Start of man-pages-5.03: renaming .Announce and .lsm files

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

diff --git a/man2/read.2 b/man2/read.2
index faa4b249089d81b58f612befc740c62f8f91781a..6e5123fa4f3bb59277565e733bb5671aa7f4ac91 100644 (file)
--- a/man2/read.2
+++ b/man2/read.2
@@ -32,13 +32,13 @@
 .\" 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
@@ -49,15 +49,15 @@ bytes from file descriptor
 .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,
@@ -72,12 +72,13 @@ does not check for errors, a
 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.
@@ -87,6 +88,8 @@ 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
 .BR read ()
 was interrupted by a signal.
+See also NOTES.
+.PP
 On error, \-1 is returned, and
 .I errno
 is set appropriately.
@@ -100,6 +103,11 @@ The file descriptor
 refers to a file other than a socket and has been marked nonblocking
 .RB ( O_NONBLOCK ),
 and the read would block.
+See
+.BR open (2)
+for further details on the
+.BR O_NONBLOCK
+flag.
 .TP
 .BR EAGAIN " or " EWOULDBLOCK
 .\" Actually EAGAIN on Linux
@@ -133,7 +141,7 @@ flag, and either the address specified in
 .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
@@ -155,6 +163,15 @@ 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.
+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
@@ -162,17 +179,24 @@ 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
-.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
+0x7ffff000 (2,147,479,552) bytes,
+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
@@ -191,13 +215,13 @@ increase server load and decrease performance.
 .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