NAME
read - 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 descrip-
tor 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 indi-
cates 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 situa-
tions this will substantially increase server load and
decrease performance.
SEE ALSO
readdir(2), write(2), write(2), lseek(2), select(2),
readlink(2), ioctl(2),