READ (2)
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 .
If
count
is zero, read() returns zero and has no other results.
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
EINTR
The call was interrupted by a signal before any data was read.
EAGAIN
Non-blocking I/O has been selected using
O_NONBLOCK
and no data was immediately available for reading.
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.
EBADF
fd
is not a valid file descriptor or is not open for reading.
EINVAL
fd
is attached to an object which is unsuitable for reading.
EFAULT
buf
is outside your accessible address space.
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, SVID, AT&T, POSIX, X/OPEN, BSD 4.3
RESTRICTIONS
On NFS file systems, reading small amounts of data will only update the
time stamp 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 atime updates to the server and client side reads satisfied from the
client's cache will not cause 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
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