NAME
pread, read - read from a file
SYNOPSIS
#include <unistd.h>
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
ssize_t read(int fildes, void *buf, size_t nbyte);
DESCRIPTION
The read() function shall attempt to read nbyte bytes from the file
associated with the open file descriptor, fildes, into the buffer
pointed to by buf. The behavior of multiple concurrent reads on the
same pipe, FIFO, or terminal device is unspecified.
Before any action described below is taken, and if nbyte is zero, the
read() function may detect and return errors as described below. In the
absence of errors, or if error detection is not performed, the read()
function shall return zero and have no other results.
On files that support seeking (for example, a regular file), the read()
shall start at a position in the file given by the file offset
associated with fildes. The file offset shall be 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.
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
read() requests is implementation-defined.
If the value of nbyte is greater than {SSIZE_MAX}, the result is
implementation-defined.
When attempting to read from an empty pipe or FIFO:
* If no process has the pipe open for writing, read() shall return 0
to indicate end-of-file.
* If some process has the pipe open for writing and O_NONBLOCK is set,
read() shall return -1 and set errno to [EAGAIN].
* If some process has the pipe open for writing and O_NONBLOCK is
clear, read() 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.
When attempting to read a file (other than a pipe or FIFO) that
supports non-blocking reads and has no data currently available:
* If O_NONBLOCK is set, read() shall return -1 and set errno to
[EAGAIN].
* If O_NONBLOCK is clear, read() shall block the calling thread until
some data becomes available.
* The use of the O_NONBLOCK flag has no effect if there is some data
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() shall return bytes with value 0. For example, lseek()
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.
Upon successful completion, where nbyte is greater than 0, read() shall
mark for update the st_atime field of the file, and shall return the
number of bytes read. This number shall 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
shall return -1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully read
some data, it shall return the number of bytes read.
For regular files, no data transfer shall occur past the offset maximum
established in the open file description associated with fildes.
If fildes refers to a socket, read() shall be equivalent to recv() with
no flags set.
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.
If fildes refers to a shared memory object, the result of the read()
function is unspecified.
If fildes refers to a typed memory object, the result of the read()
function is unspecified.
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 shall be byte-stream mode. This can be changed using the
I_SRDOPT ioctl() request, and can be tested with I_GRDOPT ioctl(). In
byte-stream mode, read() 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.
In STREAMS message-nondiscard mode, read() shall retrieve 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 shall be 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 shall be discarded, and shall not be
available for a subsequent read(), getmsg(), or getpmsg() call.
How read() handles zero-byte STREAMS messages is determined by the
current read mode setting. In byte-stream mode, read() shall accept
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 shall then return the number of bytes read, and place the
zero-byte message back on the STREAM to be retrieved by the next
read(), getmsg(), or getpmsg(). 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.
A read() 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.
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() 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 ioctl() command. In
control-data mode, read() 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, read() shall
discard message control parts but return to the process any data part
in the message.
In addition, read() shall fail if the STREAM head had processed an
asynchronous error before the call. In this case, the value of errno
shall not reflect the result of read(), but reflect the prior error. If
a hangup occurs on the STREAM being read, read() shall continue to
operate normally until the STREAM head read queue is empty. Thereafter,
it shall return 0.
The pread() function shall be equivalent to read(), except that it
shall read 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. An attempt to perform a pread() on a file that is
incapable of seeking shall result in an error.
RETURN VALUE
Upon successful completion, read() and pread() shall return a non-
negative integer indicating the number of bytes actually read.
Otherwise, the functions shall return -1 and set errno to indicate the
error.
ERRORS
The read() and pread() functions shall fail if:
EAGAIN The O_NONBLOCK flag is set for the file descriptor and the
process would be delayed.
EBADF The fildes argument is not a valid file descriptor open for
reading.
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.
EINTR The read operation was terminated due to the receipt of a
signal, and no data was transferred.
EINVAL The STREAM or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a multiplexer.
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.
EISDIR The fildes argument refers to a directory and the implementation
does not allow the directory to be read using read() or pread().
The readdir() function should be used instead.
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 read() function shall fail if:
EAGAIN or EWOULDBLOCK
The file descriptor is for a socket, is marked O_NONBLOCK, and
no data is waiting to be received.
ECONNRESET
A read was attempted on a socket and the connection was forcibly
closed by its peer.
ENOTCONN
A read was attempted on a socket that is not connected.
ETIMEDOUT
A read was attempted on a socket and a transmission timeout
occurred.
The read() and pread() functions may fail if:
EIO A physical I/O error has occurred.
ENOBUFS
Insufficient resources were available in the system to perform
the operation.
ENOMEM Insufficient memory was available to fulfill the request.
ENXIO A request was made of a nonexistent device, or the request was
outside the capabilities of the device.
The pread() function shall fail, and the file pointer shall remain
unchanged, if:
EINVAL The offset argument is invalid. The value is negative.
EOVERFLOW
The file is a regular file and an attempt was made to read at or
beyond the offset maximum associated with the file.
ENXIO A request was outside the capabilities of the device.
ESPIPE fildes is associated with a pipe or FIFO.
The following sections are informative.
EXAMPLES
Reading Data into a Buffer
The following example reads data from the file associated with the file
descriptor fd into the buffer pointed to by buf.
#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);
...
APPLICATION USAGE
None.
RATIONALE
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.
Note that a read() of zero bytes does not modify st_atime. A read()
that requests more than zero bytes, but returns zero, shall modify
st_atime.
Implementations are allowed, but not required, to perform error
checking for read() requests of zero bytes.
Input and Output
The use of I/O with large byte counts has always presented problems.
Ideas such as lread() and lwrite() (using and returning longs) were
considered at one time. The current solution is to use abstract types
on the ISO C standard function to read() and write(). 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 size_t 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 int can hold.
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.
This volume of IEEE Std 1003.1-2001 requires that no action be taken
for read() or write() when nbyte 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.
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).
There were recommendations to add format parameters to read() and
write() 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.
In 4.3 BSD, a read() or write() 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, select(), 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 select() to be used before read() in
situations (such as keyboard input) where interruption of I/O due to a
signal is desired.
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.
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.
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 read() and
write() return -1 and set errno 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 read() and write() 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.
IEEE Std 1003.1-2001 does not specify when an implementation that
buffers read()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 read()
to return a partial byte count, but rather to return -1 and set errno
to [EINTR].
Consideration was also given to combining the two previous options, and
setting errno 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 errno is set, the function
responsible shall return -1.
FUTURE DIRECTIONS
None.
SEE ALSO
fcntl() , ioctl() , lseek() , open() , pipe() , readv() , the Base
Definitions volume of IEEE Std 1003.1-2001, Chapter 11, General
Terminal Interface, <stropts.h>, <sys/uio.h>, <unistd.h>
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 .