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READ(2) Linux Programmer's Manual READ(2)
NAME
read - read from a file descriptor
SYNOPSIS
#include
ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data
was immediately available for reading.
EBADF fd is not a valid file descriptor or is not open for reading.
EFAULT buf is outside your accessible address space.
EINTR The call was interrupted by a signal before any data was read.
EINVAL 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.
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 pro-
cess 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.
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, 4.3BSD, POSIX.1-2001.
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 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 filesystems and disks were considered to be fast enough that the
implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may
not be available on files and/or disks.
SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2),
readlink(2), readv(2), select(2), write(2), fread(3)
Linux 2.0.32 1997-07-12 READ(2)
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