Tribblix: manual page: read.2

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.

If an asynchronous error occurs on a socket, it is possible for the
read() and readv() functions to return an asynchronous error, just as
in the STREAMS case described above. This might occur, for example,
if a TCP socket that is using TCP keep-alive is closed due to failing
the keep-alive check.

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

In addition to the errors documented below, if the filedes argument
refers to a socket, then an asynchronous error generated by the
underlying socket protocol may be returned. For the full list of
errors, please see the corresponding socket protocol manual page. For
example, for a list of TCP errors, please see tcp(4P).

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(7).

ATTRIBUTES