How to seek in a file "wb"?????

[hstagni]

I tried to use fseek in a file opened for writing:
------ BEGIN -------
int main()
{
char c;
FILE *fp;
fp=fopen("texto", "wb");
putc('a', fp);
putc('b', fp);
fseek(fp, -1, SEEK_CUR);
c=getc(fp); printf("%c", c);

fclose(fp);
return 0;
}
------END--------

However, the result of "printf" was the EOF char and I was expecting a
"b".
I know the problem is in fseek, because it doesn´t work(how i wish it
works) on files opened for writing.

THE QUESTION IS:
WHAT SHOULD I DO TO MAKE THE SAMPLE ABOVE WORK?

[Walter Roberson]

In article <11**********************@p77g2000hsh.googlegroups .com>,
hstagni <st****@gmail.comwrote:

>I tried to use fseek in a file opened for writing:
------ BEGIN -------
int main()
{
char c;
FILE *fp;
fp=fopen("texto", "wb");
putc('a', fp);
putc('b', fp);
fseek(fp, -1, SEEK_CUR);
c=getc(fp); printf("%c", c);

fclose(fp);
return 0;
}
------END--------

>However, the result of "printf" was the EOF char and I was expecting a
"b".
I know the problem is in fseek, because it doesn'tt work(how i wish it
works) on files opened for writing.

>THE QUESTION IS:
WHAT SHOULD I DO TO MAKE THE SAMPLE ABOVE WORK?

Change the "wb" to "wb+" so that you are in update mode instead
of write mode.

--
I was very young in those days, but I was also rather dim.
-- Christopher Priest

[Ben Pfaff]

hstagni <st****@gmail.comwrites:

I tried to use fseek in a file opened for writing:

[...]

fp=fopen("texto", "wb");

[...]

c=getc(fp); printf("%c", c);

Open the file for reading and writing (mode "w+b"). You can't
expect reading to work on a file opened only for writing.
--
"C has its problems, but a language designed from scratch would have some too,
and we know C's problems."
--Bjarne Stroustrup

[JimS]

On 22 May 2007 14:20:39 -0700, hstagni <st****@gmail.comwrote:

>I tried to use fseek in a file opened for writing:
------ BEGIN -------
int main()
{
char c;
FILE *fp;
fp=fopen("texto", "wb");
putc('a', fp);
putc('b', fp);
fseek(fp, -1, SEEK_CUR);
c=getc(fp); printf("%c", c);

fclose(fp);
return 0;
}
------END--------

However, the result of "printf" was the EOF char

Unlikely, because you've declared c as char instead of int.

Jim

[Martin Ambuhl]

hstagni wrote:

I tried to use fseek in a file opened for writing:
------ BEGIN -------
int main()
{
char c;
FILE *fp;
fp=fopen("texto", "wb");
putc('a', fp);
putc('b', fp);
fseek(fp, -1, SEEK_CUR);
c=getc(fp); printf("%c", c);

fclose(fp);
return 0;
}
------END--------

However, the result of "printf" was the EOF char and I was expecting a
"b".
I know the problem is in fseek, because it doesn´t work(how i wish it
works) on files opened for writing.

THE QUESTION IS:
WHAT SHOULD I DO TO MAKE THE SAMPLE ABOVE WORK?

The printf(..., ftell(fp)) calls have been added so you can see what is
happening. The other three changes (including a needed header, changing
the mode in which the file is opened, ending the last line of output
with an '\n') are all needed.

#include <stdio.h /* mha: added. Needed for all of FILE,
fopen, putc, fseek, getc, and fclose
in OP's code. */
int main()
{
char c;
FILE *fp;
fp = fopen("texto", "wb+"); /* mha: please note the addition of '+'
*/
printf("After open, ftell -%ld\n", ftell(fp));
putc('a', fp);
printf("After putc('a',fp), ftell -%ld\n", ftell(fp));
putc('b', fp);
printf("After putc('b',fp), ftell -%ld\n", ftell(fp));

fseek(fp, -1, SEEK_CUR);
printf("After fseek(fp, -1, SEEK_CUR), ftell -%ld\n", ftell(fp));
c = getc(fp);
printf("After getc(fp), ftell -%ld\n", ftell(fp));
printf("%c\n", c); /* mha: added the necessary '\n' */

fclose(fp);
return 0;
}

After open, ftell -0
After putc('a',fp), ftell -1
After putc('b',fp), ftell -2
After fseek(fp, -1, SEEK_CUR), ftell -1
After getc(fp), ftell -2
b

[hstagni]

THANKS A LOT

One last question:

>Unlikely, because you've declared c as char instead of int.

Should I use ints to read chars from a file? I know EOF is (-1), but
since you have "feof" function, why would I use ints?

thnx

[Chris Dollin]

hstagni wrote:

THANKS A LOT

One last question:

>>Unlikely, because you've declared c as char instead of int.

Should I use ints to read chars from a file?

If you're using `getc`, you should not that it returns an
`int`, not `char`, and that end-of-file is noted by returning
EOF, which is a value that /isn't/ a (n unsigned) char.
Stuffing the result of `getc` into a `char` will break
somethine.

I know EOF is (-1),

The Standard only requires that it's negative. It need not
be -1. (-1 is a very sensible value, but C implementations
are not required to be sensible.)

but since you have "feof" function, why would I use ints?

Because you weren't using `feof`, and you were using `getc`.

--
"He's dead, Jim, but not as we know it." Unsaid /Trek/

Hewlett-Packard Limited registered office: Cain Road, Bracknell,
registered no: 690597 England Berks RG12 1HN

[Kenneth Brody]

hstagni wrote:

>
I tried to use fseek in a file opened for writing:
------ BEGIN -------

[...]

fp=fopen("texto", "wb");

You have opened the file in write-only mode.

[...]

c=getc(fp); printf("%c", c);

Here, you attempt to read from the write-only file.

[...]

THE QUESTION IS:
WHAT SHOULD I DO TO MAKE THE SAMPLE ABOVE WORK?

The problem has nothing to do with your seek. Rather, it has
to do with attempting to read from a write-only file. You need
to open the file in read-write mode:

fp = fopen("texto","wb+");

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer.h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th*************@gmail.com>

[Keith Thompson]

hstagni <st****@gmail.comwrites:

THANKS A LOT

One last question:

>>Unlikely, because you've declared c as char instead of int.

Should I use ints to read chars from a file? I know EOF is (-1), but
since you have "feof" function, why would I use ints?

Have you read section 12 of the comp.lang.c FAQ, <http://www.c-faq.com/>?

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[CBFalconer]

hstagni wrote:

>

.... snip ...

>
Should I use ints to read chars from a file? I know EOF is (-1),
but since you have "feof" function, why would I use ints?

You should use int to receive from fgetc, getc, getchar, etc. all
of which return an int. This is to allow them to return EOF (which
may not be -1, all you know is that it will be negative).

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
<http://kadaitcha.cx/vista/dogsbreakfast/index.html>
cbfalconer at maineline dot net

--
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