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read() - Unix, Linux System Call
read - read from a file descriptor
ssize_t read(int fd, void *buf, size_t count);
read() attempts to read up to
count bytes from file descriptor
fd into the buffer starting at
count is zero, read() returns zero and has no other results.
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.
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.
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 reading.
buf is outside your accessible address space.
The call was interrupted by a signal before any data was read.
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 current file offset is not suitably aligned.
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.
SVr4, 4.3BSD, POSIX.1-2001.
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 st_atime (last file access time)
updates to the server and client side reads satisfied from the
clients 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 filesystems and disks were considered to be fast enough that the
O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may not be available on files