.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "READ" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" read 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
pread, read \- read from a file
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
\fP
.LP
\fBssize_t pread(int\fP \fIfildes\fP\fB, void *\fP\fIbuf\fP\fB, size_t\fP
\fInbyte\fP\fB,
off_t\fP \fIoffset\fP\fB); \fP
\fB
.br
ssize_t read(int\fP \fIfildes\fP\fB, void *\fP\fIbuf\fP\fB, size_t\fP
\fInbyte\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIread\fP() function shall attempt to read \fInbyte\fP bytes
from the file associated with the open file descriptor,
\fIfildes\fP, into the buffer pointed to by \fIbuf\fP. The behavior
of multiple concurrent reads on the same pipe, FIFO, or
terminal device is unspecified.
.LP
Before any action described below is taken, and if \fInbyte\fP is
zero, the \fIread\fP() function may detect and return errors
as described below. In the absence of errors, or if error detection
is not performed, the \fIread\fP() function shall return zero
and have no other results.
.LP
On files that support seeking (for example, a regular file), the \fIread\fP()
shall start at a position in the file given by
the file offset associated with \fIfildes\fP. The file offset shall
be incremented by the number of bytes actually read.
.LP
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.
.LP
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 \fIread\fP() requests is
implementation-defined.
.LP
If the value of \fInbyte\fP is greater than {SSIZE_MAX}, the result
is implementation-defined.
.LP
When attempting to read from an empty pipe or FIFO:
.IP " *" 3
If no process has the pipe open for writing, \fIread\fP() shall return
0 to indicate end-of-file.
.LP
.IP " *" 3
If some process has the pipe open for writing and O_NONBLOCK is set,
\fIread\fP() shall return -1 and set \fIerrno\fP to
[EAGAIN].
.LP
.IP " *" 3
If some process has the pipe open for writing and O_NONBLOCK is clear,
\fIread\fP() 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.
.LP
.LP
When attempting to read a file (other than a pipe or FIFO) that supports
non-blocking reads and has no data currently
available:
.IP " *" 3
If O_NONBLOCK is set, \fIread\fP() shall return -1 and set \fIerrno\fP
to [EAGAIN].
.LP
.IP " *" 3
If O_NONBLOCK is clear, \fIread\fP() shall block the calling thread
until some data becomes available.
.LP
.IP " *" 3
The use of the O_NONBLOCK flag has no effect if there is some data
available.
.LP
.LP
The \fIread\fP() 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, \fIread\fP() shall return bytes with value 0.
For example, \fIlseek\fP() 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.
.LP
Upon successful completion, where \fInbyte\fP is greater than 0, \fIread\fP()
shall mark for update the \fIst_atime\fP field
of the file, and shall return the number of bytes read. This number
shall never be greater than \fInbyte\fP. The value returned
may be less than \fInbyte\fP if the number of bytes left in the file
is less than \fInbyte\fP, if the \fIread\fP() request was
interrupted by a signal, or if the file is a pipe or FIFO or special
file and has fewer than \fInbyte\fP bytes immediately
available for reading. For example, a \fIread\fP() from a file associated
with a terminal may return one typed line of data.
.LP
If a \fIread\fP() is interrupted by a signal before it reads any data,
it shall return -1 with \fIerrno\fP set to [EINTR].
.LP
If a \fIread\fP() is interrupted by a signal after it has successfully
read some data, it shall return the number of bytes
read.
.LP
For regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with
\fIfildes\fP.
.LP
If \fIfildes\fP refers to a socket, \fIread\fP() shall be equivalent
to \fIrecv\fP()
with no flags set.
.LP
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.
.LP
If \fIfildes\fP refers to a shared memory object, the result of the
\fIread\fP() function is unspecified. 
.LP
If \fIfildes\fP refers to a typed memory object, the result of the
\fIread\fP() function is unspecified. 
.LP
A
\fIread\fP() from a STREAMS file can read data in three different
modes: \fIbyte-stream\fP mode, \fImessage-nondiscard\fP mode,
and \fImessage-discard\fP mode. The default shall be byte-stream mode.
This can be changed using the I_SRDOPT \fIioctl\fP() request, and
can be tested with I_GRDOPT \fIioctl\fP(). In byte-stream mode, \fIread\fP()
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.
.LP
In STREAMS message-nondiscard mode, \fIread\fP() shall retrieve data
until as many bytes as were requested are transferred, or
until a message boundary is reached. If \fIread\fP() 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 \fIread\fP() 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
\fIread\fP() returns shall be discarded, and shall not be available
for a subsequent \fIread\fP(), \fIgetmsg\fP(), or \fIgetpmsg\fP()
call.
.LP
How \fIread\fP() handles zero-byte STREAMS messages is determined
by the current read mode setting. In byte-stream mode,
\fIread\fP() shall accept data until it has read \fInbyte\fP bytes,
or until there is no more data to read, or until a zero-byte
message block is encountered. The \fIread\fP() 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 \fIread\fP(),
\fIgetmsg\fP(), or \fIgetpmsg\fP(). 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.
.LP
A \fIread\fP() 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.
.LP
By default, STREAMs are in control-normal mode, in which a \fIread\fP()
from a STREAMS file can only process messages that
contain a data part but do not contain a control part. The \fIread\fP()
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 \fIioctl\fP() command. In control-data
mode,
\fIread\fP() 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, \fIread\fP() shall discard
message control parts but return to the process any data
part in the message.
.LP
In addition, \fIread\fP() shall fail if the STREAM head had processed
an asynchronous error before the call. In this case, the
value of \fIerrno\fP shall not reflect the result of \fIread\fP(),
but reflect the prior error. If a hangup occurs on the STREAM
being read, \fIread\fP() shall continue to operate normally until
the STREAM head read queue is empty. Thereafter, it shall return
0. 
.LP
The \fIpread\fP() function shall be equivalent to \fIread\fP(), except
that it shall read from a given position in the file
without changing the file pointer. The first three arguments to \fIpread\fP()
are the same as \fIread\fP() with the addition of a
fourth argument \fIoffset\fP for the desired position inside the file.
An attempt to perform a \fIpread\fP() on a file that is
incapable of seeking shall result in an error. 
.SH RETURN VALUE
.LP
Upon successful completion, \fIread\fP()  and \fIpread\fP()  shall
return a non-negative integer indicating the number of bytes actually
read. Otherwise, the
functions shall return -1 and set \fIerrno\fP to indicate the error.
.SH ERRORS
.LP
The \fIread\fP() and   \fIpread\fP()  functions shall fail
if:
.TP 7
.B EAGAIN
The O_NONBLOCK flag is set for the file descriptor and the process
would be delayed.
.TP 7
.B EBADF
The \fIfildes\fP argument is not a valid file descriptor open for
reading.
.TP 7
.B EBADMSG
The file is a STREAM file that is set to control-normal mode and the
message waiting to be read includes a control part. 
.TP 7
.B EINTR
The read operation was terminated due to the receipt of a signal,
and no data was transferred.
.TP 7
.B EINVAL
The STREAM or multiplexer referenced by \fIfildes\fP is linked (directly
or indirectly) downstream from a multiplexer. 
.TP 7
.B EIO
The process is a member of a background process attempting to read
from its controlling terminal, the process is ignoring or
blocking the SIGTTIN signal, or the process group is orphaned. This
error may also be generated for implementation-defined
reasons.
.TP 7
.B EISDIR
The \fIfildes\fP argument refers to a directory and the implementation
does not allow the directory to be read using \fIread\fP()
or \fIpread\fP(). The \fIreaddir\fP() function should be used instead.
.TP 7
.B EOVERFLOW
The file is a regular file, \fInbyte\fP 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
\fIfildes\fP.
.sp
.LP
The \fIread\fP() function shall fail if:
.TP 7
.B EAGAIN \fRor\fP EWOULDBLOCK
.sp
The file descriptor is for a socket, is marked O_NONBLOCK, and no
data is waiting to be received.
.TP 7
.B ECONNRESET
A read was attempted on a socket and the connection was forcibly closed
by its peer.
.TP 7
.B ENOTCONN
A read was attempted on a socket that is not connected.
.TP 7
.B ETIMEDOUT
A read was attempted on a socket and a transmission timeout occurred.
.sp
.LP
The \fIread\fP() and   \fIpread\fP()  functions may fail
if:
.TP 7
.B EIO
A physical I/O error has occurred.
.TP 7
.B ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.TP 7
.B ENOMEM
Insufficient memory was available to fulfill the request.
.TP 7
.B ENXIO
A request was made of a nonexistent device, or the request was outside
the capabilities of the device.
.sp
.LP
The \fIpread\fP() function shall fail, and the file pointer shall
remain unchanged, if:
.TP 7
.B EINVAL
The \fIoffset\fP argument is invalid. The value is negative. 
.TP 7
.B EOVERFLOW
The file is a regular file and an attempt was made to read at or beyond
the offset maximum associated with the file. 
.TP 7
.B ENXIO
A request was outside the capabilities of the device. 
.TP 7
.B ESPIPE
\fIfildes\fP is associated with a pipe or FIFO. 
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Reading Data into a Buffer
.LP
The following example reads data from the file associated with the
file descriptor \fIfd\fP into the buffer pointed to by
\fIbuf\fP.
.sp
.RS
.nf

\fB#include <sys/types.h>
#include <unistd.h>
\&...
char buf[20];
size_t nbytes;
ssize_t bytes_read;
int fd;
\&...
nbytes = sizeof(buf);
bytes_read = read(fd, buf, nbytes);
\&...
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
This volume of IEEE\ Std\ 1003.1-2001 does not specify the value of
the file offset after an error is returned; there
are too many cases. For programming errors, such as [EBADF], the concept
is meaningless since no file is involved. For errors that
are detected immediately, such as [EAGAIN], clearly the pointer should
not change. After an interrupt or hardware error, however,
an updated value would be very useful and is the behavior of many
implementations.
.LP
Note that a \fIread\fP() of zero bytes does not modify \fIst_atime\fP.
A \fIread\fP() that requests more than zero bytes, but
returns zero, shall modify \fIst_atime\fP.
.LP
Implementations are allowed, but not required, to perform error checking
for \fIread\fP() requests of zero bytes.
.SS Input and Output
.LP
The use of I/O with large byte counts has always presented problems.
Ideas such as \fIlread\fP() and \fIlwrite\fP() (using and
returning \fBlong\fPs) were considered at one time. The current solution
is to use abstract types on the ISO\ C standard
function to \fIread\fP() and \fIwrite\fP(). The abstract types can
be declared so that
existing functions work, but can also be declared so that larger types
can be represented in future implementations. It is presumed
that whatever constraints limit the maximum range of \fBsize_t\fP
also limit portable I/O requests to the same range. This volume
of IEEE\ Std\ 1003.1-2001 also limits the range further by requiring
that the byte count be limited so that a signed return
value remains meaningful. Since the return type is also a (signed)
abstract type, the byte count can be defined by the
implementation to be larger than an \fBint\fP can hold.
.LP
The standard developers considered adding atomicity requirements to
a pipe or FIFO, but recognized that due to the nature of
pipes and FIFOs there could be no guarantee of atomicity of reads
of {PIPE_BUF} or any other size that would be an aid to
applications portability.
.LP
This volume of IEEE\ Std\ 1003.1-2001 requires that no action be taken
for \fIread\fP() or \fIwrite\fP() when \fInbyte\fP is zero. This is
not intended to take precedence over detection of
errors (such as invalid buffer pointers or file descriptors). This
is consistent with the rest of this volume of
IEEE\ Std\ 1003.1-2001, but the phrasing here could be misread to
require detection of the zero case before any other
errors. A value of zero is to be considered a correct value, for which
the semantics are a no-op.
.LP
I/O is intended to be atomic to ordinary files and pipes and FIFOs.
Atomic means that all the bytes from a single operation that
started out together end up together, without interleaving from other
I/O operations. It is a known attribute of terminals that
this is not honored, and terminals are explicitly (and implicitly
permanently) excepted, making the behavior unspecified. The
behavior for other device types is also left unspecified, but the
wording is intended to imply that future standards might choose
to specify atomicity (or not).
.LP
There were recommendations to add format parameters to \fIread\fP()
and \fIwrite\fP() in
order to handle networked transfers among heterogeneous file system
and base hardware types. Such a facility may be required for
support by the OSI presentation of layer services. However, it was
determined that this should correspond with similar C-language
facilities, and that is beyond the scope of this volume of IEEE\ Std\ 1003.1-2001.
The concept was suggested to the
developers of the ISO\ C standard for their consideration as a possible
area for future work.
.LP
In 4.3 BSD, a \fIread\fP() or \fIwrite\fP() that is interrupted by
a signal before
transferring any data does not by default return an [EINTR] error,
but is restarted. In 4.2 BSD, 4.3 BSD, and the Eighth Edition,
there is an additional function, \fIselect\fP(), whose purpose is
to pause until specified
activity (data to read, space to write, and so on) is detected on
specified file descriptors. It is common in applications written
for those systems for \fIselect\fP() to be used before \fIread\fP()
in situations (such as
keyboard input) where interruption of I/O due to a signal is desired.
.LP
The issue of which files or file types are interruptible is considered
an implementation design issue. This is often affected
primarily by hardware and reliability issues.
.LP
There are no references to actions taken following an "unrecoverable
error". It is considered beyond the scope of this volume
of IEEE\ Std\ 1003.1-2001 to describe what happens in the case of
hardware errors.
.LP
Previous versions of IEEE\ Std\ 1003.1-2001 allowed two very different
behaviors with regard to the handling of
interrupts. In order to minimize the resulting confusion, it was decided
that IEEE\ Std\ 1003.1-2001 should support only
one of these behaviors. Historical practice on AT&T-derived systems
was to have \fIread\fP() and \fIwrite\fP() return -1 and set \fIerrno\fP
to [EINTR] when interrupted after some, but not all, of
the data requested had been transferred. However, the U.S. Department
of Commerce FIPS 151-1 and FIPS 151-2 require the historical
BSD behavior, in which \fIread\fP() and \fIwrite\fP() return the number
of bytes actually
transferred before the interrupt. If -1 is returned when any data
is transferred, it is difficult to recover from the error on a
seekable device and impossible on a non-seekable device. Most new
implementations support this behavior. The behavior required by
IEEE\ Std\ 1003.1-2001 is to return the number of bytes transferred.
.LP
IEEE\ Std\ 1003.1-2001 does not specify when an implementation that
buffers \fIread\fP()ss actually moves the data into
the user-supplied buffer, so an implementation may chose to do this
at the latest possible moment. Therefore, an interrupt arriving
earlier may not cause \fIread\fP() to return a partial byte count,
but rather to return -1 and set \fIerrno\fP to [EINTR].
.LP
Consideration was also given to combining the two previous options,
and setting \fIerrno\fP to [EINTR] while returning a short
count. However, not only is there no existing practice that implements
this, it is also contradictory to the idea that when
\fIerrno\fP is set, the function responsible shall return -1.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIfcntl\fP(), \fIioctl\fP(), \fIlseek\fP(), \fIopen\fP(), \fIpipe\fP(),
\fIreadv\fP(), the Base
Definitions volume of IEEE\ Std\ 1003.1-2001, Chapter 11, General
Terminal
Interface, \fI<stropts.h>\fP, \fI<sys/uio.h>\fP, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .