read(2)read(2)NAME
read, readv, pread, pread64 - read from file
SYNOPSIS
#include <unistd.h>
ssize_t read(int fildes, void *buf, size_t nbyte);
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
ssize_t pread64(int fildes, void *buf, size_t nbyte, off64_t offset);
#include <sys/uio.h>
ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
read attempts to read nbyte bytes from the file associated with fildes
into the buffer pointed to by buf. If nbyte is zero, read returns zero
and has no other results. fildes is a file descriptor obtained from a
creat, open, dup, fcntl, pipe, or ioctl system call.
On devices capable of seeking, the read starts at a position in the file
given by the file pointer associated with fildes. On 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.
pread and pread64 are the same as read except that they do the equivalent
of an lseek (for pread) or lseek64 (for pread64) offset bytes with the
whence set to SEEK_SET before reading. On return from pread or pread64,
the file pointer is unchanged. If fildes refers to a file incapable of
seeking (a fifo or socket) then an error is returned and errno will be
set to ESPIPE.
readv performs the same action as read, but places the input data into
the iovcnt buffers specified by the members of the iov array: iov[0],
iov[1], ..., iov[iovcnt-1].
For readv, the iovec structure contains the following members:
void *iov_base;
ssize_t iov_len;
Each iovec entry specifies the base address and length of an area in
memory where data should be placed. readv always fills one buffer
completely before proceeding to the next.
On success, read and readv return the number of bytes actually read and
placed in the buffer; There are many possible reasons why this number may
be less than nbyte , without indicating end-of-file or an error. Some
common reasons include when the file is associated with a communication
line [see ioctl(2) and termio(7)], or when the number of bytes left in
the file is less than nbyte, or when the file is a pipe or a special
Page 1
read(2)read(2)
file, or the system call was interrupted. When end-of-file has been
reached, read will return a value of 0. A value greater than 0, but less
than nbyte , should not be construed as implying end-of-file, because
many devices and even filesystems can return short counts for various
reasons.
read reads data previously written to a file. If any portion of an
ordinary file prior to the end of file has not been written, read returns
the number of bytes read as 0. For example, the lseek routine allows the
file pointer to be set beyond the end of existing data in the file. If
additional data is written at this point, subsequent reads in the gap
between the previous end of data and newly written data return bytes with
a value of 0 until data is written into the gap.
When attempting to read from a file with O_DIRECT or FDIRECT set, -1 will
be returned and errno will be set to EINVAL if nbyte or the current file
position is not a multiple of the underlying device's blocksize, nbyte is
too big or buf isn't properly aligned. See also F_DIOINFO in the
fcntl(2) manual entry.
When attempting to read from a file with O_DIRECT or FDIRECT set, the
read data will by default come from an in-memory page if the data is
locked in memory by a mlock(3C) command. If you wish to fail the read
instead of having the data not be read directly from disk, set the
systune(1M) variable xfs_dio_retry to zero (0) and the read will return
-1 and errno will be set to EBUSY.
When attempting to read from a file with O_RSYNC and O_SYNC set, the read
will will wait to return until both the file data and file status to be
physically updated. When attempting to read from a file with O_RSYNC and
O_DSYNC set, the read will will wait to return until the file data has
been physically updated.
A read or readv 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(2) request [see streamio(7)], and can be tested
with the I_GRDOPT ioctl(2) request. In byte-stream mode, read and readv
usually retrieve data from the stream until they have retrieved nbyte
bytes, or until there
is no more data to be retrieved. Byte-stream mode usually ignores
message boundaries.
In STREAMS message-nondiscard mode, read and readv retrieve data until
they have read nbyte bytes, or until they reach a message boundary. If
read or readv does not retrieve all the data in a message, the remaining
data is replaced on the stream and can be retrieved by the next read or
readv 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 or readv returns is discarded,
and is not available for a subsequent read, readv, or getmsg [see
getmsg(2)].
Page 2
read(2)read(2)
When attempting to read from a regular file with mandatory file/record
locking set [see chmod(2)], and there is a write lock owned by another
process on the segment of the file to be read:
If O_NDELAY or O_NONBLOCK is set, read returns -1 and sets errno to
EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read sleeps until the blocking
record lock is removed.
When attempting to read from an empty pipe (or FIFO):
If no process has the pipe open for writing, read returns 0 to
indicate end-of-file.
If some process has the pipe open for writing and O_NDELAY is set,
read returns 0.
If some process has the pipe open for writing and O_NONBLOCK is set,
read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data is
written to the pipe or the pipe is closed by all processes that had
opened the pipe for writing.
When attempting to read a file associated with a terminal or slave pty
that has no data currently available:
If O_NDELAY is set, read returns 0.
If O_NONBLOCK is set, read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data becomes
available.
When attempting to read a file associated with a master pty or any other
stream that is not a pipe or FIFO, or terminal, and that has no data
currently available:
If O_NDELAY or O_NONBLOCK is set, read returns -1 and sets errno to
EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data becomes
available.
When attempting to read from a regular file in a DMAPI file system, if
the DMAPI application will take a considerable time to make the file data
available:
If O_NDELAY or O_NONBLOCK is set, read returns -1 and sets errno to
EAGAIN.
Page 3
read(2)read(2)
If O_NDELAY and O_NONBLOCK are clear, read sleeps until the the
DMAPI application has made the file data 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 [see
getmsg(2)]. 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 or readv from a STREAMS file returns the data in the message at
the front of the stream head read queue, regardless of the priority band
of the message.
Normally, a read from a STREAMS file can only process messages with data
and without control information. The read fails if a message containing
control information 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(2). In control-data
mode, control messages are converted to data messages by read. In
control-discard mode, control messages are discarded by read, but any
data associated with the control messages is returned to the user.
read and readv fail if one or more of the following are true:
EACCES fildes is open to a dynamic device and read permission is
denied.
EAGAIN Mandatory file/record locking was set, O_NDELAY or
O_NONBLOCK was set, and there was a blocking record lock.
EAGAIN Total amount of system memory available when reading via
raw I/O is temporarily insufficient.
EAGAIN No data is waiting to be read on a file associated with a
tty device and O_NONBLOCK was set.
EAGAIN No message is waiting to be read on a stream and O_NDELAY
or O_NONBLOCK was set.
EAGAIN A DMAPI application might delay a considerable time
retrieving the file data, and O_NDELAY or O_NONBLOCK was
set.
EBADF fildes is not a valid file descriptor open for reading.
Page 4
read(2)read(2)
EBUSY A direct I/O read was attempted on data that was locked in
memory and could not be flushed to disk before doing the
I/O.
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 to
occur.
EFAULT buf points outside the allocated address space.
EINTR A signal was caught during the read or readv system call
and no bytes had been read.
EINVAL Attempted to read from a stream linked to a multiplexor.
EINVAL fildes has O_DIRECT or FDIRECT set and either the buffer
alignment, current file pointer alignment or read request
size is not valid for direct I/O. See also F_DIOINFO on
fcntl(2).
EIO A physical I/O error has occurred, or the process is in a
background process group and is attempting to read from
its controlling terminal, and either the process is
ignoring or blocking the SIGTTIN signal or the process
group of the process is orphaned.
EIO fildes is open to a device that is in the process of
closing.
EIO fildes has O_DIRECT or FDIRECT set and the number of bytes
requested by read or readv is greater than the number of
bytes between the current file pointer position and end of
file.
ENOLCK The system record lock table was full, so the read or
readv could not go to sleep until the blocking record lock
was removed.
ENXIO The device associated with fildes is a block special or
character special file and the value of the file pointer
is out of range.
ESPIPE pread or pread64 was called on a file incapable of
seeking.
In addition, readv may return one of the following errors:
EFAULT iov points outside the allocated address space.
Page 5
read(2)read(2)
EINVAL iovcnt was less than or equal to 0 or greater than
{IOV_MAX}. {IOV_MAX} is the maximum number of iovec
structures that one process has available for use with
readv. The value {IOV_MAX} can be obtained from a call to
sysconf() [see sysconf(3C)].
EINVAL The sum of the iov_len values in the iov array overflowed
a 32-bit integer.
ETIMEDOUT The object of the read is located on a remote system which
is not available [see intro(2)].
A read from a STREAMS file also fails 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 continues to operate normally until the stream head
read queue is empty. Thereafter, it returns 0.
SEE ALSOintro(2), creat(2), dup(2), fcntl(2), getmsg(2), ioctl(2), open(2),
pipe(2), recv(3N), streamio(7), sysconf(3C), termio(7), pty(7M).
NOTES
read updates the time of last access (see stat(2)) of the file.
DIAGNOSTICS
On success 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.
Page 6
