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.
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.
Non-blocking I/O has been selected using O_NONBLOCK
and no data was immediately available for reading.
fd is not a valid file descriptor or is not open for
buf is outside your accessible address space.
The call was interrupted by a signal before any data was
fd is attached to an object which is unsuitable for
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.
fd refers to a directory.
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
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
Many filesystems and disks were considered to be fast enough that the
implementation of O_NONBLOCK was deemed unneccesary. So, O_NONBLOCK may
not be available on files and/or disks.