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[thirdparty/man-pages.git] / man2 / read.2

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.\"                               1993 Michael Haardt, Ian Jackson.
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.\" Modified Sat Jul 24 00:06:00 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Wed Jan 17 16:02:32 1996 by Michael Haardt
.\"   <michael@cantor.informatik.rwth-aachen.de>
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.\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
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.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
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.\"
.TH READ 2 1997-07-12 "Linux 2.0.32" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.sp
.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
.fi
.SH DESCRIPTION
.BR read ()
attempts to read up to
.I count
bytes from file descriptor
.I fd
into the buffer starting at
.IR buf .
.PP
If
.I count
is zero, \fBread\fP() returns zero and has no other results.
If
.I count
is greater than SSIZE_MAX, the result is unspecified.
.PP
.SH "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
\fBread\fP() was interrupted by a signal.
On error, \-1 is returned, and
.I errno
is set appropriately. In this case it is left unspecified whether
the file position (if any) changes.
.SH ERRORS
.TP
.B EAGAIN
Non-blocking I/O has been selected using
.B O_NONBLOCK
and no data was immediately available for reading.
.TP
.B EBADF
.I fd
is not a valid file descriptor or is not open for reading.
.TP
.B EFAULT
.I buf
is outside your accessible address space.
.TP
.B EINTR
The call was interrupted by a signal before any data was read.
.TP
.B EINVAL
.I fd
is attached to an object which is unsuitable for reading;
or the file was opened with the 
.B O_DIRECT
flag, and either the address specified in
.IR buf ,
the value specified in
.IR count ,
or the current file offset is not suitably aligned.
.TP
.B 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.
.TP
.B EISDIR
.I fd
refers to a directory.
.PP
Other errors may occur, depending on the object connected to
.IR fd .
POSIX allows a
.BR read ()
that is interrupted after reading some data
to return \-1 (with
.I errno
set to EINTR) or to return the number of bytes already read.
.SH "CONFORMING TO"
SVr4, SVID, AT&T, POSIX, X/OPEN, 4.3BSD
.SH 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.
.PP
Many filesystems and disks were considered to be fast enough that the 
implementation of 
.B O_NONBLOCK
was deemed unneccesary. So, O_NONBLOCK may not be available on files
and/or disks.
.SH "SEE ALSO"
.BR close (2),
.BR fcntl (2),
.BR ioctl (2),
.BR lseek (2),
.BR open (2),
.BR readdir (2),
.BR readlink (2),
.BR select (2),
.BR write (2),
.BR fread (3),
.BR readv (3)