Strange Behaviour in finding Size of a File

[felix]

This method was written to create new Log File, when the size of the Log File reaches a max size defined by user [10MB in our case]. Here is the code snippet that does this check:

//-- Code starts here : --

static size_t LogSize = 1048576;
bool CreateNewLogs = false;


if ( stat ( logFile, &results ) == 0 )
{
if ( results.st_size > LogSize )
{
CreateNewLogs = true;
}
}
//-- Code ends here : --

It is strange that the condition got satisfied when results.st_size = 2589116.
And we are sure that the size of the data that is written is between 50 to 100 bytes in one operation. And this check is done before writing into the LogFile.

I am not sure if I am missing anything in our understanding of the stat function. Any inputs or pointers on this regard will be really Helpful.


Thanks in advance, and please let me know if any other information is required.


--
Felix

[James Kuyper]

On 11/09/2012 02:09 AM, felix wrote:
> This method was written to create new Log File, when the size of the Log File reaches a max size defined by user [10MB in our case]. Here is the code snippet that does this check:
>
> //-- Code starts here : --
>
> static size_t LogSize = 1048576;
> bool CreateNewLogs = false;
>
>
> if ( stat ( logFile, &results ) == 0 )

That is presumably the POSIX stat() function, or something similar? If
so, its behavior is defined by the POSIX standard, not the C standard,
and you'll get better answers to your questions in comp.unix.programmer
than in this newsgroup.

> {
> if ( results.st_size > LogSize )
> {
> CreateNewLogs = true;
> }
> }
> //-- Code ends here : --
>
> It is strange that the condition got satisfied when results.st_size = 2589116.
> And we are sure that the size of the data that is written is between 50 to 100 bytes in one operation. And this check is done before writing into the LogFile.

Keep in mind that file I/O is normally buffered, so the buffer size is
more relevant than the size of your individual writes. Still, that seems
to be a rather large jump to explain by buffering.

> I am not sure if I am missing anything in our understanding of the stat function. Any inputs or pointers on this regard will be really Helpful.

The people in comp.unix.programming may need to know more details about
how data is written to the file, and whether or not you've used any
POSIX functions to change the file mode.
Just to get a better idea of what's going on, I'd recommend reporting
the file size somewhere (probably in a separate log file) every time you
call stat().
--
James Kuyper

[Eric Sosman]

On 11/9/2012 2:09 AM, felix wrote:
> This method was written to create new Log File, when the size of the Log File reaches a max size defined by user [10MB in our case]. Here is the code snippet that does this check:
>
> //-- Code starts here : --
>
> static size_t LogSize = 1048576;

Ah. This is obviously some strange usage of "10 MB" that
I hadn't previously been aware of.

> It is strange that the condition got satisfied when results.st_size = 2589116.

Not all *that* strange ...

--
Eric Sosman
eso...@comcast-dot-net.invalid

[John Gordon]

In <8439d649-fae9-4828...@googlegroups.com> felix <ckpr...@gmail.com> writes:

> static size_t LogSize = 1048576;

> if ( results.st_size > LogSize )

> It is strange that the condition got satisfied when
> results.st_size = 2589116.

You're surprised that 2589116 is greater than 1048576?

--
John Gordon A is for Amy, who fell down the stairs
gor...@panix.com B is for Basil, assaulted by bears
-- Edward Gorey, "The Gashlycrumb Tinies"

[Mark Bluemel]

On 09/11/2012 15:16, John Gordon wrote:
> In <8439d649-fae9-4828...@googlegroups.com> felix <ckpr...@gmail.com> writes:
>
>> static size_t LogSize = 1048576;
>
>> if ( results.st_size > LogSize )
>
>> It is strange that the condition got satisfied when
>> results.st_size = 2589116.
>
> You're surprised that 2589116 is greater than 1048576?

No. Given that "we are sure that the size of the data that is written is

between 50 to 100 bytes in one operation. And this check is done before

writing into the LogFile." I think the OP is surprised that the
condition wasn't satisfied earlier.

I think James Kuyper has given some good advice.

[James Kuyper]

On 11/09/2012 10:16 AM, John Gordon wrote:
> In <8439d649-fae9-4828...@googlegroups.com> felix <ckpr...@gmail.com> writes:
>
>> static size_t LogSize = 1048576;
>
>> if ( results.st_size > LogSize )
>
>> It is strange that the condition got satisfied when
>> results.st_size = 2589116.
>
> You're surprised that 2589116 is greater than 1048576?

No, he's surprised that, when checking this condition periodically,
separated by writes of no more than 100 bytes, that it doesn't trigger
until 2589116. Naively, it could be expected to trigger with
results.st_size no more than 100 bytes larger than LogSize. Buffering is
the simplest of the many reasons invalidating that conclusion; there's
several others, and many people who are better equipped to explain those
issues than I am, so I'll leave that explanation to them, rather than
embarrassing myself by getting it wrong.

[Greg Martin]

I would think it strange if I knew for sure that the function got called
before the write in every place that the file was possibly being written
to and that there weren't multiple process/threads that could write to it.

[Alain Ketterlin]

felix <ckpr...@gmail.com> writes:

> if ( stat ( logFile, &results ) == 0 )

Use ftell() instead (or lseek() at level 2).

-- Alain.

[Keith Thompson]

What makes you think that will yield better results, and what is "level 2"?

--
Keith Thompson (The_Other_Keith) ks...@mib.org <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

felix <ckpr...@gmail.com> writes:
> This method was written to create new Log File, when the size of the
> Log File reaches a max size defined by user [10MB in our case]. Here
> is the code snippet that does this check:
>
> //-- Code starts here : --
>
> static size_t LogSize = 1048576;
> bool CreateNewLogs = false;
>
>
> if ( stat ( logFile, &results ) == 0 )
> {
> if ( results.st_size > LogSize )
> {
> CreateNewLogs = true;
> }
> }
> //-- Code ends here : --
>
> It is strange that the condition got satisfied when results.st_size =
> 2589116. And we are sure that the size of the data that is written is
> between 50 to 100 bytes in one operation. And this check is done
> before writing into the LogFile.

It's been pointed out that 1048576 is the wrong value if you want 10 MB
(more pedantically, 10 MiB). But LogSize is also the wrong type.
It should be the same type as the st_size member.

You probably want to use "const" rather than "static" in the definition
of LogSize, unless the value can change.

Neither of these is likely to be the cause of the problem you're seeing.
Since stat() is defined by POSIX, not by C, you'll likely get better
answers in comp.unix.programmer.

[Alain Ketterlin]

Keith Thompson <ks...@mib.org> writes:

> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>> felix <ckpr...@gmail.com> writes:
>>> if ( stat ( logFile, &results ) == 0 )
>>
>> Use ftell() instead (or lseek() at level 2).
>
> What makes you think that will yield better results,

What makes you think it will not. At least ftell() is C.

> and what is "level 2"?

Unixism. Just ignore it if it hurts you.

-- Alain.

[Keith Thompson]

Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
> Keith Thompson <ks...@mib.org> writes:
>> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>>> felix <ckpr...@gmail.com> writes:
>>>> if ( stat ( logFile, &results ) == 0 )
>>>
>>> Use ftell() instead (or lseek() at level 2).
>>
>> What makes you think that will yield better results,
>
> What makes you think it will not. At least ftell() is C.

We know that stat() isn't working as the OP expects. I'd expect
ftell() to yield exactly the same results -- and it requires opening
the file and seeking to the end of it. Furthermore, there's no
guarantee in C that the fseek()/ftell() trick will accurately yield
the size of a file. Binary streams may legally be padded with an
implementation-defined number of null characters (N1370 7.21.2p3),
and "A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END" (7.21.9.2p3). For text streams,
the value returned by ftell() isn't necessarily meaningful except
as an argument to fseek() (7.21.9.4p2).

A POSIX environment makes more guarantees -- but as long as you're
depending on POSIX, there's no good reason not to use stat()
(or fstat() or lstat()).

My point is that you suggested ftell() as a solution to the OP's
problem. It isn't.

>> and what is "level 2"?
>
> Unixism. Just ignore it if it hurts you.

If you're referring to section 2 of the manual (ftell(2),
documentation available via "man 2 ftell"), I've never heard of
that being referred to as "level 2". Try being a little less
condescending and actually answering the question.

[Alain Ketterlin]

Keith Thompson <ks...@mib.org> writes:

> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>> Keith Thompson <ks...@mib.org> writes:
>>> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>>>> felix <ckpr...@gmail.com> writes:
>>>>> if ( stat ( logFile, &results ) == 0 )
>>>>
>>>> Use ftell() instead (or lseek() at level 2).
>>>
>>> What makes you think that will yield better results,
>>
>> What makes you think it will not. At least ftell() is C.
>
> We know that stat() isn't working as the OP expects. I'd expect
> ftell() to yield exactly the same results -- and it requires opening
> the file and seeking to the end of it.

If stat() doesn't give the correct result in the OP's use case (writing
chunks, and testing whether the size has reached a limit), it's probably
because the file is still open. And several people have suggested that
the problem was probably with the buffering. The doc of ftell() says (on
my system):

| The ftell() function obtains the current value of the file position
| indicator for the stream pointed to by stream.

In my opinion, that's a pretty good hint given the problem description.

> Furthermore, there's no guarantee in C that the fseek()/ftell() trick
> will accurately yield the size of a file. Binary streams may legally
> be padded with an implementation-defined number of null characters
> (N1370 7.21.2p3), and "A binary stream need not meaningfully support
> fseek calls with a whence value of SEEK_END" (7.21.9.2p3). For text
> streams, the value returned by ftell() isn't necessarily meaningful
> except as an argument to fseek() (7.21.9.4p2).

I know all this, thank you, but I have no indication that the OP is in
any of these cases. I just gave the OP a track to follow. He/she would
surely come back with another (and maybe more precise) question if
things turn out to be more difficult.

> A POSIX environment makes more guarantees -- but as long as you're
> depending on POSIX, there's no good reason not to use stat()
> (or fstat() or lstat()).

Keeping the file open is a good reason to not use stat() (see code
above).

> My point is that you suggested ftell() as a solution to the OP's
> problem. It isn't.

OK, call it a hint if you want...

>>> and what is "level 2"?
>>
>> Unixism. Just ignore it if it hurts you.
>
> If you're referring to section 2 of the manual (ftell(2),
> documentation available via "man 2 ftell"), I've never heard of
> that being referred to as "level 2".

OK now you have. I didn't think it was that hard to understand, given
that lseek was two words away.

> Try being a little less condescending and actually answering the
> question.

Try being a little less condescending and actually helping people asking
for help instead of being pedantic (your words) on size units and
everything but the OP's problem.

-- Alain.

[Philip Lantz]

Alain Ketterlin wrote:
> Keith Thompson <ks...@mib.org> writes:
> > Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
> >> Keith Thompson <ks...@mib.org> writes:
> >>> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
> >>>> felix <ckpr...@gmail.com> writes:
> >>>>> if ( stat ( logFile, &results ) == 0 )
> >>>>
> >>>> Use ftell() instead (or lseek() at level 2).

> >>> and what is "level 2"?
> >>
> >> Unixism. Just ignore it if it hurts you.
> >
> > If you're referring to section 2 of the manual (ftell(2),
> > documentation available via "man 2 ftell"), I've never heard of
> > that being referred to as "level 2".
>
> OK now you have. I didn't think it was that hard to understand, given
> that lseek was two words away.

I didn't understand it either--I thought you meant to use a value of 2
for whence, and I thought it was a strange way to express that. It never
occurred to me you were referring to section 2 of the Unix Programmer's
Manual.

[Kenny McCormack]

In article <MPG.2b075cebd...@news.eternal-september.org>,
Philip Lantz <p...@canterey.us> wrote:
...

>I didn't understand it either--I thought you meant to use a value of 2
>for whence, and I thought it was a strange way to express that. It never
>occurred to me you were referring to section 2 of the Unix Programmer's
>Manual.

Well, live & learn!

See, the Usenet can be a helpful thing after all.

--
Just for a change of pace, this sig is *not* an obscure reference to
comp.lang.c...

[James Kuyper]

On 11/09/2012 03:33 PM, Keith Thompson wrote:
> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>> Keith Thompson <ks...@mib.org> writes:
>>> Alain Ketterlin <al...@dpt-info.u-strasbg.fr> writes:
>>>> felix <ckpr...@gmail.com> writes:
>>>>> if ( stat ( logFile, &results ) == 0 )
>>>>
>>>> Use ftell() instead (or lseek() at level 2).
>>>
>>> What makes you think that will yield better results,
>>
>> What makes you think it will not. At least ftell() is C.
>
> We know that stat() isn't working as the OP expects. I'd expect
> ftell() to yield exactly the same results -- and it requires opening

> the file and seeking to the end of it. ...

Opening the file and seeking to the end is not a problem; it's already
open, or at least it was at the time of the last write, and the current
write position was presumably at the end of the file, otherwise it
wouldn't be growing. I'd expect ftell() to give a better indication of
the bytes written than stat()=>st_size, since I wouldn't expect the
value returned by ftell() to be affected by issues such as buffering.
--
James Kuyper

[Keith Thompson]

Ok, good point.

That's assuming that the program that's querying the size of the file
is the same one that's writing to the file, and that the querying
code has access to the relevant FILE*. That's not entirely obvious
from the original post, but it seems likely.

(As I said earlier, C doesn't guarantee that the value returned by
ftell() is meaningful for text streams, but that's unlikely to be
an issue for the OP.)

[James Kuyper]

On 11/09/2012 10:24 PM, Keith Thompson wrote:

> James Kuyper <james...@verizon.net> writes:
...
>> Opening the file and seeking to the end is not a problem; it's already
>> open, or at least it was at the time of the last write, and the current
>> write position was presumably at the end of the file, otherwise it
>> wouldn't be growing. I'd expect ftell() to give a better indication of
>> the bytes written than stat()=>st_size, since I wouldn't expect the
>> value returned by ftell() to be affected by issues such as buffering.
>
> Ok, good point.
>
> That's assuming that the program that's querying the size of the file
> is the same one that's writing to the file, and that the querying
> code has access to the relevant FILE*. That's not entirely obvious
> from the original post, but it seems likely.

The behavior that's controlled by the results of this query is the
creation of a new log file, so it never even occurred to me to consider
that the log file might be being written by some other program.
--
James Kuyper

[Ian Collins]

I didn't understand it either. I've never seen the expression used in
the context of man pages.

--
Ian Collins

[Philip Lantz]

Kenny McCormack wrote:
>
> In article <MPG.2b075cebd...@news.eternal-september.org>,
> Philip Lantz <p...@canterey.us> wrote:
> ...
> >I didn't understand it either--I thought you meant to use a value of 2
> >for whence, and I thought it was a strange way to express that. It never
> >occurred to me you were referring to section 2 of the Unix Programmer's
> >Manual.
>
> Well, live & learn!
>
> See, the Usenet can be a helpful thing after all.

What am I supposed to have learned? Is the term "level" commonly used to
identify a section of the Unix Programmer's Manual?

[Eric Sosman]

On 11/9/2012 10:24 PM, Keith Thompson wrote:
>[...]

> (As I said earlier, C doesn't guarantee that the value returned by
> ftell() is meaningful for text streams, but that's unlikely to be
> an issue for the OP.)

If the all the logging happens in one place (or a small
number of nearby places), and if it all happens during one
execution of the program, the O.P. can do the entire job in
purely portable C. Something like

static FILE *logStream;
static size_t logLength;

void writeLog(const char *format, ...) {
if (logStream == NULL) {
logStream = openLog(...);
logLength = 0;
}

va_list ap;
va_start(ap, format);
logLength += vfprintf(format, ap);

va_end(ap);
if (logLength > LIMIT) {
closeLog(logStream);
logStream = NULL;
}
}

... should do it, along with a little error-checking and such.

(Okay, okay: The number of characters written to a stream is
not necessarily the same thing as the number of bytes stored on
a disk. Nonetheless, for "start a new log when the old one gets
too big" purposes it should be close enough.)

--
Eric Sosman
eso...@comcast-dot-net.invalid

[Nick Keighley]

On Nov 9, 1:24 pm, Eric Sosman <esos...@comcast-dot-net.invalid>
wrote:

this is why you don't put numbers like that in your code

#define K 1024
#define M (K * K)
#define LOG_SIZE (10 * M)

[Nick Keighley]

On Nov 10, 7:29 am, Philip Lantz <p...@canterey.us> wrote:
> Kenny McCormack wrote:
>

> > In article <MPG.2b075cebd6906329989...@news.eternal-september.org>,

no

[Ben Bacarisse]

Nick Keighley <nick_keigh...@hotmail.com> writes:

> On Nov 9, 1:24 pm, Eric Sosman <esos...@comcast-dot-net.invalid>
> wrote:
>> On 11/9/2012 2:09 AM, felix wrote:

<snip>

>> > //-- Code starts here : --
>>
>> > static size_t LogSize = 1048576;
>>
>>      Ah. This is obviously some strange usage of "10 MB" that
>> I hadn't previously been aware of.
>>
>> > It is strange that the condition got satisfied when results.st_size
>> > = 2589116.
>>
>>      Not all *that* strange ...
>
> this is why you don't put numbers like that in your code
>
> #define K 1024
> #define M (K * K)
> #define LOG_SIZE (10 * M)

I'd put an 'L' suffix on the 1024 since 10 * M is outside the minimum
range or plain int arithmetic.

--
Ben.

[Eric Sosman]

Ugh. If you simply *must* do that sort of thing, at least
use identifiers like KILO and MEGA (or KIBI and MIBI). Myself,
I'd prefer to write

#define LOG_SIZE (10L * 1024 * 1024)

... with special emphasis on the "L".

--
Eric Sosman
eso...@comcast-dot-net.invalid

[Ike Naar]

On 2012-11-10, Eric Sosman <eso...@comcast-dot-net.invalid> wrote:
> On 11/9/2012 10:24 PM, Keith Thompson wrote:
>>[...]
>> (As I said earlier, C doesn't guarantee that the value returned by
>> ftell() is meaningful for text streams, but that's unlikely to be
>> an issue for the OP.)
>
> If the all the logging happens in one place (or a small
> number of nearby places), and if it all happens during one
> execution of the program, the O.P. can do the entire job in
> purely portable C. Something like
>
> static FILE *logStream;
> static size_t logLength;
>
> void writeLog(const char *format, ...) {
> if (logStream == NULL) {
> logStream = openLog(...);
> logLength = 0;
> }
>
> va_list ap;
> va_start(ap, format);
> logLength += vfprintf(format, ap);

You mean
logLength += vfprintf(logStream, format, ap);
?

[Ian Collins]

Or lust use

const size_t oneK = 1024;
const size_t oneM = oneK * 1024;
const size_t logSize = 10 * oneM;

--
Ian Collins

[Eric Sosman]

On 11/10/2012 12:42 PM, Ike Naar wrote:
> On 2012-11-10, Eric Sosman <eso...@comcast-dot-net.invalid> wrote:

>>[...]

>> logLength += vfprintf(format, ap);
>
> You mean
> logLength += vfprintf(logStream, format, ap);
> ?

No, I *like* error messages ... ;-)

--
Eric Sosman
eso...@comcast-dot-net.invalid

[Ben Bacarisse]

Check the newsgroup! That's more use in C++ than in C -- you can't put
these at file scope in C.

--
Ben.

[Kenny McCormack]

In article <87liea8...@dpt-info.u-strasbg.fr>,
Alain Ketterlin <al...@dpt-info.u-strasbg.fr> wrote:
...

>> Try being a little less condescending and actually answering the
>> question.
>
>Try being a little less condescending and actually helping people asking
>for help instead of being pedantic (your words) on size units and
>everything but the OP's problem.

That's Kiki. It wouldn't be Kiki if it didn't exhibit those sorts of
behaviors.

--
They say compassion is a virtue, but I don't have the time!

- David Byrne -

[James Kuyper]

Neither have I.
--
James Kuyper

[Phil Carmody]

Ditto, clearly our presumably-more-than-two decades of unix experience
are all for nought. And it's not even being compact, as the expression
used is way more characters than the *absolutely standard* notation
which Keith followed up with, viz. ftell(2).

Looks like we've got a new level 2 usenet poster, IYKWIM...

Phil
--
Regarding TSA regulations:
How are four small bottles of liquid different from one large bottle?
Because four bottles can hold the components of a binary liquid explosive,
whereas one big bottle can't. -- camperdave responding to MacAndrew on /.

[felix]

On Friday, 9 November 2012 17:26:28 UTC+5:30, James Kuyper wrote:

> On 11/09/2012 02:09 AM, felix wrote:
>
> > This method was written to create new Log File, when the size of the Log File reaches a max size defined by user [10MB in our case]. Here is the code snippet that does this check:
>
> >
>

> > //-- Code starts here : --
>
> >
>
> > static size_t LogSize = 1048576;
>

> > bool CreateNewLogs = false;

>
> >
>
> >
>
> > if ( stat ( logFile, &results ) == 0 )
>
>
>

> That is presumably the POSIX stat() function, or something similar? If
>
> so, its behavior is defined by the POSIX standard, not the C standard,
>
> and you'll get better answers to your questions in comp.unix.programmer
>
> than in this newsgroup.

>
>
>
> > {
>
> > if ( results.st_size > LogSize )
>
> > {
>
> > CreateNewLogs = true;
>
> > }
>
> > }
>
> > //-- Code ends here : --
>
> >
>

> > It is strange that the condition got satisfied when results.st_size = 2589116.
>

> > And we are sure that the size of the data that is written is between 50 to 100 bytes in one operation. And this check is done before writing into the LogFile.
>
>
>

> Keep in mind that file I/O is normally buffered, so the buffer size is
>
> more relevant than the size of your individual writes. Still, that seems
>
> to be a rather large jump to explain by buffering.
>
>
>
> > I am not sure if I am missing anything in our understanding of the stat function. Any inputs or pointers on this regard will be really Helpful.
>
>
>
> The people in comp.unix.programming may need to know more details about
>
> how data is written to the file, and whether or not you've used any
>
> POSIX functions to change the file mode.
>
> Just to get a better idea of what's going on, I'd recommend reporting
>
> the file size somewhere (probably in a separate log file) every time you
>
> call stat().
>
> --
>
> James Kuyper


Thanks a Lot James and all others that helped me understand this problem.

As said above, the file I/O is buffered. The file size was not getting updated after each fwrite(). We tried to fflush() the after writing the data into log files. And now we see the correct size of the file.

We are trying to understand ftell()/fseek() also as suggested.

And thanks for suggestions regarding the constant - LogSize.

--
Felix

[James Kuyper]

On 11/13/2012 01:23 AM, felix wrote:
> On Friday, 9 November 2012 17:26:28 UTC+5:30, James Kuyper wrote:

...

> As said above, the file I/O is buffered. The file size was not
> getting updated after each fwrite(). We tried to fflush() the after
> writing the data into log files. And now we see the correct size of
> the file.

Note: the size that you saw before was also the correct size. That is -
it was the actual size currently taken up by the file on disk. It didn't
include the part that was still in the buffers, and had not yet been
written to the disk (that explanation is somewhat incomplete, but it's
good enough for this context). That might not have been the quantity you
wanted to know, but that doesn't make it incorrect.

> We are trying to understand ftell()/fseek() also as suggested.

If fflush() caused stat() to give you the size that you wanted, calling
ftell() is almost certainly a more efficient way of getting that size
information - at least if you can call it from within the same program
that's writing to the log file. According to the C standard, this is
only guaranteed if the file is opened in binary mode, but your use of
stat() suggests that you may be using a Unix-like system, in which case
text mode is indistinguishable from binary mode.

Your wording implies that you're having trouble understand ftell(). Can
you explain the nature of your confusion?
--
James Kuyper

[felix]

First, Sorry for this delay in my response.

James, you did explain the information that we were looking for.
We were trying to understand which of those subroutines (ftell(3) or fflush(3)) would be more effective when used within the code. We wanted to make sure that we knew why we are using what we are using.

Once again, thank you James.

Thanks and Regards,
Felix

[James Kuyper]

On 11/19/2012 04:52 AM, felix wrote:
> We were trying to understand which of those subroutines (ftell(3) or
> fflush(3)) would be more effective when used within the code. We
> wanted to make sure that we knew why we are using what we are using.

That's trivial: ftell() just retrieves data that's stored in the FILE
object (conceptually, at least - implementation details may differ) -
it's very quick. fflush() forces the buffers to be flushed earlier than
they otherwise would be. Assuming a reasonably competent implementation
of the standard I/O library (if you can't make that assumption, change
which library you're using), the flushing policy is presumably close to
optimum, and randomly interfering with that policy will cause a
significant reduction in I/O efficiency. You should interfere with the
buffering only when you have to - and in this case, you don't have to.
Also, ftell(), if executed after the immediately preceding write to the
file, will (on a binary stream, at least) returns exactly the quantity
that you're interested in. Calling fflush() improves the relevance of
the numbers you can get from stat(), but it's still not guaranteed to
give you the same number that ftell() would give you.
--
James Kuyper