READ(2) READ(2)
NAME
read - read from file
SYNOPSIS
int read (fildes, buf, nbyte)
int fildes;
char *buf;
unsigned nbyte;
DESCRIPTION
Fildes is a file descriptor obtained from a creat(2),
open(2), dup(2), fcntl(2), or pipe(2) system call.
read attempts to read nbyte bytes from the file associated
with fildes into the buffer pointed to by buf.
On devices capable of seeking, the read starts at a position
in the file given by the file pointer associated with
fildes. Upon return from read, the file pointer is
incremented by the number of bytes actually read.
Devices that are incapable of seeking always read from the
current position. The value of a file pointer associated
with such a file is undefined.
Upon successful completion, read returns the number of bytes
actually read and placed in the buffer; this number may be
less than nbyte if the file is associated with a
communication line [see ioctl(2) and termio(7)], or if the
number of bytes left in the file is less than nbyte bytes.
A value of 0 is returned when an end-of-file has been
reached.
A read from a STREAMS [see intro(2)] file can operate in
three different modes: "byte-stream" mode, "message-
nondiscard" mode, and "message-discard" mode. The default
is byte-stream mode. This can be changed using the I_SRDOPT
ioctl request [see streamio(7)], and can be tested with the
I_GRDOPT ioctl. In byte-stream mode, read will retrieve
data from the stream until it has retrieved nbyte bytes, or
until there is no more data to be retrieved. Byte-stream
mode ignores message boundaries.
In STREAMS message-nondiscard mode, read retrieves data
until it has read nbyte bytes, or until it reaches a message
boundary. If the read does not retrieve all the data in a
message, the remaining data are replaced on the stream, and
can be retrieved by the next read or getmsg(2) call.
Message-discard mode also retrieves data until it has
retrieved nbyte bytes, or it reaches a message boundary.
However, unread data remaining in a message after the read
returns are discarded, and are not available for a
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READ(2) READ(2)
subsequent read or getmsg.
When attempting to read from a regular file with mandatory
file/record locking set [see chmod(2)], and there is a
blocking (i.e. owned by another process) write lock on the
segment of the file to be read:
If O_NDELAY is set, the read will return a -1 and set
errno to EAGAIN.
If O_NDELAY is clear, the read will sleep until the
blocking record lock is removed.
When attempting to read from an empty pipe (or FIFO):
If O_NDELAY is set, the read will return a 0.
If O_NDELAY is clear, the read will block until data is
written to the file or the file is no longer open for
writing.
When attempting to read a file associated with a tty that
has no data currently available:
If O_NDELAY is set, the read will return a 0.
If O_NDELAY is clear, the read will block until data
becomes available.
When attempting to read a file associated with a stream that
has no data currently available:
If O_NDELAY is set, the read will return a -1 and set
errno to EAGAIN.
If O_NDELAY is clear, the read will block until data
becomes available.
When reading from a STREAMS file, handling of zero-byte
messages is determined by the current read mode setting. In
byte-stream mode, read accepts 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. read then returns
the number of bytes read, and places the zero-byte message
back on the stream to be retrieved by the next read or
getmsg. In the two other modes, a zero-byte message returns
a value of 0 and the message is removed from the stream.
When a zero-byte message is read as the first message on a
stream, a value of 0 is returned regardless of the read
mode.
A read from a STREAMS file can only process data messages.
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READ(2) READ(2)
It cannot process any type of protocol message and will fail
if a protocol message is encountered at the stream head.
read will fail if one or more of the following are true:
[EAGAIN] Mandatory file/record locking was set,
O_NDELAY was set, and there was a blocking
record lock.
[EAGAIN] Total amount of system memory available when
reading via raw IO is temporarily
insufficient.
[EAGAIN] No message waiting to be read on a stream and
O_NDELAY flag set.
[EBADF] Fildes is not a valid file descriptor open
for reading.
[EBADMSG] Message waiting to be read on a stream is not
a data message.
[EDEADLK] The read was going to go to sleep and cause a
deadlock situation to occur.
[EFAULT] Buf points outside the allocated address
space.
[EINTR] A signal was caught during the read system
call.
[EINVAL] Attempted to read from a stream linked to a
multiplexor.
[ENOLCK] The system record lock table was full, so the
read could not go to sleep until the blocking
record lock was removed.
[ENOLINK] Fildes is on a remote machine and the link to
that machine is no longer active.
A read from a STREAMS file will also fail if an error
message is received at the stream head. In this case, errno
is set to the value returned in the error message. If a
hangup occurs on the stream being read, read will continue
to operate normally until the stream head read queue is
empty. Thereafter, it will return 0.
SEE ALSO
creat(2), dup(2), fcntl(2), ioctl(2), intro(2), open(2),
pipe(2), getmsg(2).
streamio(7), termio(7) in the System Administrator's
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READ(2) READ(2)
Reference Manual.
DIAGNOSTICS
Upon successful completion a non-negative integer is
returned indicating the number of bytes actually read.
Otherwise, a -1 is returned and errno is set to indicate the
error.
ORIGIN
AT&T V.3
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