READ(2) Linux Programmer's Manual READ(2)
NAME
read - read from a file descriptor
SYNOPSIS
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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
EBADF fd
EFAULT buf
EINTR The call was interrupted by a signal before any data was read; see signal
EINVAL fd is attached to an object which is unsuitable for reading; or the file was opened with the O_DIRECT
fied 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 informa‐
tion.
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.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
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.
SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2), readv(2), select(2), write(2), fread(3)
COLOPHON
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/.
Linux 2013-02-12 READ(2)
