end-of-file problem

[Markus Pitha]

Hello,

I read a simple bmp-file with this loop:

while ( !feof(fp) ) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

Everything seems to be correct, but at the end of the file, I get a weird
"ffffffff" output. How does this come? Actually the loop must have been
finished before printf can print some data, or when will the file
pointer increased in this case?
Thanks,
Markus

[boa]

* Markus Pitha wrote, On 20.08.2006 15:01:

Hello,

I read a simple bmp-file with this loop:

while ( !feof(fp) ) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

Everything seems to be correct,

It isn't, see the faq for details. http://c-faq.com/stdio/feof.html

but at the end of the file, I get a weird
"ffffffff" output. How does this come? Actually the loop must have been
finished before printf can print some data, or when will the file
pointer increased in this case?

ffffffff happens to be the same as -1 on most machines. EOF also happens
to be -1 on most machines. So you're printing the value of EOF, returned
from fgetc(). On the next iteration, feof() returns true and your loop
stops.

boa

>

Thanks,
Markus

[Markus Pitha]

Hello,

thanks for the link but I couldn't handle it as in this example. I
got warnings of comparing pointers with NULL and therefore a bad program
behaviour like endless loops.
Now I decided to use the following construct:

fseek(fp, 0, SEEK_END);
filesize = ftell(fp);
rewind(fp);

while (ftell(fp) < filesize) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

ffffffff happens to be the same as -1 on most machines.
to be -1 on most machines. So you're printing the value of EOF, returned
from fgetc(). On the next iteration, feof() returns true and your loop
stops.

Thanks, that sounds plausible to me.

[boa]

* Markus Pitha wrote, On 20.08.2006 16:36:

Hello,

thanks for the link but I couldn't handle it as in this example. I
got warnings of comparing pointers with NULL and therefore a bad program
behaviour like endless loops.
Now I decided to use the following construct:

fseek(fp, 0, SEEK_END);
filesize = ftell(fp);
rewind(fp);

while (ftell(fp) < filesize) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

Please don't.

How about something like this instead?
int c;

while( (c = fgetc(fp)) != EOF)
printf("%x\n"; c);
Boa

[snip]

[Markus Pitha]

boa wrote:

while( (c = fgetc(fp)) != EOF)
printf("%x\n"; c);

I thought this is only allowed with text files because of the fact that
EOF is defined as -1 and binary files could contain "-1"?

[spibou]

Markus Pitha wrote:

boa wrote:

while( (c = fgetc(fp)) != EOF)
printf("%x\n"; c);

I thought this is only allowed with text files because of the fact that
EOF is defined as -1 and binary files could contain "-1"?

No , it is allowed with all files. EOF will be defined in a
way that the value cannot appear in a file on your platform.

[Harald van Dijk]

Markus Pitha wrote:

boa wrote:

> while( (c = fgetc(fp)) != EOF)
printf("%x\n"; c);

I thought this is only allowed with text files because of the fact that
EOF is defined as -1 and binary files could contain "-1"?

fgetc() always returns either an unsigned char converted to int, or EOF. As
long as c is wide enough to hold both EOF and 0 ... UCHAR_MAX, for example
with c declared as int as it was in the message you replied to, there's no
problem. With c declared as char, there would be a problem.

[CBFalconer]

Markus Pitha wrote:

>
I read a simple bmp-file with this loop:

while ( !feof(fp) ) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

Everything seems to be correct, but at the end of the file, I get
a weird "ffffffff" output. How does this come? Actually the loop
must have been finished before printf can print some data, or when
will the file pointer increased in this case?

Nothing weird about it. It is simple misuse of the feof function.

int ch;

while (EOF != (ch = getc(fp)) {
printf("%x\n", fgetc(fp));
}
if (!feof(fp)) puts("Hardware error occured");

feof does not look forward, as in better languages. It
distinguishes between i/o errors and reaching EOF when an input
statement fails. To distinguish between EOF and all normal chars
it is necessary to receive those chars into an int, rather than a
char.

getc is normally preferable to fgetc when the file argument can be
evaluated more than once.

--
Chuck F (cb********@yah oo.com) (cb********@mai neline.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.netUSE maineline address!

[Markus Pitha]

Harald van Dijk wrote:

fgetc() always returns either an unsigned char converted to int, or EOF. As
long as c is wide enough to hold both EOF and 0 ... UCHAR_MAX, for example
with c declared as int as it was in the message you replied to, there's no
problem. With c declared as char, there would be a problem.

I understand, thanks to everybody.
Markus

[Flash Gordon]

CBFalconer wrote:

Markus Pitha wrote:

>I read a simple bmp-file with this loop:

while ( !feof(fp) ) {
printf("%x\n", fgetc(fp));
}
fclose(fp);

Everything seems to be correct, but at the end of the file, I get
a weird "ffffffff" output. How does this come? Actually the loop
must have been finished before printf can print some data, or when
will the file pointer increased in this case?

Nothing weird about it. It is simple misuse of the feof function.

int ch;

while (EOF != (ch = getc(fp)) {
printf("%x\n", fgetc(fp));

Did you really mean to read another character and print it, thereby
printing every other character? * think you meant:
printf("%x\n", ch);

}
if (!feof(fp)) puts("Hardware error occured");

feof does not look forward, as in better languages. It
distinguishes between i/o errors and reaching EOF when an input
statement fails. To distinguish between EOF and all normal chars
it is necessary to receive those chars into an int, rather than a
char.

Agreed.

getc is normally preferable to fgetc when the file argument can be
evaluated more than once.

I would say that it is extremely rare to need fgetc rather than getc. I
understand the code constructs which would require it, e.g.
fgetc(fparr[i++), I just can't think of anywhere where they would have
been useful to me. Has anyone here actually done something where they
had to use fgetc rather than getc?
--
Flash Gordon
Still sigless on this computer.