READ(2) System Calls READ(2)


NAME


read, readv, pread, preadv - 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);


#include <sys/uio.h>

ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);

ssize_t preadv(int fildes, const struct iovec *iov, int iovcnt, off_t offset);


DESCRIPTION


The read() function attempts to read nbyte bytes from the file associated
with the open file descriptor, fildes, into the buffer pointed to by buf.


If nbyte is 0, read() returns 0 and has no other results.


On files that support seeking (for example, a regular file), the read()
starts at a position in the file given by the file offset associated with
fildes. The file offset is 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.


If fildes refers to a socket, read() is equivalent to recv(3SOCKET) with
no flags set.


No data transfer will occur past the current end-of-file. If the
starting position is at or after the end-of-file, 0 will be returned. If
the file refers to a device special file, the result of subsequent read()
requests is implementation-dependent.


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:

o If O_NDELAY or O_NONBLOCK is set, read() returns -1 and sets
errno to EAGAIN.

o 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):

o If no process has the pipe open for writing, read() returns 0
to indicate end-of-file.

o If some process has the pipe open for writing and O_NDELAY is
set, read() returns 0.

o If some process has the pipe open for writing and O_NONBLOCK
is set, read() returns -1 and sets errno to EAGAIN.

o 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 that has no
data currently available:

o If O_NDELAY is set, read() returns 0.

o If O_NONBLOCK is set, read() returns -1 and sets errno to
EAGAIN.

o If O_NDELAY and O_NONBLOCK are clear, read() blocks until data
become available.


When attempting to read a file associated with a socket or a stream that
is not a pipe, a FIFO, or a terminal, and the file has no data currently
available:

o If O_NDELAY or O_NONBLOCK is set, read() returns -1 and sets
errno to EAGAIN.

o If O_NDELAY and O_NONBLOCK are clear, read() blocks until data
becomes 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() returns bytes with value 0. For example, lseek(2) 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 will return bytes with
value 0 until data is written into the gap.


For regular files, no data transfer will occur past the offset maximum
established in the open file description associated with fildes.


Upon successful completion, where nbyte is greater than 0, read() will
mark for update the st_atime field of the file, and return the number of
bytes read. This number will 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 will
return -1 with errno set to EINTR.


If a read() is interrupted by a signal after it has successfully read
some data, it will return the number of bytes read.


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 is byte-stream mode. This can be changed using the I_SRDOPT
ioctl(2) request, and can be tested with the I_GRDOPT ioctl(). In byte-
stream mode, read() retrieves 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() retrieves 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 is 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 is discarded, and is not available for a subsequent read(),
readv() or getmsg(2) call.


How read() handles zero-byte streams 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. The read() function then
returns the number of bytes read, and places the zero-byte message back
on the stream to be retrieved by the next read(), readv() or getmsg(2).
In message-nondiscard mode or message-discard mode, a zero-byte message
returns 0 and the message is removed from the stream. When a zero-byte
message is read as the first message on a stream, the message is removed
from the stream and 0 is returned, regardless of the read mode.


A read() 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.


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() fails 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() converts any control part to data and passes it to the application
before passing any data part originally present in the same message. In
control-discard mode, read() discards message control parts but returns
to the process any data part in the message.


In addition, read() and readv() will fail if the stream head had
processed an asynchronous error before the call. In this case, the value
of errno does not reflect the result of read() or readv() but reflects
the prior error. 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.

readv()
The readv() function is equivalent to 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]. The iovcnt argument is valid if
greater than 0 and less than or equal to {IOV_MAX}.


The iovec structure contains the following members:

caddr_t iov_base;
int iov_len;


Each iovec entry specifies the base address and length of an area in
memory where data should be placed. The readv() function always fills an
area completely before proceeding to the next.


Upon successful completion, readv() marks for update the st_atime field
of the file.

pread()
The pread() function performs the same action as read(), except that it
reads from a given position in the file without changing the file
pointer. 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. pread() will read up to the maximum offset value that can be
represented in an off_t for regular files. An attempt to perform a
pread() on a file that is incapable of seeking results in an error.

preadv()
The preadv() function performs the same action as readv() except it reads
from a given position in the file without changing the file pointer. The
first three arguments are the same as readv() with the addition of a
fourth argument offset for the desired position inside the file.
preadv() will read up to the maximum offset value that can be represented
in an off_t for regular files. An attempt to perform a preadv() on a file
that is incapable of seeking results in an error.


RETURN VALUES


Upon successful completion, read() and readv() return a non-negative
integer indicating the number of bytes actually read. Otherwise, the
functions return -1 and set errno to indicate the error.

ERRORS


The read(), readv(), pread(), and preadv() functions will fail if:

EAGAIN
Mandatory file/record locking was set, O_NDELAY or O_NONBLOCK
was set, and there was a blocking record lock; total amount of
system memory available when reading using raw I/O is
temporarily insufficient; no data is waiting to be read on a
file associated with a tty device and O_NONBLOCK was set; or
no message is waiting to be read on a stream and O_NDELAY or
O_NONBLOCK was set.


EBADF
The fildes argument is not a valid file descriptor open for
reading.


EBADMSG
Message waiting to be read on a stream is not a data message.


ECONNRESET
The filedes argument refers to a connection oriented socket
and the connection was forcibly closed by the peer and is no
longer valid. I/O can no longer be performed to filedes.


EDEADLK
The read was going to go to sleep and cause a deadlock to
occur.


EINTR
A signal was caught during the read operation and no data was
transferred.


EINVAL
An attempt was made to read from a stream linked to a
multiplexor.


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.


EISDIR
The fildes argument refers to a directory on a file system
type that does not support read operations on directories.


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.


ENOLINK
The fildes argument is on a remote machine and the link to
that machine is no longer active.


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.


The read() and pread() functions will fail if:

EFAULT
The buf argument points to an illegal address.


EINVAL
The nbyte argument overflowed an ssize_t.


The read() and readv() functions will fail if:

EOVERFLOW
The file is a regular file, nbyte is greater than 0, the
starting position is before the end-of-file, and the
starting position is greater than or equal to the offset
maximum established in the open file description associated
with fildes.


The readv() and preadv() functions may fail if:

EFAULT
The iov argument points outside the allocated address space.


EINVAL
The iovcnt argument was less than or equal to 0 or greater than
{IOV_MAX}. See Intro(2) for a definition of {IOV_MAX}).

One of the iov_len values in the iov array was negative, or the
sum of the iov_len values in the iov array overflowed an
ssize_t.


The pread() and preadv() functions will fail and the file pointer remain
unchanged if:

ESPIPE
The fildes argument is associated with a pipe or FIFO.


USAGE


The pread() function has a transitional interface for 64-bit file
offsets. See lf64(5).

ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------------------+
|Interface Stability | Committed |
+--------------------+-----------------------------+
|MT-Level | read() is Async-Signal-Safe |
+--------------------+-----------------------------+
|Standard | See standards(5). |
+--------------------+-----------------------------+

SEE ALSO


Intro(2), chmod(2), creat(2), dup(2), fcntl(2), getmsg(2), ioctl(2),
lseek(2), open(2), pipe(2), recv(3SOCKET), attributes(5), lf64(5),
standards(5), streamio(7I), termio(7I)


January 15, 2015 READ(2)