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READ(3P) POSIX Programmer's Manual READ(3P)
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.
pread, read — read from a file
#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);
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 last data access timestamp 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.
Upon successful completion, these functions 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.
These functions shall fail if:
EAGAIN The file is neither a pipe, nor a FIFO, nor a socket, the
O_NONBLOCK flag is set for the file descriptor, and the thread
would be delayed in the read operation.
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 group
attempting to read from its controlling terminal, and either
the calling thread is blocking SIGTTIN or the process is
ignoring SIGTTIN or the process group of the process 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 pread() function shall fail if:
EINVAL The file is a regular file or block special file, and the
offset argument is negative. The file pointer shall remain
unchanged.
ESPIPE The file is a pipe, FIFO, or socket.
The read() function shall fail if:
EAGAIN The file is a pipe or FIFO, the O_NONBLOCK flag is set for the
file descriptor, and the thread would be delayed in the read
operation.
EAGAIN or EWOULDBLOCK
The file is a socket, the O_NONBLOCK flag is set for the file
descriptor, and the thread would be delayed in the read
operation.
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.
These 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 following sections are informative.
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);
...
None.
This volume of POSIX.1‐2008 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 the last data access
timestamp. A read() that requests more than zero bytes, but returns
zero, is required to modify the last data access timestamp.
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 POSIX.1‐2008 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 POSIX.1‐2008 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 POSIX.1‐2008, 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
POSIX.1‐2008. 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
POSIX.1‐2008 to describe what happens in the case of hardware errors.
Earlier versions of this standard allowed two very different
behaviors with regard to the handling of interrupts. In order to
minimize the resulting confusion, it was decided that POSIX.1‐2008
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 US 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
POSIX.1‐2008 is to return the number of bytes transferred.
POSIX.1‐2008 does not specify when an implementation that buffers
read()s actually moves the data into the user-supplied buffer, so an
implementation may choose 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.
This volume of POSIX.1‐2008 intentionally does not specify any
pread() errors related to pipes, FIFOs, and sockets other than
[ESPIPE].
None.
fcntl(3p), ioctl(3p), lseek(3p), open(3p), pipe(3p), readv(3p)
The Base Definitions volume of POSIX.1‐2008, Chapter 11, General
Terminal Interface, stropts.h(0p), sys_uio.h(0p), unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 READ(3P)
Pages that refer to this page: aio.h(0p), stropts.h(0p), sys_uio.h(0p), unistd.h(0p), aio_error(3p), aio_fsync(3p), aio_read(3p), aio_return(3p), fchmod(3p), fdatasync(3p), fstatvfs(3p), futimens(3p), getmsg(3p), ioctl(3p), lio_listio(3p), lockf(3p), open(3p), pipe(3p), poll(3p), pread(3p), pselect(3p), putmsg(3p), readv(3p), recv(3p), recvfrom(3p), shutdown(3p), stdin(3p), ungetc(3p), ungetwc(3p), write(3p)