nbyte if the number of bytes left in the file is less than nbyte,
if the read() request was interrupted by a signal, or if the file
is a pipe or FIFO or special file and has fewer than nbyte bytes
immediately available for reading. For example, a read() from a
file associated with a terminal may return one typed line of
data.
If a read() is interrupted by a signal before it reads any data,
it shall return -1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully
read some data, it shall return the number of bytes read.
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, read() shall be equivalent to
recv() with no flags set.
If the O_DSYNC and O_RSYNC bits have been set, read I/O
operations on the file descriptor shall complete as defined by
synchronized I/O data integrity completion. If the O_SYNC and
O_RSYNC bits have been set, read 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
read() function is unspecified.
If fildes refers to a typed memory object, the result of the
read() function is unspecified.
A read() from a STREAMS file can read data in three different
modes: byte-stream mode, message-nondiscard mode, and message-
discard mode. The default shall be byte-stream mode. This can be
changed using the I_SRDOPT ioctl() request, and can be tested
with I_GRDOPT ioctl(). In byte-stream mode, read() shall
retrieve data from the STREAM until as many bytes as were
requested are transferred, or until there is no more data to be
retrieved. Byte-stream mode ignores message boundaries.
In STREAMS message-nondiscard mode, read() shall retrieve data
until as many bytes as were requested are transferred, or until a
message boundary is reached. If read() does not retrieve all the
data in a message, the remaining data shall be left on the
STREAM, and can be retrieved by the next read() call. Message-
discard mode also retrieves data until as many bytes as were
requested are transferred, or a message boundary is reached.
However, unread data remaining in a message after the read()
returns shall be discarded, and shall not be available for a
subsequent read(), getmsg(), or getpmsg() call.
How read() handles zero-byte STREAMS messages is determined by
the current read mode setting. In byte-stream mode, read() shall
accept data until it has read nbyte bytes, or until there is no
more data to read, or until a zero-byte message block is
encountered. The read() function shall then return the number of
bytes read, and place the zero-byte message back on the STREAM to
be retrieved by the next read(), getmsg(), or getpmsg(). In
message-nondiscard mode or message-discard mode, a zero-byte
message shall return 0 and the message shall be removed from the
STREAM. When a zero-byte message is read as the first message on
a STREAM, the message shall be removed from the STREAM and 0
shall be returned, regardless of the read mode.
A read() from a STREAMS file shall return the data in the message
at the front of the STREAM head read queue, regardless of the
priority band of the message.
By default, STREAMs are in control-normal mode, in which a read()
from a STREAMS file can only process messages that contain a data
part but do not contain a control part. The read() shall fail if
a message containing a control part is encountered at the STREAM
head. This default action can be changed by placing the STREAM in
either control-data mode or control-discard mode with the
I_SRDOPT ioctl() command. In control-data mode, read() shall
convert any control part to data and pass it to the application
before passing any data part originally present in the same
message. In control-discard mode, read() shall discard message
control parts but return to the process any data part in the
message.
In addition, read() shall fail if the STREAM head had processed
an asynchronous error before the call. In this case, the value of
errno shall not reflect the result of read(), but reflect the
prior error. If a hangup occurs on the STREAM being read, read()
shall continue to operate normally until the STREAM head read
queue is empty. Thereafter, it shall return 0.
The pread() function shall be equivalent to read(), except that
it shall read from a given position in the file without changing
the file offset. The first three arguments to pread() are the
same as read() with the addition of a fourth argument offset for
the desired position inside the file. An attempt to perform a
pread() on a file that is incapable of seeking shall result in an
error.
