FILE pointer

**hbn** · Oct 19 '06, 11:15 AM

Re: FILE pointer

rajus wrote:

I have made a simple program to read the contents of a file.The FILE
pointer returns NULL at the very beginning eventhough the file is
present.Now if I modify my program and use command line arguments
instead, I get the desired output.Now after I have run this program
with argv; and I shift back to the one without command line arguments
and run it ,it works.Why doesn't it work in the beginning itself?
I am using Turbo C++ (DOS).I tried it in devc++ but there it doesn't
work at all.

Your error is probably in line 42.

Please provide some code, we have a very hard to guessing what your
code looks like. Other than that it sounds like you are either trying
to open a file you don't have permissions to open or you are giving
fopen() an incorrect first and/or second argument.

hbn

**Nick Keighley** · Oct 19 '06, 12:45 PM

Re: FILE pointer

rajus wrote:

I have made a simple program to read the contents of a file.The FILE
pointer returns NULL at the very beginning eventhough the file is
present.Now if I modify my program and use command line arguments
instead, I get the desired output.Now after I have run this program
with argv; and I shift back to the one without command line arguments
and run it ,it works.Why doesn't it work in the beginning itself?
I am using Turbo C++ (DOS).I tried it in devc++ but there it doesn't
work at all.

if ((inf = fopen (filename, "r")) == NULL)
{
fprintf (stderr, "can't open file \"%s\" %s\n", filename,
strerror (errno));
exit (1);
}

note 1: printing the filename is handy because it may not be the one
you think it is. You may have \n on the end for instance.

note 2: strerror() relies on fprintf() setting errno. This is *not*
guarenteed
by the standard, but nevertheless works on many implementations . (eg.
every unix I've come across and windows)

--
Nick Keighley

**Kenneth Brody** · Oct 19 '06, 01:45 PM

Re: FILE pointer

rajus wrote:

>
I have made a simple program to read the contents of a file.The FILE
pointer returns NULL at the very beginning eventhough the file is
present.Now if I modify my program and use command line arguments
instead, I get the desired output.Now after I have run this program
with argv; and I shift back to the one without command line arguments
and run it ,it works.Why doesn't it work in the beginning itself?
I am using Turbo C++ (DOS).I tried it in devc++ but there it doesn't
work at all.

Well, my psychic reader is a little flakey, but it's telling me that
the problem is you forgot that, in order to include a backslash in a
string literal, you need to use two backslashes.

In other words, you cannot use:

char filename[] = "c:\filename.tx t";

but rather:

char filename[] = "c:\\filename.t xt";

Or, simply use the "better" form of:

char filename[] = "c:/filename.txt";

Either that, or there is an error on line 42. In that case, please
include some actual source code.

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

**Keith Thompson** · Oct 19 '06, 07:35 PM

Re: FILE pointer

"Nick Keighley" <nick_keighley_ nospam@hotmail. comwrites:
[...]

if ((inf = fopen (filename, "r")) == NULL)
{
fprintf (stderr, "can't open file \"%s\" %s\n", filename,
strerror (errno));
exit (1);
}

[...]

note 2: strerror() relies on fprintf() setting errno. This is *not*
guarenteed by the standard, but nevertheless works on many
implementations . (eg. every unix I've come across and windows)

Correction: it depends on fopen() setting errno.

--
Keith Thompson (The_Other_Keit h) kst-u@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.

**Nick Keighley** · Oct 20 '06, 07:55 AM

Re: FILE pointer

Keith Thompson wrote:

"Nick Keighley" <nick_keighley_ nospam@hotmail. comwrites:

[...]

if ((inf = fopen (filename, "r")) == NULL)
{
fprintf (stderr, "can't open file \"%s\" %s\n", filename,
strerror (errno));
exit (1);
}

[...]

note 2: strerror() relies on fprintf() setting errno. This is *not*
guarenteed by the standard, but nevertheless works on many
implementations . (eg. every unix I've come across and windows)

>
Correction: it depends on fopen() setting errno.

arg! how did I do that!?

--
Nick keighley

**CBFalconer** · Oct 20 '06, 12:35 PM

Re: FILE pointer

Nick Keighley wrote:

Keith Thompson wrote:

>"Nick Keighley" <nick_keighley_ nospam@hotmail. comwrites:

>

>[...]

>> if ((inf = fopen (filename, "r")) == NULL)
>> {
>> fprintf (stderr, "can't open file \"%s\" %s\n",
>> filename, strerror (errno));
>> exit (1);
>> }

>[...]

note 2: strerror() relies on fprintf() setting errno. This is *not*
guarenteed by the standard, but nevertheless works on many
implementations . (eg. every unix I've come across and windows)

>>
>Correction: it depends on fopen() setting errno.

>
arg! how did I do that!?

And to detect that you need to precede the fopen statement with
"errno = 0;" statement.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

**rajus** · Oct 20 '06, 05:55 PM

Re: FILE pointer

My code is:

#include<stdio. h>
#include<conio. h>
#include<stdlib .h>

void main( )
{
FILE *fp,*ft;
char ch;
//clrscr();
fp=fopen("txt1. txt","r");
if(fp==NULL)
{
printf("Error opening file");
fclose(fp);
exit(1);
}
ft=fopen("txt2. txt","w");
while((ch=fgetc (fp))!=EOF)
{
if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')
{
ch=ch-32;
fputc(ch,ft);
}
else
{
fputc(ch,ft);
}

}

fclose(ft);
fclose(fp);

}

This program just converts to uppercase.But this doesn't work in the
beginning.In fact none of the programs using fopen work.All return NULL
at the very beginning.

But then if i run this code.
#include<stdio. h>
#include<conio. h>
#include<stdlib .h>

void main(int argc,char* argv[])
{
FILE *fp,*ft;
char ch;
//clrscr();
fp=fopen(argv[1],"r");
if(fp==NULL)
{
printf("Error opening file");
fclose(fp);
exit(1);
}
ft=fopen(argv[2],"w");
while((ch=fgetc (fp))!=EOF)
{
if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')
{
ch=ch-32;
fputc(ch,ft);
}
else
{
fputc(ch,ft);
}

}

fclose(ft);
fclose(fp);

}

and run the previous one it works.Also all other fopen programs start
working?
What's the problem?

hbn wrote:

rajus wrote:

I have made a simple program to read the contents of a file.The FILE
pointer returns NULL at the very beginning eventhough the file is
present.Now if I modify my program and use command line arguments
instead, I get the desired output.Now after I have run this program
with argv; and I shift back to the one without command line arguments
and run it ,it works.Why doesn't it work in the beginning itself?
I am using Turbo C++ (DOS).I tried it in devc++ but there it doesn't
work at all.

>
Your error is probably in line 42.
>
Please provide some code, we have a very hard to guessing what your
code looks like. Other than that it sounds like you are either trying
to open a file you don't have permissions to open or you are giving
fopen() an incorrect first and/or second argument.
>
hbn

**Kenny McCormack** · Oct 20 '06, 06:45 PM

Re: FILE pointer

In article <1161367671.203 152.91170@i3g20 00cwc.googlegro ups.com>,
rajus <rajas3@gmail.c omwrote:

>My code is:
>
>#include<stdio .h>
>#include<conio .h>
>#include<stdli b.h>
>
>void main( )

You know the drill...

**Keith Thompson** · Oct 20 '06, 07:55 PM

Re: FILE pointer

"rajus" <rajas3@gmail.c omwrites:

My code is:
>
#include<stdio. h>
#include<conio. h>

Delete this. It's non-standard, and you don't use it.

#include<stdlib .h>
>
void main( )

main returns int. The correct declaration is:

int main(void)

{
FILE *fp,*ft;
char ch;

You've declared ch as a variable of type char (see below).

//clrscr();

"//" comments can cause problems on Usenet. If a long line is
wrapped, which can easily happen, a "//" comment can create a syntax
error; "/*...*/" comments are less likely to cause this. And "//"
comments are new in C99; they're not supported in C90, and some
compilers, in some modes, do not support them.

Why would you want to clear the screen anyway? I might have valuable
information there.

fp=fopen("txt1. txt","r");
if(fp==NULL)
{
printf("Error opening file");

Usually error messages should be printed to stderr, and output
should end in a newline:

fprintf(stderr, "Error opening file\n");

fclose(fp);
exit(1);

exit(1) is non-portable. The only portable values for exit() are 0,
EXIT_SUCCESS, and EXIT_FAILURE. (On some systems, exit(1) indicates
that your program terminated successfully.)

}
ft=fopen("txt2. txt","w");

You didn't check whether this fopen() succeeded. You should *always*
check the value returned by fopen().

while((ch=fgetc (fp))!=EOF)

fgetc() returns a result of type int, but you assign it to a variable
of type char. The reason fgetc() returns int is so that the result
can represent *either* any possible character value (represented as an
unsigned char) *or* the unique value EOF. You need to change the
declaration of ch to "int ch;".

{
if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')
{
ch=ch-32;
fputc(ch,ft);
}
else
{
fputc(ch,ft);
}
>
}

Magic numbers. The above code depends on the numeric values of
various character codes; these are not specified by the standard. It
also, I think, assumes that the codes for the letters are contiguous;
this also isn't guaranteed. Fortunately, there's no need to make
these assumptions. You say later that the program is supposed to
convert to upper case; see the toupper() function in <ctype.h>.

fclose(ft);
fclose(fp);
>

Add here:
return 0;

}

[snip]

The comp.lang.c FAQ is at <http://www.c-faq.com/>; I think you'll find
it very useful.

--
Keith Thompson (The_Other_Keit h) kst-u@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.

**CBFalconer** · Oct 20 '06, 08:55 PM

Re: FILE pointer

rajus wrote:

>
My code is:
>
#include<stdio. h>
#include<conio. h>

No such header as <conio.h>. Delete this.

#include<stdlib .h>
>
void main( )

main returns int. Use "int main(void)

{
FILE *fp,*ft;
char ch;

This should be an int. A char can't hold the value of EOF.

//clrscr();
fp=fopen("txt1. txt","r");
if(fp==NULL)
{
printf("Error opening file");
fclose(fp);

Don't close a NULL fp. Delete this.

exit(1);

Invalid argument to exit. Use EXIT_FAILURE and #include <stdlib.h>

}
ft=fopen("txt2. txt","w");

How do you know this fopen worked. You didn't test it.

while((ch=fgetc (fp))!=EOF)
{
if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')

Horrors. What have 32, 65, 90, etc. got to do with it? See the
is* family of tests, prototyped in <ctype.h>. isupper, islower,
toupper, tolower, etc.

{
ch=ch-32;

Ugh.

fputc(ch,ft);
}
else
{
fputc(ch,ft);
}
>
}
>
fclose(ft);
fclose(fp);

You forgot to return 0, or EXIT_SUCCESS.

>
}
>
This program just converts to uppercase.But this doesn't work in
the beginning.In fact none of the programs using fopen work.All
return NULL at the very beginning.

Also end your sentences with ". ". And don't top-post, your
answer belongs below (or intermixed with) the _snipped_ material to
which you reply. The snipping removes anything that is not germane
to your reply.

You are also allowed to use blanks in your source. They are no
longer a scarce resource.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net>

**Kenneth Brody** · Oct 23 '06, 04:45 PM

Re: FILE pointer

CBFalconer wrote:

>
rajus wrote:

[...]

if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')

>
Horrors. What have 32, 65, 90, etc. got to do with it? See the
is* family of tests, prototyped in <ctype.h>. isupper, islower,
toupper, tolower, etc.

[...]

Am I missing something, or have all the other posters missed what I'm
seeing?

Won't the above statement be true for every character except '\n',
regardless of whether it's lower-case or not, because of the "||"
rather than "&&"? Every number is ">=65" or "<=90"

(Not to mention the already-mentioned horrors of the use of such
magic numbers here and elsewhere in the program.)

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

**Richard Bos** · Oct 25 '06, 08:05 AM

Re: FILE pointer

Kenneth Brody <kenbrody@spamc op.netwrote:

CBFalconer wrote:

rajus wrote:

[...]

if((ch-32)>=65 || (ch-32)<=90 && ch!='\n')

Horrors. What have 32, 65, 90, etc. got to do with it? See the
is* family of tests, prototyped in <ctype.h>. isupper, islower,
toupper, tolower, etc.

>
Am I missing something, or have all the other posters missed what I'm
seeing?
>
Won't the above statement be true for every character except '\n',
regardless of whether it's lower-case or not, because of the "||"
rather than "&&"? Every number is ">=65" or "<=90"

No, that's just another good reason to use the Standard <ctype.h>
functions, rather than getting it wrong your own way.

Richard

**Hallvard B Furuseth** · Oct 26 '06, 11:35 AM

Which standards do the implementation support?

A recent thread mentioned that the C standard does not guarantee
that failed file operations set errno. To see if it did, set
errno=0 before the file operation and test errno afterwards.

I don't want to clutter my entire program with errno settings,
so is there a way for a general error reporting routine check
for _common_ implementations which set errno? Then use perror()
if yes, otherwise just output the user-supplied error phrase.

POSIX says errno should be set (or at least the POSIX version
I'm looking at) - is there an official 'if (running on POSIX)'
test to use? How about X/Open, XPG3/4, and whatever other
"standard" buzzwords there are to check for?

I need code for at least Windows and preferably Mac as well,
but I don't know them. I seem to remember there is some weird
Windows error code one should use instead of errno?

--
Hallvard

**Al Balmer** · Oct 26 '06, 03:15 PM

Re: Which standards do the implementation support?

On Thu, 26 Oct 2006 13:52:48 +0200, Hallvard B Furuseth
<h.b.furuseth@u sit.uio.nowrote :

>POSIX says errno should be set (or at least the POSIX version
>I'm looking at) - is there an official 'if (running on POSIX)'
>test to use?

<OTYes, there are defined macros which tell what versions and what
particular features of POSIX are available if compiling on a POSIX
implementation, and functions that report the POSIX support at
runtime. Any good POSIX book should detail the steps. If you need
further help, comp.unix.progr ammer is probably a good place to ask.

--
Al Balmer
Sun City, AZ

FILE pointer

FILE pointer

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