read() builtin doesn't read integer value /proc files (but bash's does)

read() builtin doesn't read integer value /proc files (but bash's does)

[Steve Schnepp]

Hi, I opened bug 595063 on the debian BTS [1] and I was suggested to
resend the email upstream.

So I copied the body of the bug below :

dash's read() builtin seems to read the underlying file 1 char at a
time. This doesn't work with some files under /proc, since procfs isn't
fully POSIX compliant.

With bash it works :

$ bash -c 'read MAX < /proc/sys/kernel/pid_max; echo $MAX'
32768

With dash it only reads the first character :

$ dash -c 'read MAX < /proc/sys/kernel/pid_max; echo $MAX'
3

If we use the cat(1) external program it works :

$ dash -c 'MAX=$(cat /proc/sys/kernel/pid_max); echo $MAX'
32768

After a little digging, it only appears on files that contains just an
integer value. When asked to read with a non-null offset (*ppos != 0),
__do_proc_dointvec() just returns 0 (meaning an EOF) as shown on [2].

I'm aware that the issue isn't strictly a dash one, since it has the
right to read one character at a time. But since fixing procfs to be
conforming to POSIX isn't a realistic option, would it be possible to
have a workaround that doesn't involve an external tool like cat(1) ?

[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=595063
[2] http://lxr.linux.no/#linux+v2.6.32/kernel/sysctl.c#L2371
--
Steve Schnepp
http://blog.pwkf.org/

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Steve Schnepp]

[-- Attachment #1: Type: text/plain, Size: 424 bytes --]

2010/9/1 Steve Schnepp <steve.schnepp@gmail.com>:
> conforming to POSIX isn't a realistic option, would it be possible to
> have a workaround that doesn't involve an external tool like cat(1) ?

Hi, I just hacked & attached a little patch away to be able to solve this case.
Feel free to reply with your comments.

NB: I just targeted dash-0.5.5.1, but it might apply to any version.

--
Steve Schnepp
http://blog.pwkf.org/

[-- Attachment #2: chunked_bltin_read.diff --]
[-- Type: application/octet-stream, Size: 1476 bytes --]

--- dash-0.5.5.1/src/miscbltin.c.orig   2009-01-14 00:37:13.000000000 +0100
+++ dash-0.5.5.1/src/miscbltin.c        2010-09-02 16:54:29.330830859 +0200
@@ -58,6 +58,34 @@

 #undef rflag

+#define CHUNK_READ_SIZE 32
+
+/*
+ * Reads from fd 0, with a CHUNK_READ_SIZE,
+ * but emitting one char at a time
+ */
+static int _read_bufferred(char* c) {
+       static char buffer[CHUNK_READ_SIZE];
+       static int buffer_offset = 0;
+       static int buffer_len = 0;
+
+       if (buffer_len == 0) {
+               // No caracter left, resetting buffer & read some more
+               buffer_offset = 0;
+               buffer_len = read(0, buffer, CHUNK_READ_SIZE);
+
+               if (buffer_len == 0) {
+                       // Nothing to read anymore
+                       return 0;
+               }
+       }
+
+       // Still some character left
+       *c = buffer[buffer_offset++];
+       buffer_len--;
+
+       return 1;
+}


 /*
@@ -65,6 +93,7 @@
  * following character.
  *
  * This uses unbuffered input, which may be avoidable in some cases.
+ * XXX - Uses _read_bufferred() that chunks read(), but emits one char at a time
  */

 int
@@ -104,7 +133,7 @@
        backslash = 0;
        STARTSTACKSTR(p);
        for (;;) {
-               if (read(0, &c, 1) != 1) {
+               if (_read_bufferred(&c) != 1) {
                        status = 1;
                        break;
                }

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Jilles Tjoelker]

On Thu, Sep 02, 2010 at 05:02:55PM +0200, Steve Schnepp wrote:
> 2010/9/1 Steve Schnepp <steve.schnepp@gmail.com>:
> > conforming to POSIX isn't a realistic option, would it be possible to
> > have a workaround that doesn't involve an external tool like cat(1) ?

> Hi, I just hacked & attached a little patch away to be able to solve
> this case.
> Feel free to reply with your comments.

> NB: I just targeted dash-0.5.5.1, but it might apply to any version.

This patch assumes that the file descriptor is discarded afterwards (its
position does not matter). Therefore the very common construct
  while read x; do
    ...
  done
stops working.

A possible fix is to check first if the input supports seeking. If it
does, use the buffering and at the end of the line seek backwards for
the number of bytes remaining in the buffer. If it does not, read one
byte at a time.

-- 
Jilles Tjoelker

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Steve Schnepp]

[-- Attachment #1: Type: text/plain, Size: 1020 bytes --]

2010/9/3 Jilles Tjoelker <jilles@stack.nl>:
> This patch assumes that the file descriptor is discarded afterwards (its
> position does not matter). Therefore the very common construct
>  while read x; do
>    ...
>  done
> stops working.

Ohh.. thanks for that, I didn't see it.

Actually "while read x" continues to work.
But "reopening the file" doesn't as in :

read a b < datafile
echo ${a} ${b}
read a b < datafile
echo ${a} ${b}

I attached an updated patch that corrects this pb by discarding the
buffer when opening a new file.
I also put everything in new files (bufreadcmd.c & .h), in order to
ease its understanding.

--
Steve Schnepp
http://blog.pwkf.org/


>
> A possible fix is to check first if the input supports seeking. If it
> does, use the buffering and at the end of the line seek backwards for
> the number of bytes remaining in the buffer. If it does not, read one
> byte at a time.
>
> --
> Jilles Tjoelker
>


--
Steve Schnepp
http://blog.pwkf.org/

[-- Attachment #2: dash_readbuf.diff --]
[-- Type: text/x-patch, Size: 6637 bytes --]

Common subdirectories: dash-0.5.4/src/bltin and dash-0.5.4-patched/src/bltin
diff -puN dash-0.5.4/src/bufreadcmd.c dash-0.5.4-patched/src/bufreadcmd.c
--- dash-0.5.4/src/bufreadcmd.c	1970-01-01 01:00:00.000000000 +0100
+++ dash-0.5.4-patched/src/bufreadcmd.c	2010-09-04 12:31:46.000000000 +0200
@@ -0,0 +1,59 @@
+/*
+ * Offers a buffered read builtin
+ */
+
+#include <unistd.h>
+#include <stdlib.h>
+
+#include "bufreadcmd.h"
+
+#ifdef BUF_READ_BUILTIN_DISABLED
+int dup2_wrapper(int old, int new) {
+	return dup2(old, new);
+}
+int read_stdin_bufferred(char *c) {
+	return read(0, buffer, 1);
+}
+#else // BUF_READ_BUILTIN_DISABLED
+
+/*
+ * Reads from fd 0, with a CHUNK_READ_SIZE,
+ * but emitting one char at a time
+ */
+#define CHUNK_READ_SIZE 32
+static char buffer[CHUNK_READ_SIZE];
+static int buffer_offset = 0;
+static int buffer_len = 0;
+
+int read_stdin_bufferred(char* c) {
+       if (buffer_len == 0) {
+               // No caracter left, resetting buffer & read some more
+               buffer_offset = 0;
+               buffer_len = read(0, buffer, CHUNK_READ_SIZE);
+
+               if (buffer_len == 0) {
+                       // Nothing to read anymore
+                       return 0;
+               }
+       }
+
+       // Still some character left
+       *c = buffer[buffer_offset++];
+       buffer_len--;
+
+       return 1;
+}
+
+static void _flush_readcmd(int fd) {
+	if (fd == 0) { 
+		// Flush the buffer, discarding its content
+		buffer_len = 0;
+	}
+}
+
+/* Intercept dup2() calls */
+int dup2_wrapper(int old, int new) {
+	_flush_readcmd(new);
+	return dup2(old, new);
+}
+#endif // BUF_READ_BUILTIN_DISABLED
diff -puN dash-0.5.4/src/bufreadcmd.h dash-0.5.4-patched/src/bufreadcmd.h
--- dash-0.5.4/src/bufreadcmd.h	1970-01-01 01:00:00.000000000 +0100
+++ dash-0.5.4-patched/src/bufreadcmd.h	2010-09-04 12:23:53.000000000 +0200
@@ -0,0 +1,3 @@
+/* Used for flushing the readcmd read() buffer */
+int dup2_wrapper(int to, int from);
+int read_stdin_bufferred(char *c);
Common subdirectories: dash-0.5.4/src/.deps and dash-0.5.4-patched/src/.deps
diff -puN dash-0.5.4/src/eval.c dash-0.5.4-patched/src/eval.c
--- dash-0.5.4/src/eval.c	2007-07-13 10:26:42.000000000 +0200
+++ dash-0.5.4-patched/src/eval.c	2010-09-04 12:24:55.000000000 +0200
@@ -64,6 +64,8 @@
 #include "myhistedit.h"
 #endif
 
+#include "bufreadcmd.h"
+
 
 /* flags in argument to evaltree */
 #define EV_EXIT 01		/* exit after evaluating tree */
@@ -543,11 +545,12 @@ evalpipe(union node *n, int flags)
 				close(pip[0]);
 			}
 			if (prevfd > 0) {
-				dup2(prevfd, 0);
+				dup2_wrapper(prevfd, 0);
+
 				close(prevfd);
 			}
 			if (pip[1] > 1) {
-				dup2(pip[1], 1);
+				dup2_wrapper(pip[1], 1);
 				close(pip[1]);
 			}
 			evaltreenr(lp->n, flags);
@@ -625,7 +628,7 @@ evalbackcmd(union node *n, struct backcm
 			FORCEINTON;
 			close(pip[0]);
 			if (pip[1] != 1) {
-				dup2(pip[1], 1);
+				dup2_wrapper(pip[1], 1);
 				close(pip[1]);
 			}
 			eflag = 0;
diff -puN dash-0.5.4/src/Makefile dash-0.5.4-patched/src/Makefile
--- dash-0.5.4/src/Makefile	2010-09-04 13:06:05.000000000 +0200
+++ dash-0.5.4-patched/src/Makefile	2010-09-04 13:08:26.000000000 +0200
@@ -57,7 +57,7 @@ am__objects_1 = alias.$(OBJEXT) arith_yy
 	miscbltin.$(OBJEXT) mystring.$(OBJEXT) options.$(OBJEXT) \
 	parser.$(OBJEXT) redir.$(OBJEXT) show.$(OBJEXT) trap.$(OBJEXT) \
 	output.$(OBJEXT) printf.$(OBJEXT) system.$(OBJEXT) \
-	test.$(OBJEXT) times.$(OBJEXT) var.$(OBJEXT)
+	test.$(OBJEXT) times.$(OBJEXT) var.$(OBJEXT) bufreadcmd.$(OBJEXT)
 am_dash_OBJECTS = $(am__objects_1) arith.$(OBJEXT)
 dash_OBJECTS = $(am_dash_OBJECTS)
 dash_DEPENDENCIES = builtins.o init.o nodes.o signames.o syntax.o
@@ -169,14 +169,14 @@ dash_CFILES = \
 	alias.c arith_yylex.c cd.c error.c eval.c exec.c expand.c \
 	histedit.c input.c jobs.c mail.c main.c memalloc.c miscbltin.c \
 	mystring.c options.c parser.c redir.c show.c trap.c output.c \
-	bltin/printf.c system.c bltin/test.c bltin/times.c var.c
+	bltin/printf.c system.c bltin/test.c bltin/times.c var.c bufreadcmd.c
 
 dash_SOURCES = \
 	$(dash_CFILES) arith.y \
 	alias.h bltin/bltin.h cd.h error.h eval.h exec.h expand.h hetio.h \
 	init.h input.h jobs.h machdep.h mail.h main.h memalloc.h miscbltin.h \
 	myhistedit.h mystring.h options.h output.h parser.h redir.h shell.h \
-	show.h system.h trap.h var.h
+	show.h system.h trap.h var.h bufreadcmd.h
 
 dash_LDADD = builtins.o init.o nodes.o signames.o syntax.o
 HELPERS = mkinit mksyntax mknodes mksignames
diff -puN dash-0.5.4/src/miscbltin.c dash-0.5.4-patched/src/miscbltin.c
--- dash-0.5.4/src/miscbltin.c	2007-07-13 10:26:43.000000000 +0200
+++ dash-0.5.4-patched/src/miscbltin.c	2010-09-04 12:23:21.000000000 +0200
@@ -55,16 +55,17 @@
 #include "miscbltin.h"
 #include "mystring.h"
 #include "main.h"
+#include "bufreadcmd.h"
 
 #undef rflag
 
 
-
 /*
  * The read builtin.  The -e option causes backslashes to escape the
  * following character.
  *
  * This uses unbuffered input, which may be avoidable in some cases.
+ * XXX - Uses _read_bufferred() that chunks read(), but emits one char at a time
  */
 
 int
@@ -104,7 +105,7 @@ readcmd(int argc, char **argv)
 	backslash = 0;
 	STARTSTACKSTR(p);
 	for (;;) {
-		if (read(0, &c, 1) != 1) {
+		if (read_stdin_bufferred(&c) != 1) {
 			status = 1;
 			break;
 		}
diff -puN dash-0.5.4/src/miscbltin.h dash-0.5.4-patched/src/miscbltin.h
--- dash-0.5.4/src/miscbltin.h	2007-07-13 10:26:43.000000000 +0200
+++ dash-0.5.4-patched/src/miscbltin.h	2010-09-04 10:46:02.000000000 +0200
@@ -29,3 +29,6 @@
 int readcmd(int, char **);
 int umaskcmd(int, char **);
 int ulimitcmd(int, char **);
+
+/* Used for flushing the readcmd read() buffer */
+void flush_readcmd(int fd);
diff -puN dash-0.5.4/src/redir.c dash-0.5.4-patched/src/redir.c
--- dash-0.5.4/src/redir.c	2007-07-13 10:26:43.000000000 +0200
+++ dash-0.5.4-patched/src/redir.c	2010-09-04 12:25:49.000000000 +0200
@@ -56,6 +56,8 @@
 #include "memalloc.h"
 #include "error.h"
 
+#include "bufreadcmd.h"
+
 
 #define REALLY_CLOSED -3	/* fd that was closed and still is */
 #define EMPTY -2		/* marks an unused slot in redirtab */
@@ -265,14 +267,14 @@ dupredirect(redir, f)
 				memory[fd] = 1;
 			else
 #endif
-				if (dup2(f, fd) < 0) {
+				if (dup2_wrapper(f, fd) < 0) {
 					err = errno;
 					goto err;
 				}
 			return;
 		}
 		f = fd;
-	} else if (dup2(f, fd) < 0)
+	} else if (dup2_wrapper(f, fd) < 0)
 		err = errno;
 
 	close(f);
@@ -354,7 +356,7 @@ popredir(int drop)
 			break;
 		default:
 			if (!drop)
-				dup2(rp->renamed[i], i);
+				dup2_wrapper(rp->renamed[i], i);
 			close(rp->renamed[i]);
 			break;
 		}

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Jilles Tjoelker]

On Sat, Sep 04, 2010 at 08:20:33PM +0200, Steve Schnepp wrote:
> 2010/9/3 Jilles Tjoelker <jilles@stack.nl>:
> > This patch assumes that the file descriptor is discarded afterwards (its
> > position does not matter). Therefore the very common construct
> >  while read x; do
> >    ...
> >  done
> > stops working.

> Ohh.. thanks for that, I didn't see it.

> Actually "while read x" continues to work.
> But "reopening the file" doesn't as in :

> read a b < datafile
> echo ${a} ${b}
> read a b < datafile
> echo ${a} ${b}

You're right, it's even stranger than I expected.

> I attached an updated patch that corrects this pb by discarding the
> buffer when opening a new file.

This discarding is still bad as it throws away valid data if the open
file description is shared. This happens if stdin is redirected inside a
while read... loop.

Furthermore, I think constructions like
  read x; cat
and
  read x; (read y); read z
should keep working. This requires that the input's file position be
synced whenever another process may see it (fork/exit). Due to the
highly dynamic character of the shell and the common use of fd 0, this
probably means that you can't do better than syncing after each read
builtin. (For example, 'read' could be overridden with a function after
the third line.)

Another thought:
  exec 3<&0; read x; read y <&3
or even
  sh -c 'read x; read y <&3' 3<&0
Different file descriptors may refer to the same open file description
and the shell may not know this.

-- 
Jilles Tjoelker

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Herbert Xu]

On Sat, Sep 04, 2010 at 07:35:04PM +0000, Jilles Tjoelker wrote:
> 
> > I attached an updated patch that corrects this pb by discarding the
> > buffer when opening a new file.
> 
> This discarding is still bad as it throws away valid data if the open
> file description is shared. This happens if stdin is redirected inside a

I'm with Jilles on this.  I also don't particularly feel like
bloating dash just because of the borked /proc interface when
there is a perfectly adequate work-around in "cat".

	value=$(cat /proc/file)

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: read() builtin doesnt read integer value /proc files (but bashs does)

[Cristian Ionescu-Idbohrn]

On Sun, 28 Nov 2010, Herbert Xu wrote:
> On Sat, Sep 04, 2010 at 07:35:04PM +0000, Jilles Tjoelker wrote:
> >
> > > I attached an updated patch that corrects this pb by discarding the
> > > buffer when opening a new file.
> >
> > This discarding is still bad as it throws away valid data if the open
> > file description is shared. This happens if stdin is redirected inside a
>
> I'm with Jilles on this.  I also don't particularly feel like
> bloating dash just because of the borked /proc interface when
> there is a perfectly adequate work-around in "cat".
>
> 	value=$(cat /proc/file)

I wouldn't call that "a perfectly adequate work-around", but a painful and
unadequate work-around.  And this example will hopefully show why:

$ dash -c 'loops=10000; while [ $loops -gt 0 ];do read MAX
</proc/sys/kernel/pid_max; loops=$(($loops - 1)); done; times'
0m0.180000s 0m0.100000s
0m0.000000s 0m0.000000s

total: 0.28s

$ dash -c 'loops=10000; while [ $loops -gt 0 ];do MAX=$(cat
/proc/sys/kernel/pid_max); loops=$(($loops - 1)); done; times'
0m0.280000s 0m1.330000s
0m3.840000s 0m1.560000s

total: 7.01s

That is, the first example is 24x more efficient than the second.  And
that realy _matters_, I would say.


Cheers,

-- 
Cristian

Re: read() builtin doesnt read integer value /proc files (but bashs does)

[Jonathan Nieder]

Hi Cristian,

Cristian Ionescu-Idbohrn wrote:
> On Sun, 28 Nov 2010, Herbert Xu wrote:
> > On Sat, Sep 04, 2010 at 07:35:04PM +0000, Jilles Tjoelker wrote:

>>> This discarding is still bad as it throws away valid data if the open
>>> file description is shared. This happens if stdin is redirected inside a
>>
>> I'm with Jilles on this.  I also don't particularly feel like
>> bloating dash just because of the borked /proc interface when
>> there is a perfectly adequate work-around in "cat".
>>
>> 	value=$(cat /proc/file)
>
> I wouldn't call that "a perfectly adequate work-around", but a painful and
> unadequate work-around.

For what it's worth, here's what bash does (based on strace):

1. Determine whether the file is seekable.  That is, seek using
SEEK_CUR with offset 0.

2. If seekable, read a nice big chunk and then seek back to put the
file offset back in the right place.  If not seekable, read one byte
at a time.

This works in /proc because files in /proc are seekable.

That said, I don't think borked /proc is a great reason to do this
(it's a better reason to fix /proc).  Speeding up the read builtin
might be a good reason.

Regards,
Jonathan

Re: read() builtin doesnt read integer value /proc files (but bashs does)

[Cristian Ionescu-Idbohrn]

Jonathan,

On Wed, 15 Dec 2010, Jonathan Nieder wrote:
> Cristian Ionescu-Idbohrn wrote:
> > On Sun, 28 Nov 2010, Herbert Xu wrote:
> >>
> >> I'm with Jilles on this.  I also don't particularly feel like
> >> bloating dash just because of the borked /proc interface when
> >> there is a perfectly adequate work-around in "cat".
> >>
> >> 	value=$(cat /proc/file)
> >
> > I wouldn't call that "a perfectly adequate work-around", but a painful
> > and unadequate work-around.
>
> This works in /proc because files in /proc are seekable.
>
> That said, I don't think borked /proc is a great reason to do this
> (it's a better reason to fix /proc).  Speeding up the read builtin
> might be a good reason.

Right.  So, there are 2 options here.  One is to to make dash work like
bash on a proc filesystem, the other to "fix" the kernel.

How many linux distributions depend on a "working" dash?
Which alternative is the more realistic one?
What are the ETAs odds?
How do we proceed?


Cheers,

-- 
Cristian

Re: read() builtin doesnt read integer value /proc files (but bashs does)

[Jilles Tjoelker]

On Wed, Dec 15, 2010 at 12:55:51PM -0600, Jonathan Nieder wrote:
> Cristian Ionescu-Idbohrn wrote:
> > On Sun, 28 Nov 2010, Herbert Xu wrote:
> > > On Sat, Sep 04, 2010 at 07:35:04PM +0000, Jilles Tjoelker wrote:

> >>> This discarding is still bad as it throws away valid data if the open
> >>> file description is shared. This happens if stdin is redirected inside a

> >> I'm with Jilles on this.  I also don't particularly feel like
> >> bloating dash just because of the borked /proc interface when
> >> there is a perfectly adequate work-around in "cat".

> >> 	value=$(cat /proc/file)

> > I wouldn't call that "a perfectly adequate work-around", but a painful and
> > unadequate work-around.

> For what it's worth, here's what bash does (based on strace):

> 1. Determine whether the file is seekable.  That is, seek using
> SEEK_CUR with offset 0.

> 2. If seekable, read a nice big chunk and then seek back to put the
> file offset back in the right place.  If not seekable, read one byte
> at a time.

> This works in /proc because files in /proc are seekable.

> That said, I don't think borked /proc is a great reason to do this
> (it's a better reason to fix /proc).  Speeding up the read builtin
> might be a good reason.

The optimization is of limited benefit (still way more syscalls than
strictly necessary) and does not apply to the common use case of reading
from a pipe. Generally, if 'read' is too slow, it is better to spend a
fork on a tool like grep, sed or awk which processes large amounts of
text much more efficiently.

As for value=$(cat /proc/file), there are at least two ways to make this
faster. The traditional ksh way is the extension value=$(</proc/file)
which is permitted but not required by POSIX. I do not really like this
as it makes the scripts not POSIX compliant. In recent mksh I noticed
another way: by making cat(1) a builtin under certain circumstances.
These circumstances include the absence of options (other than "--") and
should probably also exclude foreground commands in interactive job
control shells (as builtins cannot be suspended). To avoid needing to
implement extensions like FreeBSD cat's ability to read from Unix domain
sockets, named files could also be excluded, requiring value=$(cat
</proc/file).

-- 
Jilles Tjoelker

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Jilles Tjoelker]

On Wed, Sep 01, 2010 at 10:10:11AM +0200, Steve Schnepp wrote:
> Hi, I opened bug 595063 on the debian BTS [1] and I was suggested to
> resend the email upstream.

> So I copied the body of the bug below :

> dash's read() builtin seems to read the underlying file 1 char at a
> time. This doesn't work with some files under /proc, since procfs isn't
> fully POSIX compliant.

> [snip]

> After a little digging, it only appears on files that contains just an
> integer value. When asked to read with a non-null offset (*ppos != 0),
> __do_proc_dointvec() just returns 0 (meaning an EOF) as shown on [2].

> I'm aware that the issue isn't strictly a dash one, since it has the
> right to read one character at a time. But since fixing procfs to be
> conforming to POSIX isn't a realistic option, would it be possible to
> have a workaround that doesn't involve an external tool like cat(1) ?

Given that other files in /proc do work, I don't see why the ones that
only contain an integer value cannot be fixed. All the necessary state
to produce the second and further bytes is available.

Choosing a powerful abstraction like a regular file has its
implications.

Note that a change in the file between the single-byte reads will cause
an inconsistent value to be read. This is also the case with regular
files on a filesystem, so it is acceptable.

If single-byte reads are really unacceptable, then the proper way to
read these files needs to be documented, and clear violations that will
not work properly should cause an error (in this case, this means that
reading one byte from offset 0 should fail like reading one byte from
offset 1 does).

-- 
Jilles Tjoelker

Re: read() builtin doesn't read integer value /proc files (but bash's does)

[Steve Schnepp]

2010/9/2 Jilles Tjoelker <jilles@stack.nl>:

Thanks for your prompt reply.

> Note that a change in the file between the single-byte reads will cause
> an inconsistent value to be read. This is also the case with regular
> files on a filesystem, so it is acceptable.

Are you implying that:
- if the procfs is made to support char per char reads, dash reading
an inconsistent value is actually a feature ?
- buffering should, therefore, always be explicit ?

On a side note, the whole procfs seems to be designed around one
unique page read if possible (1x 4K).
I think it does so in order to be able to vastly simplify its
usage/implementation by kernel modules.

> If single-byte reads are really unacceptable, then the proper way to
> read these files needs to be documented, and clear violations that will
> not work properly should cause an error (in this case, this means that
> reading one byte from offset 0 should fail like reading one byte from
> offset 1 does).

+1 for "the proper way to read these files needs to be documented" and
I also think that emitting an error would be better than silently
returning erroneous data. [ EOVERFLOW is coming to my mind ]

--
Steve Schnepp
http://blog.pwkf.org/