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 buf.
If count is zero, read() returns zero and has no other
results. If count is greater than SSIZE_MAX, the result
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 hap
pen 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
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
EISDIR fd refers to a directory.
EBADF fd is not a valid file descriptor or is not open
EINVAL fd is attached to an object which is unsuitable for
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.
SVr4, SVID, AT&T, POSIX, X/OPEN, BSD 4.3
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 sub
stantially increase server load and decrease performance.
close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), read
link(2), select(2), write(2), fread(3)