From 1dcacd2cfdce9695c9e66e21d6ec0292d66da400 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date: Fri, 12 Jun 2020 12:11:39 +0200
Subject: [PATCH 4/4] readfile.2

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+.\" This manpage is Copyright (C) 2020 Greg Kroah-Hartman;
+.\"  and Copyright (C) 2020 The Linux Foundation
+.\"
+.\" %%%LICENSE_START(VERBATIM)
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\"
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein.  The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\"
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
+.\"
+.TH READFILE 2 2020-06-12 "Linux" "Linux Programmer's Manual"
+.SH NAME
+readfile \- read a file into a buffer
+.SH SYNOPSIS
+.nf
+.B #include <unistd.h>
+.PP
+.BI "ssize_t readfile(int " dirfd ", const char *" pathname ", void *" buf \
+", size_t " count ", int " flags );
+.fi
+.SH DESCRIPTION
+.BR readfile ()
+attempts to open the file specified by
+.IR pathname
+and to read up to
+.I count
+bytes from the file into the buffer starting at
+.IR buf .
+It is to be a shortcut of doing the sequence of
+.BR open ()
+and then
+.BR read ()
+and then
+.BR close ()
+for small files that are read frequently, such as those in
+.B procfs
+or
+.BR sysfs .
+.PP
+If the size of file is smaller than the value provided in
+.I count
+then the whole file will be copied into
+.IR buf .
+.PP
+If the file is larger than the value provided in
+.I count
+then only
+.I count
+number of bytes will be copied into
+.IR buf .
+.PP
+The argument
+.I flags
+may contain one of the following
+.IR "access modes" :
+.BR O_NOFOLLOW ", or " O_NOATIME .
+.PP
+If the pathname given in
+.I pathname
+is relative, then it is interpreted relative to the directory
+referred to by the file descriptor
+.IR dirfd .
+.PP
+If
+.I pathname
+is relative and
+.I dirfd
+is the special value
+.BR AT_FDCWD ,
+then
+.I pathname
+is interpreted relative to the current working
+directory of the calling process (like
+.BR open ()).
+.PP
+If
+.I pathname
+is absolute, then
+.I dirfd
+is ignored.
+.SH RETURN VALUE
+On success, the number of bytes read is returned.
+It is not an error if this number is smaller than the number of bytes
+requested; this can happen if the file is smaller than the number of
+bytes requested.
+.PP
+On error, \-1 is returned, and
+.I errno
+is set appropriately.
+.SH ERRORS
+.TP
+.B EAGAIN
+The file descriptor
+.I fd
+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
+The file descriptor
+.I fd
+refers to a socket and has been marked nonblocking
+.RB ( O_NONBLOCK ),
+and the read would block.
+POSIX.1-2001 allows either error to be returned for this case,
+and does not require these constants to have the same value,
+so a portable application should check for both possibilities.
+.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; see
+.BR signal (7).
+.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 file offset is not suitably aligned.
+.TP
+.B EINVAL
+.I fd
+was created via a call to
+.BR timerfd_create (2)
+and the wrong size buffer was given to
+.BR read ();
+see
+.BR timerfd_create (2)
+for further information.
+.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 terminal,
+and either it is ignoring or blocking
+.B 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.
+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
+refers to a directory.
+.PP
+Other errors may occur, depending on the object connected to
+.IR fd .
+.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
+by client side attribute caching, because most if not all NFS clients
+leave
+.I st_atime
+(last file access time)
+updates to the server, and client side reads satisfied from the
+client's cache will not cause
+.I 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.
+.SH BUGS
+None yet!
+.SH SEE ALSO
+.BR close (2),
+.BR open (2),
+.BR openat (2),
+.BR read (2),
+.BR fread (3)