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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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read(2) System Calls Manual read(2)
read - read from a file descriptor
Standard C library (libc, -lc)
#include <unistd.h>
ssize_t read(int fd, void buf[.count], 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 to indicate the error.
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. A further possible cause of EIO on
networked filesystems is when an advisory lock had been
taken out on the file descriptor and this lock has been
lost. See the Lost locks section of fcntl(2) for further
details.
EISDIR fd refers to a directory.
Other errors may occur, depending on the object connected to fd.
POSIX.1-2008.
SVr4, 4.3BSD, POSIX.1-2001.
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, before Linux
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 to 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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.10.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-02-02. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.10 2024-07-23 read(2)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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