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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.
The file descriptor fd refers to a file other
than a socket and has been marked nonblocking
(O_NONBLOCK), and the
read would block.
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.
fd is not a
valid file descriptor or is not open for reading.
buf is
outside your accessible address space.
The call was interrupted by a signal before any data was read; see signal(7).
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.
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.
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.
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.
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.
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 3.54 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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This manpage is Copyright (C) 1992 Drew Eckhardt; and Copyright (C) 1993 Michael Haardt, Ian Jackson. %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faithcs.unc.edu> Modified Wed Jan 17 16:02:32 1996 by Michael Haardt <michaelcantor.informatik.rwth-aachen.de> Modified Thu Apr 11 19:26:35 1996 by Andries Brouwer <aebcwi.nl> Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aebcwi.nl> Modified Fri Jan 31 16:47:33 1997 by Eric S. Raymond <esrthyrsus.com> Modified Sat Jul 12 20:45:39 1997 by Michael Haardt <michaelcantor.informatik.rwth-aachen.de> |