.\" Modified Sat Jul 12 20:45:39 1997 by Michael Haardt
.\" <michael@cantor.informatik.rwth-aachen.de>
.\"
-.TH READ 2 2014-05-04 "Linux" "Linux Programmer's Manual"
+.TH READ 2 2018-02-02 "Linux" "Linux Programmer's Manual"
.SH NAME
read \- read from a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
-.sp
+.PP
.BI "ssize_t read(int " fd ", void *" buf ", size_t " count );
.fi
.SH DESCRIPTION
.I fd
into the buffer starting at
.IR buf .
-
+.PP
On files that support seeking,
-the read operation commences at the current file offset,
+the read operation commences at the file offset,
and the file offset is incremented by the number of bytes read.
-If the current file offset is at or past the end of file,
+If the file offset is at or past the end of file,
no bytes are read, and
.BR read ()
returns zero.
-
+.PP
If
.I count
is zero,
with a
.I count
of 0 returns zero and has no other effects.
-
-If
+.PP
+According to POSIX.1, if
.I count
is greater than
.BR SSIZE_MAX ,
-the result is unspecified.
+the result is implementation-defined;
+see NOTES for the upper limit on Linux.
.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.
.BR read ()
was interrupted by a signal.
See also NOTES.
-
+.PP
On error, \-1 is returned, and
.I errno
is set appropriately.
.IR buf ,
the value specified in
.IR count ,
-or the current file offset is not suitably aligned.
+or the file offset is not suitably aligned.
.TP
.B EINVAL
.I fd
is orphaned.
It may also occur when there is a low-level I/O error
while reading from a disk or tape.
+A further possible cause of
+.B EIO
+on networked filesystems is when an advisory lock had been taken
+out on the file descriptor and this lock has been lost.
+See the
+.I "Lost locks"
+section of
+.BR fcntl (2)
+for further details.
.TP
.B EISDIR
.I fd
.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
-.BR EINTR )
-or to return the number of bytes already read.
.SH CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
.SH NOTES
+The types
+.I size_t
+and
+.I ssize_t
+are, respectively,
+unsigned and signed integer data types specified by POSIX.1.
+.PP
On Linux,
.BR read ()
(and similar system calls) will transfer at most
returning the number of bytes actually transferred.
.\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
(This is true on both 32-bit and 64-bit systems.)
-
+.PP
On NFS filesystems, reading small amounts of data will update the
timestamp only the first time, subsequent calls may not do so.
This is caused
.SH BUGS
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
("Thread Interactions with Regular File Operations"):
-
+.PP
.RS 4
All of the following functions shall be atomic with respect to
each other in the effects specified in POSIX.1-2008 when they
operate on regular files or symbolic links: ...
.RE
-
+.PP
Among the APIs subsequently listed are
.BR read ()
and