read(2) - Linux manual page (original) (raw)
read(2) System Calls Manual read(2)
NAME top
read - read from a file descriptorLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <unistd.h>**
**ssize_t read(int** _fd_**, void** _buf_**[.**_count_**], size_t** _count_**);**DESCRIPTION top
**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.RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.
In this case, it is left unspecified whether the file position (if
any) changes.ERRORS top
**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)](../man2/open.2.html) 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)](../man7/signal.7.html).
**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)](../man2/timerfd%5Fcreate.2.html) and the
wrong size buffer was given to **read**(); see
[timerfd_create(2)](../man2/timerfd%5Fcreate.2.html) 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)](../man2/fcntl.2.html) for further
details.
**EISDIR** _fd_ refers to a directory.
Other errors may occur, depending on the object connected to _fd_.STANDARDS top
POSIX.1-2008.HISTORY top
SVr4, 4.3BSD, POSIX.1-2001.NOTES top
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 _statime_ (last file access time) updates
to the server, and client side reads satisfied from the client's
cache will not cause _statime_ 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.BUGS top
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)](../man2/readv.2.html). 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)](../man2/open.2.html)) perform a **read**() (or [readv(2)](../man2/readv.2.html)) 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.SEE ALSO top
[close(2)](../man2/close.2.html), [fcntl(2)](../man2/fcntl.2.html), [ioctl(2)](../man2/ioctl.2.html), [lseek(2)](../man2/lseek.2.html), [open(2)](../man2/open.2.html), [pread(2)](../man2/pread.2.html),
[readdir(2)](../man2/readdir.2.html), [readlink(2)](../man2/readlink.2.html), [readv(2)](../man2/readv.2.html), [select(2)](../man2/select.2.html), [write(2)](../man2/write.2.html), [fread(3)](../man3/fread.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 read(2)
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