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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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.
On files that support seeking, the read operation commences at the
file offset, and the file offset is incremented by the number of
bytes read. If the 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.
According to POSIX.1, if count is greater than SSIZE_MAX, the result
is implementation-defined; see NOTES for the upper limit on Linux.
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. See also NOTES.
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. See open(2) for further details on the
O_NONBLOCK flag.
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
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.
SVr4, 4.3BSD, POSIX.1-2001.
The types size_t and ssize_t are, respectively, unsigned and signed
integer data types specified by POSIX.1.
On Linux, read() (and similar system calls) will transfer at most
0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
actually transferred. (This is true on both 32-bit and 64-bit
systems.)
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 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2017-03-13 READ(2)
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