File Seeking / Overwriting bytes

[Jason]

Hi,

I need to open an existing file, seek to a position at X number of
bytes, and write out Y number of bytes overwriting any existing bytes,
but no erasing any other data. Is this possible?

I've opened a file in append "a" mode, and then used fseek with
SEEK_SET to seek to 0 bytes into the file, but this sets the position
at the end of existing data, not actually at the start of the file.

So say I have a file of 8 bytes: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF
0xFF
I want to open it, seek to a specific offset, lets say 2 bytes in,
then write out 4 bytes with the value 1D overwriting existing bytes,
leaving the file with: 0xFF 0xFF 0x1D 0x1D 0x1D 0x1D 0xFF 0xFF

Heres my testing code.

FILE *cont;
cont = fopen("/home/jk/test.txt", "a");
fseek(cont, 0, SEEK_SET);
fputs("T", cont);
fclose(cont);
Please can someone point me in the right direction, I want to avoid
reading everything in, editing it and rewriting it all out because the
file could be huge.

Many thanks,
Jason

[Phil Carmody]

Ian Collins <ia******@hotma il.comwrites:

la************@ siemens.com wrote:

>In regards to C99: 7.19.9.2p4:

For a text stream, either offset shall be zero, or offset shall
be a value returned by an earlier successful call to the ftell
function on a stream associated with the same file and whence
shall be SEEK_SET.

Keith Thompson <ks***@mib.orgw rote:

>>I didn't find it ambiguous until you mentioned it. 8-)}

For the second interpretation, I think there would have to be a comma
after "the same file".

And no comma after "offset shall be zero". Because English doesn't have
grouping operators, the standard is very carefully punctuated and also
makes judicious use of words like "either" and "both" to indicate the
correct parsing.

In non-American English commas are seldom, if ever, placed before an and.

What about the Oxford comma? OUP and Fowler both recommend its use.
The Guardian's neutral, advising its use if it would aid understanding.
I'm a Brit, and I'm very pro the Oxford comma; this is surprising, as
I tend to disagree with Fowler practically every time he's used as
support for some contruct.

Phil
--
I tried the Vista speech recognition by running the tutorial. I was
amazed, it was awesome, recognised every word I said. Then I said the
wrong word ... and it typed the right one. It was actually just
detecting a sound and printing the expected word! -- pbhj on /.

[Ben Bacarisse]

la************@ siemens.com writes:

In regards to C99: 7.19.9.2p4:

For a text stream, either offset shall be zero, or offset shall
be a value returned by an earlier successful call to the ftell
function on a stream associated with the same file and whence
shall be SEEK_SET.

Keith Thompson <ks***@mib.orgw rote:

>>
I didn't find it ambiguous until you mentioned it. 8-)}

For the second interpretation, I think there would have to be a comma
after "the same file".

And no comma after "offset shall be zero". Because English doesn't have
grouping operators, the standard is very carefully punctuated and also
makes judicious use of words like "either" and "both" to indicate the
correct parsing.

To me, the most compelling argument is that if the other meaning were
intended the restriction on whence would surely come first: "For a
text stream, whence shall be SEEK_SET and either offset shall be zero
or offset shall be a value returned by a ...".

I think you can get rid of any trace of an alternate parse simply by
putting the whence restriction sooner:

For a text stream, either offset shall be zero, or whence shall be
SEEK_SET and offset shall be a value returned by an earlier
successful call to the ftell function on a stream associated with
the same file.

Now any attempt to make the "and offset shall be a value..." clause
apply to both parts of the preceding "or" results in a double
restriction on offset (it shall be zero and the result of ftell).
Readers naturally avoid such illogical parses.

I don't think the original is ambiguous, but it seems to have proved to
be so despite careful punctuation!

--
Ben.

[Keith Thompson]

Ben Bacarisse <be********@bsb .me.ukwrites:
[...]

To me, the most compelling argument is that if the other meaning were
intended the restriction on whence would surely come first: "For a
text stream, whence shall be SEEK_SET and either offset shall be zero
or offset shall be a value returned by a ...".

[...]

Better yet, "For a text stream, whence shall be SEEK_SET and offset
shall be either zero or a value returned by a ...".

(And yes, I'd probably put a comma after SEEK_SET.)

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[lawrence.jones]

Ian Collins <ia******@hotma il.comwrote:

>
http://en.wikipedia.org/wiki/Serial_comma a fairly balanced account.

The standard does use serial commas, but the usage we're discussing
(mixed "and" and "or" clauses) isn't really serial.
--
Larry Jones

This sounds suspiciously like one of Dad's plots to build my character.
-- Calvin

[lawrence.jones]

Ben Bacarisse <be********@bsb .me.ukwrote:

>
I think you can get rid of any trace of an alternate parse simply by
putting the whence restriction sooner:

For a text stream, either offset shall be zero, or whence shall be
SEEK_SET and offset shall be a value returned by an earlier
successful call to the ftell function on a stream associated with
the same file.

That's better, but I think it would be better still to capture the "and"
between the "either" and the "or":

For a text stream, either whence shall be SEEK_SET and offset
shall be a value returned by an earlier successful call to the
ftell function on a stream associated with the same file, or
offset shall be zero.
--
Larry Jones

What a waste to be going to school on a morning like this. -- Calvin

[Ian Collins]

la************@ siemens.com wrote:

Ian Collins <ia******@hotma il.comwrote:

>http://en.wikipedia.org/wiki/Serial_comma a fairly balanced account.

The standard does use serial commas, but the usage we're discussing
(mixed "and" and "or" clauses) isn't really serial.

Is there a standard or guidelines for the language in ISO standards?

--
Ian Collins

[Ben Bacarisse]

la************@ siemens.com writes:

Ben Bacarisse <be********@bsb .me.ukwrote:

>>
I think you can get rid of any trace of an alternate parse simply by
putting the whence restriction sooner:

For a text stream, either offset shall be zero, or whence shall be
SEEK_SET and offset shall be a value returned by an earlier
successful call to the ftell function on a stream associated with
the same file.

That's better, but I think it would be better still to capture the "and"
between the "either" and the "or":

For a text stream, either whence shall be SEEK_SET and offset
shall be a value returned by an earlier successful call to the
ftell function on a stream associated with the same file, or
offset shall be zero.

That does not works so well to my mind -- maybe simply because of the
of two short phases with such a long one in the middle. It seems
harder to hold them meaning until it is all resolved. However, I am
biased!

--
Ben.

[lawrence.jones]

Ian Collins <ia******@hotma il.comwrote:

>
Is there a standard or guidelines for the language in ISO standards?

Part 2 of the ISO/IEC Directives (Rules for the structure and drafting
of International Standard) provides some guidelines.
--
Larry Jones

I thought my life would seem more interesting with a musical
score and a laugh track. -- Calvin