| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
READ(2) Linux Programmer's Manual READ(2)
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.
If count is zero, read() returns zero and has no other results. 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 tty, 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 file systems, reading small amounts of data will only update the
timestamp 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.
Many file systems and disks were considered to be fast enough that the
implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may not
be available on files and/or disks.
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 3.32 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/.
Linux 2009-02-23 READ(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface