Problem with FCLOSE

[Marcus Jacobs]

Dear Group

I have encountered a problem with fclose and I am wondering if anyone
could provide some insight about this problem to me. Currently, I am
working on a small personal project that is requiring for me to alter
some existing code to a program. While running the unaltered code, I
kept encountering an application (The exception unknown software
exception(0xc00000fd) etc. .. etc... etc... I am running W2K Pro).
Through a bit of debugging I singled out where the problem is
occuring. It is occuring when it attemps to close a file that had
previously been open. While tinkering with this problem, I noticed
that no error occured while closing when I manually entered the file
name into the source code. The error only occured when the user has to
key-in the file name from a command line (this is how I wish for the
program to run). What is causing this problem? Below I have provided
some source code to the function that is causing the problem. Here is
the version of the source in which no error occurs. I have set the
string, finc_name, to the file name "all.inc". Normally, the array is
set to a default of size 80 similar to fpov_name. I have also
commented out the portion of the code which prompts the user to key-in
the file name.

Thanks


Marcus D. Jacobs






#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include "numtypes.h"
/* #include "numtable.h"*/

typedef struct
{
Word x,r,g,b;
} colorinfo;


FILE *finc, *fpov, *frad;
char ch, frad_name[80], finc_name[] = "all.inc", fpov_name[80] ,
is_num;
char *pov_word;
char chpov , colorend='f';
colorinfo color;
Word texture[2600];


void reads_triangle(Word pov_num);
void reads_smoothtriangle(Word pov_num);
void reads_inc(void);
void reads_color(void);
void reads_texture(void);



main(void)
{
int m=1, n, j;
char chtext, finish;
char *inc_word;
int isize;

int j2;

/*
printf("Enter POV include file name ( *.inc ) : ");
gets(finc_name);
printf("\n");
*/

finc=fopen(finc_name, "r");
chtext = getc(finc);

if (finc==NULL)
{ printf(" Can't open %s\n", finc);
exit(EXIT_FAILURE);
}
while (chtext != EOF )
{
finish ='f';
if ( ( inc_word = malloc(25)) == NULL )
{ printf("Out of memory");
exit(EXIT_FAILURE);;
}
while (isspace(chtext)) chtext = getc(finc);
j=0;
while (chtext != EOF && isspace(chtext) == 0)
{ inc_word[j++] = chtext;
chtext = getc(finc);
}

inc_word[j] = '\0';

if (strcmp(inc_word, "texture") == 0 && finish == 'f')
{ while (isspace(chtext)) chtext = getc(finc);
n=0;
chtext = getc(finc);
while ( chtext != '}' )
{
texture[m][n++] = chtext;
chtext = getc(finc);
}
texture[m][n] = '\0';
/* printf(" texture = %s ", texture[m]);*/
m++;
finish = 't';
}
free(inc_word);
}

fclose(finc);
}

 

[Eric Sosman]

[...]
char ch, frad_name[80], finc_name[] = "all.inc", fpov_name[80] ,
[...]
gets(finc_name);

Here's at least one problem; there may be others. The
`finc_name' array is only eight characters long, so it can't
hold a string any longer than seven characters plus the
terminating '\0'. If the user types a longer name, the extra
characters won't fit in the array, and will most likely stomp
all over the nearby memory. What happens next is unpredictable,
but is unlikely to be a Happy Experience.

By the way, don't use gets(). See Question 12.23 in the
comp.lang.c Frequently Asked Questions (FAQ) list

http://www.eskimo.com/~scs/C-faq/top.html

 

[Mark Gordon]

On 5 Aug 2003 14:53:20 -0700

Dear Group

I have encountered a problem with fclose and I am wondering if anyone
could provide some insight about this problem to me. Currently, I am
working on a small personal project that is requiring for me to alter
some existing code to a program. While running the unaltered code, I
kept encountering an application (The exception unknown software
exception(0xc00000fd) etc. .. etc... etc... I am running W2K Pro).
Through a bit of debugging I singled out where the problem is
occuring. It is occuring when it attemps to close a file that had
previously been open. While tinkering with this problem, I noticed
that no error occured while closing when I manually entered the file
name into the source code. The error only occured when the user has to
key-in the file name from a command line (this is how I wish for the
program to run). What is causing this problem? Below I have provided
some source code to the function that is causing the problem. Here is
the version of the source in which no error occurs. I have set the
string, finc_name, to the file name "all.inc". Normally, the array is
set to a default of size 80 similar to fpov_name. I have also
commented out the portion of the code which prompts the user to key-in
the file name.

Thanks


Marcus D. Jacobs






#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include "numtypes.h"
/* #include "numtable.h"*/

typedef struct
{
Word x,r,g,b;
} colorinfo;


FILE *finc, *fpov, *frad;
char ch, frad_name[80], finc_name[] = "all.inc", fpov_name[80] ,
is_num;
char *pov_word;
char chpov , colorend='f';
colorinfo color;
Word texture[2600];

You have not shown us the definition of Word, so we don't know what type
texture is.

void reads_triangle(Word pov_num);
void reads_smoothtriangle(Word pov_num);
void reads_inc(void);
void reads_color(void);
void reads_texture(void);



main(void)

int main(void)

{
int m=1, n, j;
char chtext, finish;
char *inc_word;
int isize;

int j2;

/*
printf("Enter POV include file name ( *.inc ) : ");
gets(finc_name);

When this is uncommented, if the user enters a file name longer than the
buffer you have just blown up your program.

printf("\n");
*/

finc=fopen(finc_name, "r");
chtext = getc(finc);

Surely this should be after you've checked the open worked? Otherwise
*BANG*

if (finc==NULL)
{ printf(" Can't open %s\n", finc);
exit(EXIT_FAILURE);
}
while (chtext != EOF )

EOF might not fit in a char so chtext should have been declared as an
int rather than a char.

{
finish ='f';
if ( ( inc_word = malloc(25)) == NULL )
{ printf("Out of memory");
exit(EXIT_FAILURE);;
}
while (isspace(chtext)) chtext = getc(finc);
j=0;
while (chtext != EOF && isspace(chtext) == 0)
{ inc_word[j++] = chtext;
chtext = getc(finc);
}

inc_word[j] = '\0';

If a 25 character (or longer) word was entered you've just blown things
up again. You might want to either limit the above loop or use realloc.

if (strcmp(inc_word, "texture") == 0 && finish == 'f')
{ while (isspace(chtext)) chtext = getc(finc);
n=0;
chtext = getc(finc);
while ( chtext != '}' )
{
texture[m][n++] = chtext;
chtext = getc(finc);
}

Again, not check on going passed the end of the array. Also you are not
checking to see if getc succeeds.

texture[m][n] = '\0';
/* printf(" texture = %s ", texture[m]);*/
m++;
finish = 't';
}
free(inc_word);
}

fclose(finc);
}

There are probably other errors with the program, I could not be
bothered to check it carefully since it is so bad. I would also suggest
checking the FAQ for comp.lang.c (google will find it) for the correct
way to handle checking the results of file operations and probably
almost everything else in the program.

 

[Jens.Toerring]

I have encountered a problem with fclose and I am wondering if anyone
could provide some insight about this problem to me. Currently, I am
working on a small personal project that is requiring for me to alter
some existing code to a program. While running the unaltered code, I
kept encountering an application (The exception unknown software
exception(0xc00000fd) etc. .. etc... etc... I am running W2K Pro).
Through a bit of debugging I singled out where the problem is
occuring. It is occuring when it attemps to close a file that had
previously been open. While tinkering with this problem, I noticed
that no error occured while closing when I manually entered the file
name into the source code. The error only occured when the user has to
key-in the file name from a command line (this is how I wish for the
program to run). What is causing this problem? Below I have provided
some source code to the function that is causing the problem. Here is
the version of the source in which no error occurs. I have set the
string, finc_name, to the file name "all.inc". Normally, the array is
set to a default of size 80 similar to fpov_name. I have also
commented out the portion of the code which prompts the user to key-in
the file name.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include "numtypes.h"
/* #include "numtable.h"*/

typedef struct
{
Word x,r,g,b;
} colorinfo;

FILE *finc, *fpov, *frad;
char ch, frad_name[80], finc_name[] = "all.inc", fpov_name[80] ,
is_num;
char *pov_word;
char chpov , colorend='f';
colorinfo color;
Word texture[2600];

void reads_triangle(Word pov_num);
void reads_smoothtriangle(Word pov_num);
void reads_inc(void);
void reads_color(void);
void reads_texture(void);

main(void)

int main( void )

since main() is supposed to return an int.

{
int m=1, n, j;
char chtext, finish;
char *inc_word;
int isize;

int j2;

/*
printf("Enter POV include file name ( *.inc ) : ");

You better add here

fflush( stdout );

or the user may not see the request.

gets(finc_name);

Never, ever use gets(). You will never be able to ascertain that the
user does not enter a file name that's longer than the length of the
buffer you supply. Use fgets().

printf("\n");
*/

finc=fopen(finc_name, "r");
chtext = getc(finc);

if (finc==NULL)
{
printf(" Can't open %s\n", finc);
exit(EXIT_FAILURE);
}

You need to check if fopen() was successful *before* you call getc().
And you must make 'chtext' an int, not a char. Otherwise you won't be
able to check for EOF. EOF is a return value of getc() that can't be
stored in a char.

while (chtext != EOF )
{
finish ='f';
if ( ( inc_word = malloc(25)) == NULL )
{
printf("Out of memory");
exit(EXIT_FAILURE);;
}
while (isspace(chtext)) chtext = getc(finc);

What is supposed to happen when getc() returns EOF, e.g. because the
input file is broken and does not look like you expect it?

j=0;
while (chtext != EOF && isspace(chtext) == 0)
{
inc_word[j++] = chtext;
chtext = getc(finc);
}

You never check that you don't get more than 24 characters in a row.
If this happens you write past the end of the memory you allocated.

inc_word[j] = '\0';

if (strcmp(inc_word, "texture") == 0 && finish == 'f')
{
while (isspace(chtext)) chtext = getc(finc);
n=0;
chtext = getc(finc);
while ( chtext != '}' )
{
texture[m][n++] = chtext;

'texture' is declared as an array of 'Word' (whatever that's supposed
to be). Unless 'Word' is some kind of array (of chars?) and not just a
pointer to an array this looks fishy, to say the least. Even assuming
that 'Word' is typedef'ed to be an array of chars you don't check that
you don't write past its end. And if you do anything can happen.

Finally, since you start with m = 1, why don't you want to use the
first element of 'texture'?

chtext = getc(finc);
}
texture[m][n] = '\0';
/* printf(" texture = %s ", texture[m]);*/
m++;
finish = 't';
}

And you also don't make sure that you don't read more elements into
'texture' than the 2600 'Word's you declared.

free(inc_word);
}

fclose(finc);

You probably have been written into some memory you don't own,
overwriting some important structure for the file pointer. And
now you get what you deserve ;-) You might be unlucky in not
getting an error when you comment out the stuff for getting
the file name from the user because of a different layout of
the executable. But that does not mean that your program is
correct.

}

And finally you're missing returning an int.

Regards, Jens
--
_ _____ _____
| ||_ _||_ _| (e-mail address removed)-berlin.de
_ | | | | | |
| |_| | | | | | http://www.physik.fu-berlin.de/~toerring
\___/ens|_|homs|_|oerring

 

[Andre Hinrichs]

[...]
{ printf(" Can't open %s\n", finc);
[...]

In addition to the other hints already given you probably want to use
{ printf(" Can't open %s\n", finc_name);

Additionally most times it's wise to write this information to standard
error:
{ fprintf(stderr," Can't open %s\n", finc_name);




Andre

 

[Jens.Toerring]

Beside what Artie has been already told you (about removing the '\n'
from the input) there are still several things that won't work:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#ifndef NUMTYPES_H
#define NUMTYPES_H

#define MAX_WORD_LEN 20
typedef char Word[MAX_WORD_LEN+1] ;

#endif

Where's the #if (or similar) for this #endif?

typedef struct
{
Word x,r,g,b;
} colorinfo;

FILE *finc, *fpov, *frad;
char ch, frad_name[80], finc_name[80], fpov_name[80] , is_num;
char *pov_word;
char * finame;
char chpov , colorend='f';
colorinfo color;
Word texture[2600];

void reads_triangle(Word pov_num);
void reads_smoothtriangle(Word pov_num);
void reads_inc(void);
void reads_color(void);
void reads_texture(void);

int main(void)
{
int m=1, n, j;
char chtext, finish;
char *inc_word;
int j2;

printf("Enter POV include file name ( *.inc ) : ");
fgets(finc_name, 80, stdin);
printf("%s", finc_name);
printf("\n");

finc=fopen(finc_name, "r");

if (finc==NULL)
{ printf(" Can't open %s\n", finc);
exit(EXIT_FAILURE);
}

chtext = getc(finc);

while (chtext != EOF )

As long as you don't make 'chtext' an int you definitely will
have only a near-to-zero chance of it being equal to EOF. EOF is
typically an -1 *integer* (but don't rely on this, but you can
rely on *not* being able to store it in a char). So the above
comparison will nearly always fail and you never will stop looping.

{
finish ='f';
if ( ( inc_word = malloc(25)) == NULL )
{ printf("Out of memory");
exit(EXIT_FAILURE);;
}
while (isspace(chtext)) chtext = getc(finc);

When you reached the end of the file (and you can't detect it with
'chtext' being a char) you just created an infinite loop because
EOF is typically (int) -1. If you implicitely cast it to a char
'chtext' is going to be a (char) -1 (or perhaps something else,
e.g. a 255 if on your system chars are unsigned), at least it
usually isn't going to be a white-space character.

j=0;
while (chtext != EOF && isspace(chtext) == 0)
{ inc_word[j++] = chtext;
chtext = getc(finc);
}

inc_word[j] = '\0';

Unless you are lucky you will end up with a sequence of more than
24 non-whitespace characters and write past the end of the memory
you allocated. In this case literally everything can happen. So
*do* the checks or use a different language where you can't make
these kinds of mistakes (you should think of using a lexer like
lex or flex for these kinds of tasks anyway).

// printf("string are %s\n", inc_word);
if (strcmp(inc_word, "texture") == 0 && finish == 'f')
{ while (isspace(chtext)) chtext = getc(finc);
n=0;
chtext = getc(finc);
while ( chtext != '}' )
{
texture[m][n++] = chtext;

And here is the same problem: if you're unlucky enough there are
going to be more than MAX_WORD_LEN characters not being '}' and
you end up with writing over the end of texture[m] (and this looks
like reading in a povray file where e.g. additional white-space
characters are syntactically correct). It just takes a very small
bit of care to avoid this, so why don't you just do it?

chtext = getc(finc);
}
texture[m][n] = '\0';
/* printf(" texture = %s ", texture[m]);*/
m++;

How can you be that sure that you won't get more than 2599 of
these texture strings? Checking is cheap and simple and avoids
lots of unnecessary grief that comes from invoking undefined
behavior.

finish = 't';
}
free(inc_word);
}

fclose(finc);

return 0;
}

Regards, Jens
--
_ _____ _____
| ||_ _||_ _| (e-mail address removed)-berlin.de
_ | | | | | |
| |_| | | | | | http://www.physik.fu-berlin.de/~toerring
\___/ens|_|homs|_|oerring

 

[Barry Schwarz]

Marcus Jacobs wrote: snip

You haven't stripped the newline ('\n') character.
*strstr(finc_name, "\n") = '\0';

You need to make sure that fgets inserted a \n before you strip it.
strstr can return NULL.

snip


<<Remove the del for email>>

 

[Barry Schwarz]

Thanks everyone for all of your suggestions. I have revised some of
the source code (primarily getting rid of gets()). I have also
included here the definition of type WORD for everyone to view. The
other stuff I'll get to when I get this working properly. BTW, I am
compiling this with gcc included with MinGW. Similar issue arose when
I compiled this using MS Visual Studio. For starters, the problem that
I am encountering now is that even though fname_inc does list itself
as the filename given from fgets(), for some reason when I attempt to
fopen() a file with that name, it comes up NULL. Why does this happen.
In my case when I attempt to run the program I use a file named
"all.inc". As part of my debugging, I manually initialize the value of
fname_inc to "all.inc". When I do this, fopen() does work properly. I
can not understand why this program does not like to use any of the
data that is to be entered. As for the original problem with fclose(),
while debugging the program it seems to me that this isn't what is
causing the proble. The program works it way to the end and then at
the end the following error message is displayed:

"The instruction at "0x77fcb03d" reference memory at "0x00000000". The
memory could not be "written"."

I, for the life of me, can not figure out what is causing this. I do
like to post my questions to this ng but I do try very hard to fix
these problems myself before coming to everyone else. I am stuck. Here
is the source as I have revised it. If it would be helpful, I can post
a portion of the file "all.inc" to this ng such that someone may test
out the code. It is only an ASCII text file.


Thanks to everyone for your help.


Marcus

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#ifndef NUMTYPES_H
#define NUMTYPES_H

#define MAX_WORD_LEN 20
typedef char Word[MAX_WORD_LEN+1] ;

#endif

typedef struct
{
Word x,r,g,b;
} colorinfo;


FILE *finc, *fpov, *frad;
char ch, frad_name[80], finc_name[80], fpov_name[80] , is_num;
char *pov_word;
char * finame;
char chpov , colorend='f';
colorinfo color;
Word texture[2600];


void reads_triangle(Word pov_num);
void reads_smoothtriangle(Word pov_num);
void reads_inc(void);
void reads_color(void);
void reads_texture(void);



int main(void)
{
int m=1, n, j;
char chtext, finish;
char *inc_word;
int j2;

printf("Enter POV include file name ( *.inc ) : ");
fgets(finc_name, 80, stdin);

If the data input by the user is less than 79 characters, fgets will
include the \n (corresponding to the enter key) in the string. You
need something like

if ((inc_word = strchr(finc_name, '\n')) == NULL)
/* error logic - you do not have the complete file name */
else
*inc_word = '\0';

printf("%s", finc_name);
printf("\n");

finc=fopen(finc_name, "r");

if (finc==NULL)
{ printf(" Can't open %s\n", finc);

This should be finc_name. finc is NULL and attempting to print what
it points to invokes undefined behavior which may be the cause of your
problem.

exit(EXIT_FAILURE);
}

chtext = getc(finc);

getc returns an int for a reason. You should respect that. It is
entirely possible that EOF will not fit in a char.

snip


<<Remove the del for email>>

 

[bd]

Beside what Artie has been already told you (about removing the '\n'
from the input) there are still several things that won't work:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#ifndef NUMTYPES_H
#define NUMTYPES_H

#define MAX_WORD_LEN 20
typedef char Word[MAX_WORD_LEN+1] ;

#endif

Where's the #if (or similar) for this #endif?

Right below #include <string.h>.

 

[bd]

[...]
{ printf(" Can't open %s\n", finc);
[...]

In addition to the other hints already given you probably want to use
{ printf(" Can't open %s\n", finc_name);

Additionally most times it's wise to write this information to standard
error:
{ fprintf(stderr," Can't open %s\n", finc_name);

Better yet:
{
const static char *prefix = "Can't open ";
char *buf = malloc(strlen(prefix) + strlen(finc_name) + 1);
sprintf(buf, "%s%s", prefix, finc_name);
perror(buf); /* Give more details on why the error occured */
free(buf);

 

[Artie Gold]

You need to make sure that fgets inserted a \n before you strip it.
strstr can return NULL.

snip


<<Remove the del for email>>

Yup. You're absolutely correct, of course.
I wasn't being as thorough as I should have been (but then I would have
had to rewrite _everything_ for the OP ;-)).

--ag

 

[Chris Torek]

[On issuing error messages after a failed fopen()]

Better yet:
const static char *prefix = "Can't open ";
char *buf = malloc(strlen(prefix) + strlen(finc_name) + 1);
sprintf(buf, "%s%s", prefix, finc_name);
perror(buf); /* Give more details on why the error occured */
free(buf);

Much simpler, and more likely to work (malloc() might fail, and
malloc() and sprintf() might overwrite errno, even on success --
this is not entirely theoretical; real implementations really do
overwrite errno on successful calls to fprintf(), for instance):

fprintf(stderr, "cannot open %s for reading: %s\n", finc_name,
strerror(errno));

Here strerror(errno) grabs the value of errno before fprintf() can
overwrite it, so that as long as fopen() sets errno based on the
underlying reason the system was unable to open the supplied file,
you will get a useful message, rather than -- as actually happens
when fprintf() overwrites things -- something like:

could not send mail to user@host: not a typewriter

(The program that conspicuously produced this kind of error message
was sendmail. "Well of course he's not a typewriter, what kind of
stupid error is that?" )

The drawback to using strerror() here is that the ANSI C standard
(either one, C89 or C99) does not *require* systems to set errno
on failure to open a file, and some systems sometimes do not. In
this case, errno can be "left over" from a previous failure, so you
may get messages like:

cannot open foo.bar for reading: Numerical argument out of domain

if the last errno was (say) EDOM. One can attempt to cure this by
clearing errno before calling fopen, and using strerror(errno) only
if errno is nonzero; or one can simply leave the interpretation up
to the user:

fprintf(stderr, "Cannot open %s for reading.\n"
"The system's most recent reported error was %s, which\n"
"is not necessarily the reason the open failed.\n",
finc_name, strerror(errno));

Or, of course, one can just restrict oneself to implementations that
do always set errno appropriately.