attempts to read up to
bytes from file descriptor
into the buffer starting at
returns zero and has no other results.
is greater than
the result is unspecified.
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
was interrupted by a signal.
On error, -1 is returned, and
is set appropriately.
In this case it is left unspecified whether
the file position (if any) changes.
Non-blocking I/O has been selected using
and no data was immediately available for reading.
is not a valid file descriptor or is not open for reading.
is outside your accessible address space.
The call was interrupted by a signal before any data was read; see
is attached to an object which is unsuitable for reading;
or the file was opened with the
flag, and either the address specified in
the value specified in
or the current file offset is not suitably aligned.
was created via a call to
and the wrong size buffer was given to
for further information.
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
or its process group
It may also occur when there is a low-level I/O error
while reading from a disk or tape.
refers to a directory.
Other errors may occur, depending on the object connected to
POSIX allows a
that is interrupted after reading some data
to return -1 (with
or to return the number of bytes already read.
SVr4, 4.3BSD, POSIX.1-2001.
On NFS file systems, reading small amounts of data will only update the
timestamp 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.
Many file systems and disks were considered to be fast enough that the
was deemed unnecessary.
may not be available on files