READ(2) manual page
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read - read from a file descriptor
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
read() attempts
to read up to count bytes from file descriptor fd into the buffer starting
at buf.
On files that support seeking, the read operation commences at
the current file offset, and the file offset is incremented by the number
of bytes read. If the current file offset is at or past the end of file,
no bytes are read, and read() returns zero.
If count is zero, read() may
detect the errors described below. In the absence of any errors, or if read()
does not check for errors, a read() with a count of 0 returns zero and
has no other effects.
If count is greater than SSIZE_MAX, the result is
unspecified.
On success, the number of bytes read is returned
(zero indicates end of file), and the file position is advanced by this
number. It is not an error if this number is smaller than the number of
bytes requested; this may happen for example because fewer bytes are actually
available right now (maybe because we were close to end-of-file, or because
we are reading from a pipe, or from a terminal), or because read() was
interrupted by a signal. On error, -1 is returned, and errno is set appropriately.
In this case, it is left unspecified whether the file position (if any)
changes.
- EAGAIN
- The file descriptor fd refers to a file other than
a socket and has been marked nonblocking (O_NONBLOCK), and the read would
block.
- EAGAIN or EWOULDBLOCK
- The file descriptor fd refers to a socket
and has been marked nonblocking (O_NONBLOCK), and the read would block.
POSIX.1-2001 allows either error to be returned for this case, and does not
require these constants to have the same value, so a portable application
should check for both possibilities.
- EBADF
- fd is not a valid file descriptor
or is not open for reading.
- EFAULT
- buf is outside your accessible address
space.
- EINTR
- The call was interrupted by a signal before any data was read;
see signal(7)
.
- EINVAL
- fd is attached to an object which is unsuitable for
reading; or the file was opened with the O_DIRECT flag, and either the
address specified in buf, the value specified in count, or the current
file offset is not suitably aligned.
- EINVAL
- fd was created via a call to
timerfd_create(2)
and the wrong size buffer was given to read(); see timerfd_create(2)
for further information.
- EIO
- I/O error. This will happen for example when
the process is in a background process group, tries to read from its controlling
terminal, and either it is ignoring or blocking SIGTTIN or its process
group is orphaned. It may also occur when there is a low-level I/O error
while reading from a disk or tape.
- EISDIR
- fd refers to a directory.
Other
errors may occur, depending on the object connected to fd. POSIX allows
a read() that is interrupted after reading some data to return -1 (with
errno set to EINTR) or to return the number of bytes already read.
SVr4, 4.3BSD, POSIX.1-2001.
On NFS filesystems, reading small amounts
of data will update the timestamp only the first time, subsequent calls
may not do so. This is caused by client side attribute caching, because
most if not all NFS clients leave st_atime (last file access time) updates
to the server and client side reads satisfied from the client’s cache will
not cause st_atime updates on the server as there are no server side reads.
UNIX semantics can be obtained by disabling client side attribute caching,
but in most situations this will substantially increase server load and
decrease performance.
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
("Thread Interactions with Regular File Operations"):
All of the following
functions shall be atomic with respect to each other in the effects specified
in POSIX.1-2008 when they operate on regular files or symbolic links: ...
Among
the APIs subsequently listed are read() and readv(2)
. And among the effects
that should be atomic across threads (and processes) are updates of the
file offset. However, on Linux before version 3.14, this was not the case:
if two processes that share an open file description (see open(2)
) perform
a read() (or readv(2)
) at the same time, then the I/O operations were not
atomic with respect updating the file offset, with the result that the
reads in the two processes might (incorrectly) overlap in the blocks of
data that they obtained. This problem was fixed in Linux 3.14.
close(2)
, fcntl(2)
, ioctl(2)
, lseek(2)
, open(2)
, pread(2)
, readdir(2)
,
readlink(2)
, readv(2)
, select(2)
, write(2)
, fread(3)
This page
is part of release 3.78 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this
page, can be found at http://www.kernel.org/doc/man-pages/.
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