[PATCH] dir: do all size checks before seeking back and fix file closing

[PATCH] dir: do all size checks before seeking back and fix file closing

[Jonas Fonseca]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
---
 dir.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/dir.c b/dir.c
index d53d48f..ff8a2fb 100644
--- a/dir.c
+++ b/dir.c
@@ -122,11 +122,11 @@ static int add_excludes_from_file_1(cons
 	size = lseek(fd, 0, SEEK_END);
 	if (size < 0)
 		goto err;
-	lseek(fd, 0, SEEK_SET);
 	if (size == 0) {
 		close(fd);
 		return 0;
 	}
+	lseek(fd, 0, SEEK_SET);
 	buf = xmalloc(size+1);
 	if (read(fd, buf, size) != size)
 		goto err;
@@ -146,7 +146,7 @@ static int add_excludes_from_file_1(cons
 	return 0;
 
  err:
-	if (0 <= fd)
+	if (0 >= fd)
 		close(fd);
 	return -1;
 }
-- 
1.4.2.GIT

-- 
Jonas Fonseca

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Mitchell Blank Jr]

Jonas Fonseca wrote:
>   err:
> -	if (0 <= fd)
> +	if (0 >= fd)
>  		close(fd);

Could you explain that piece?  You now only close "fd" if it's zero (stdin)
or negative (invalid).  The old code (close if its >=0) make more sense.

-Mitch

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Jonas Fonseca]

Mitchell Blank Jr <mitch@sfgoth.com> wrote Sat, Aug 26, 2006:
> Jonas Fonseca wrote:
> >   err:
> > -	if (0 <= fd)
> > +	if (0 >= fd)
> >  		close(fd);
> 
> Could you explain that piece?  You now only close "fd" if it's zero (stdin)
> or negative (invalid).  The old code (close if its >=0) make more sense.

Ah, yes, sorry for the noise. I read the old code as (fd <= 0).

-- 
Jonas Fonseca

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Linus Torvalds]

On Sat, 26 Aug 2006, Jonas Fonseca wrote:
> 
> diff --git a/dir.c b/dir.c
> index d53d48f..ff8a2fb 100644
> --- a/dir.c
> +++ b/dir.c
> @@ -122,11 +122,11 @@ static int add_excludes_from_file_1(cons
>  	size = lseek(fd, 0, SEEK_END);
>  	if (size < 0)
>  		goto err;
> -	lseek(fd, 0, SEEK_SET);
>  	if (size == 0) {
>  		close(fd);
>  		return 0;
>  	}
> +	lseek(fd, 0, SEEK_SET);

I really think you'd be better off rewriting that to use "fstat()" 
instead. I don't know why it uses two lseek's, but it's wrong, and looks 
like some bad habit Junio picked up at some point.

> @@ -146,7 +146,7 @@ static int add_excludes_from_file_1(cons
>  	return 0;
>  
>   err:
> -	if (0 <= fd)
> +	if (0 >= fd)
>  		close(fd);

That's wrong. 

Now, admittedly it's wrong because another bad habit Junio picked up 
(doing comparisons with constants in the wrong order), so please write it 
as

	if (fd >= 0)
		close(fd);

instead.

Junio: I realize that you claim that you learnt that syntax from an 
authorative source, but he was _wrong_. Doing the constant first is more 
likely to cause bugs, rather than less. Compilers will warn about the

	if (x = 0)
		..

kind of bug, and putting the constant first just confuses humans.

It's more important to _not_ confuse humans than it is to try to avoid an 
uncommon error that compilers can and do warn about anyway.

			Linus

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Jakub Narebski]

Linus Torvalds wrote:

> Now, admittedly it's wrong because another bad habit Junio picked up 
> (doing comparisons with constants in the wrong order), so please write it 
> as
> 
>         if (fd >= 0)
>                 close(fd);
> 
> instead.
> 
> Junio: I realize that you claim that you learnt that syntax from an 
> authorative source, but he was _wrong_. Doing the constant first is more 
> likely to cause bugs, rather than less. Compilers will warn about the
> 
>         if (x = 0)
>                 ..
> 
> kind of bug, and putting the constant first just confuses humans.

Well, perhaps except checking if variable is in given range, e.g.

        if (0 <= x && x <= 5)
 
> It's more important to _not_ confuse humans than it is to try to avoid an 
> uncommon error that compilers can and do warn about anyway.
 

-- 
Jakub Narebski
Warsaw, Poland
ShadeHawk on #git

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Junio C Hamano]

Linus Torvalds <torvalds@osdl.org> writes:

> I really think you'd be better off rewriting that to use "fstat()" 
> instead. I don't know why it uses two lseek's, but it's wrong, and looks 
> like some bad habit Junio picked up at some point.

I think the code was written to avoid getting confused by
unseekable input (pipes) but was done in early morning before
the first shot of caffeine.

> Now, admittedly it's wrong because another bad habit Junio picked up 
> (doing comparisons with constants in the wrong order)

I think you misunderstand the rationale used to encourage the
comparison used there.  It does not have anything to do with
having comparison on the left.

The comparison order is done in textual order.  You list smaller
things on the left and then larger things on the right (iow, you
almost never use >= or >).

> Junio: I realize that you claim that you learnt that syntax from an 
> authorative source, but he was _wrong_....

This does not come from any authoritative source, but I picked
it up because I felt it made a lot of sense.

> ... Doing the constant first is more likely to cause bugs,
> rather than less.

That's a funny thing to say, because I was about to send out a
comment that touches this exact topic.

I spotted the bug the patch was trying to introduce right away
_because_ the original comparison was written in textual order.

The patch changed the comparison operator which first confused
me for a handful seconds, and then after I swapped everything in
my head to read as

	if (fd <= 0)
        	close(fd);

it became blatantly obvious it was a bogus change.  In the
message I was about to send out, I would have said "a fine
example that using texual order comparison consistently avoids
bugs".  So it is really relative to what you are used to.

Get used to it, please ;-).

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Junio C Hamano]

Junio C Hamano <junkio@cox.net> writes:

> Linus Torvalds <torvalds@osdl.org> writes:
>
>> Now, admittedly it's wrong because another bad habit Junio picked up 
>> (doing comparisons with constants in the wrong order)
>
> I think you misunderstand the rationale used to encourage the
> comparison used there.  It does not have anything to do with
> having comparison on the left.

s/comparison/constant/; I cannot spell.

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Linus Torvalds]

On Sat, 26 Aug 2006, Junio C Hamano wrote:
> 
> > Now, admittedly it's wrong because another bad habit Junio picked up 
> > (doing comparisons with constants in the wrong order)
> 
> I think you misunderstand the rationale used to encourage the
> comparison used there.  It does not have anything to do with
> having comparison on the left.
> 
> The comparison order is done in textual order.  You list smaller
> things on the left and then larger things on the right (iow, you
> almost never use >= or >).

Ahh. A number of people do the "0 == x" thing, because they want to be 
caught if they use "=" instead of "==" by mistake. I thought it was the 
same thing.

> This does not come from any authoritative source, but I picked
> it up because I felt it made a lot of sense.

To anybody who has _ever_ done any math at all, it makes no sense at all. 

You _always_ put constants on the right-hand side (or, possibly last on 
the left-hand side, in order to make the right-hand side be "0").

Similarly, if you say it out loud, you'd always say "if 'x' is larger than 
or equal to zero", not "if zero is smaller or less than 'x'". That's 
because "zero" obviously never varies, so you'd never talk about "zero" 
being compared to anything else.

The only exception would be the mathematical "0 < x < 10" kind of thing, 
which some languages (not C, of course) allows in that form. I can imagine 
that people would just do that as "0 < x && x < 10" just to keep the C 
form as close to the mathematical form, although I would at least 
personally do it as

	if (x > 0 &&
	    x < 10)

especially if I ever needed to write it that way on multiple lines due to 
some of the expressions being more complicated.

		Linus

Re: [PATCH] dir: do all size checks before seeking back and fix file closing

[Junio C Hamano]

Linus Torvalds <torvalds@osdl.org> writes:

>> The comparison order is done in textual order.  You list smaller
>> things on the left and then larger things on the right (iow, you
>> almost never use >= or >).
>
> Ahh. A number of people do the "0 == x" thing, because they want to be 
> caught if they use "=" instead of "==" by mistake. I thought it was the 
> same thing.

The rest is repeating what you said 15 months ago, so I did not
quote that part, but interested parties can follow this thread:

http://thread.gmane.org/gmane.comp.version-control.git/3907/focus=4126

[PATCH] Use fstat instead of fseek

[Jonas Fonseca]

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
---

 dir.c |    8 +++-----
 1 files changed, 3 insertions(+), 5 deletions(-)

Junio C Hamano <junkio@cox.net> wrote Sat, Aug 26, 2006:
> Linus Torvalds <torvalds@osdl.org> writes:
> 
> > I really think you'd be better off rewriting that to use "fstat()" 
> > instead. I don't know why it uses two lseek's, but it's wrong, and looks 
> > like some bad habit Junio picked up at some point.
> 
> I think the code was written to avoid getting confused by
> unseekable input (pipes) but was done in early morning before
> the first shot of caffeine.

I take it that you want this change, so here's a little addition to the
"use X instead of Y" series.

diff --git a/dir.c b/dir.c
index d53d48f..5a40d8f 100644
--- a/dir.c
+++ b/dir.c
@@ -112,17 +112,15 @@ static int add_excludes_from_file_1(cons
 				    int baselen,
 				    struct exclude_list *which)
 {
+	struct stat st;
 	int fd, i;
 	long size;
 	char *buf, *entry;
 
 	fd = open(fname, O_RDONLY);
-	if (fd < 0)
+	if (fd < 0 || fstat(fd, &st) < 0)
 		goto err;
-	size = lseek(fd, 0, SEEK_END);
-	if (size < 0)
-		goto err;
-	lseek(fd, 0, SEEK_SET);
+	size = st.st_size;
 	if (size == 0) {
 		close(fd);
 		return 0;
-- 
1.4.2.g2f76-dirty

-- 
Jonas Fonseca