seeking in binary files

[REH]

I thought this would have been easy...

Is there a portable way to seek to the end of an open, binary file?

Thanks,

REH

[Ben Pfaff]

"REH" <bo***@nowhere.net> writes:

Is there a portable way to seek to the end of an open, binary file?

No, assuming that you don't know the file's length.
Quoting C99, section 7.19.9.2:

A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.
--
"Your correction is 100% correct and 0% helpful. Well done!"
--Richard Heathfield

[Lawrence Kirby]

On Tue, 21 Jun 2005 14:41:00 -0700, Ben Pfaff wrote:

"REH" <bo***@nowhere.net> writes:

Is there a portable way to seek to the end of an open, binary file?

No, assuming that you don't know the file's length.
Quoting C99, section 7.19.9.2:

A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.

On the other hand of an implementation doesn't support that it likely
means that it isn't an easy or sensible operation. This is almost
certainly the nearest thing that exists to a portable solution.

If a "file" isn't seekable it won't meaningfully support *any* seek
operations, standard or otherwise.

Lawrence

[Dik T. Winter]

In article <pa****************************@netactive.co.uk> Lawrence Kirby <lk****@netactive.co.uk> writes:

On Tue, 21 Jun 2005 14:41:00 -0700, Ben Pfaff wrote:

"REH" <bo***@nowhere.net> writes:

Is there a portable way to seek to the end of an open, binary file?

No, assuming that you don't know the file's length.
Quoting C99, section 7.19.9.2:

A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.

On the other hand of an implementation doesn't support that it likely
means that it isn't an easy or sensible operation.

Indeed, there are anough binary streams where it makes no sense. Think
sockets or pipes or whatever.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, [phone]
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

[Jean-Claude Arbaut]

On 22/06/2005 03:13, Dik T. Winter wrote:

In article <pa****************************@netactive.co.uk> Lawrence Kirby
<lk****@netactive.co.uk> writes:

On Tue, 21 Jun 2005 14:41:00 -0700, Ben Pfaff wrote:

"REH" <bo***@nowhere.net> writes:

Is there a portable way to seek to the end of an open, binary file?

No, assuming that you don't know the file's length.
Quoting C99, section 7.19.9.2:

A binary stream need not meaningfully support fseek calls
with a whence value of SEEK_END.

On the other hand of an implementation doesn't support that it likely
means that it isn't an easy or sensible operation.

Indeed, there are anough binary streams where it makes no sense. Think
sockets or pipes or whatever.

So that depends on what your application is meant to deal with. Anyway, you
should not ask for the size of a socket or a pipe. And if fseek returns a
sensible error value, you know it has failed, at least.

[Lawrence Kirby]

On Wed, 22 Jun 2005 03:26:58 +0200, Jean-Claude Arbaut wrote:

....

So that depends on what your application is meant to deal with. Anyway, you
should not ask for the size of a socket or a pipe.
In standard C you can't tell whether it is a socket or a pipe.
And if fseek returns a
sensible error value, you know it has failed, at least.

That's a good point, there is an important difference between
failing an operation with a proper failure indication and performing an
operation "successfully" but in a way that is not "meaningful".

Lawrence

[REH]

"Lawrence Kirby" <lk****@netactive.co.uk> wrote in message
news:pa****************************@netactive.co.u k...

On Wed, 22 Jun 2005 03:26:58 +0200, Jean-Claude Arbaut wrote:

...

So that depends on what your application is meant to deal with. Anyway, you should not ask for the size of a socket or a pipe.

In standard C you can't tell whether it is a socket or a pipe.

I believe you can. Any seek will fail on a socket or pipe. If you seek to
the start of the "file" and it fails, you will get an errno value of ESPIPE,
which indicates an invalid seek.

REH

[Richard Tobin]

In article <II********@cwi.nl>, Dik T. Winter <Di********@cwi.nl> wrote:

Indeed, there are anough binary streams where it makes no sense. Think
sockets or pipes or whatever.

Why are binary streams any different from text streams in that respect?
You can't seek on a text socket or pipe either.

-- Richard

[Ben Pfaff]

"REH" <bo***@nowhere.net> writes:

"Lawrence Kirby" <lk****@netactive.co.uk> wrote in message
news:pa****************************@netactive.co.u k...

On Wed, 22 Jun 2005 03:26:58 +0200, Jean-Claude Arbaut wrote:

> So that depends on what your application is meant to deal with. Anyway, you > should not ask for the size of a socket or a pipe.

In standard C you can't tell whether it is a socket or a pipe.

I believe you can. Any seek will fail on a socket or pipe. If you seek to
the start of the "file" and it fails, you will get an errno value of ESPIPE,
which indicates an invalid seek.

A strictly conforming program cannot check for an errno value of
ESPIPE.
--
"This is a wonderful answer.
It's off-topic, it's incorrect, and it doesn't answer the question."
--Richard Heathfield