K&R2 , exercise 7.6

[arnuld]

PURPOSE :: see statement in comments

GOT: Segmentation Fault

I guess the segfault is sourced in the compile-time warning but I am
giving a char* to the function already.




/* K&R2, section 7.7, exercise 7.6
*
* write a program to compare 2 files, printing that first line
* where they differ.
*
*/


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

enum MAXSIZE { ARRSIZE=1000 };

void compare_files( FILE*, FILE* );
void print_line( char* );


int main( int argc, char* argv[] )
{
FILE *pf1, *pf2;

if( argc !=3 )
{
fprintf( stderr, "You iDiOT, I expect 2 files as input. \n" );
exit(EXIT_FAILU RE);
}


/* open files */
pf1 = fopen( *++argv, "r" );
pf2 = fopen( *++argv, "r" );

/* error check */
if( pf1 == NULL || pf2 == NULL )
{
fprintf( stderr, "error opening files\n");
exit(EXIT_FAILU RE);
}
else
{
compare_files( pf1,pf2 );
}



/* don't forget to close the files */
fclose( pf1 );
fclose( pf2 );


return 0;
}



/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
int c1, c2, match;
char *line1, *line2;
char *begin_line1, *begin_line2;

match = 1;

while( ((line1 = fgets( line1, ARRSIZE, pf1 )) != NULL) ||
((line2 = fgets( line2, ARRSIZE, pf2 )) != NULL))
{
begin_line1 = line1;
begin_line2 = line2;

for( c1 = *line1, c2 = *line2; c1 != '\0' && c2 != '\0'; ++line1,
++line2 )
{
if ( c1 != c2 )
{
match = 0;
print_line( begin_line1 );
printf("\n-----------------------\n"); print_line( begin_line2 );
}
}
}
}



void print_line( char* line )
{
printf("%s\n", *line++);
}

=============== === OUTPUT =============== =======
[arnuld@raj C]$ gcc -ansi -pedantic -Wall -Wextra 7-6.c
7-6.c: In function `print_line':
7-6.c:88: warning: format argument is not a pointer (arg 2)

[arnuld@raj C]$ ./a.out
You iDiOT, I expect 2 files as input.

[arnuld@raj C]$ ./a.out 7-6.c 5-4.c
Segmentation fault


--

http://lispmachine.wordpress.com/

my email ID is at the above address

[Richard Heathfield]

Re: K&amp;R2 , exercise 7.6

arnuld said:

PURPOSE :: see statement in comments
>
GOT: Segmentation Fault
>

<snip>

int c1, c2, match;
char *line1, *line2;

Where do these point? (Hint: nowhere in particular.)

char *begin_line1, *begin_line2;
>
match = 1;
>
while( ((line1 = fgets( line1, ARRSIZE, pf1 )) != NULL) ||
((line2 = fgets( line2, ARRSIZE, pf2 )) != NULL))

They certainly don't point to ARRSIZE bytes of memory that you have the
right to update.

<snip>

void print_line( char* line )
{
printf("%s\n", *line++);

void print_line(cons t char *line)
{
puts(line);
}

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[arnuld]

Re: K&amp;R2 , exercise 7.6

On Mon, 21 Apr 2008 10:02:16 +0000, Richard Heathfield wrote:

arnuld said:

Where do these point? (Hint: nowhere in particular.)

OK, here is the 2nd version:


/* K&R2, section 7.7, exercise 7.6
*
* write a program to compare 2 files, printing that first line
* where they differ.
*
* version 1.1
*/


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

enum MAXSIZE { ARRSIZE=1000 };

void compare_files( FILE*, FILE* );
void print_line( const char* );


int main( int argc, char* argv[] )
{
FILE *pf1, *pf2;

if( argc !=3 )
{
fprintf( stderr, "You iDiOT, I expect 2 files as input. \n" );
exit(EXIT_FAILU RE);
}


/* open files */
pf1 = fopen( *++argv, "r" );
pf2 = fopen( *++argv, "r" );

/* error check */
if( pf1 == NULL || pf2 == NULL )
{
fprintf( stderr, "error opening files\n");
exit(EXIT_FAILU RE);
}
else
{
compare_files( pf1,pf2 );
}



/* don't forget to close the files */
fclose( pf1 );
fclose( pf2 );


return 0;
}



/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
int match;
char c1, c2;
char line1[ARRSIZE];
char line2[ARRSIZE];
char *begin_line1, *begin_line2, *p1, *p2;


match = 1;

while( (fgets( line1, ARRSIZE, pf1 ) != NULL) ||
(fgets( line2, ARRSIZE, pf2 ) != NULL))
{
begin_line1 = line1;
begin_line2 = line2;

p1 = line1;
p2 = line2;
for( c1 = *p1, c2 = *p2; c1 != '\0' && c2 != '\0'; ++p1, ++p2 )
{
if ( c1 != c2 )
{
match = 0;
print_line( begin_line1 );
printf("\n-----------------------\n"); print_line( begin_line2 );
}
}
}
}



void print_line( const char* line )
{
while( *line != '\0')
{
puts(*line++);
}
}

=============== = OUTPUT =============== ========

[arnuld@raj C]$ gcc -ansi -pedantic -Wall -Wextra 7-6.c
7-6.c: In function `print_line':
7-6.c:96: warning: passing arg 1 of `puts' makes pointer from integer
without a cast
[arnuld@raj C]$ ./a.out 7-6.c replace-blanks.c Segmentationfau lt
[arnuld@raj C]$







--

http://lispmachine.wordpress.com/

my email ID is at the above address

[Chris Dollin]

Re: K&amp;R2 , exercise 7.6

arnuld wrote:

void print_line( const char* line )
{
while( *line != '\0')
{
puts(*line++);
}
}
>
=============== = OUTPUT =============== ========
>
[arnuld@raj C]$ gcc -ansi -pedantic -Wall -Wextra 7-6.c
7-6.c: In function `print_line':
7-6.c:96: warning: passing arg 1 of `puts' makes pointer from integer
without a cast
[arnuld@raj C]$ ./a.out 7-6.c replace-blanks.c Segmentationfau lt
[arnuld@raj C]$

Ask yourself: what does `puts` accept as an parameter? What is
the type of the expression `*line++`? Are C characters a kind
of integer? Do you really expect an integer to be freely and
automatically converted to an integer?

--
"It took a very long time, much longer than the most /Sector General/
generous estimates."

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

[Joachim Schmitz]

Re: K&amp;R2 , exercise 7.6

arnuld wrote:

>On Mon, 21 Apr 2008 10:02:16 +0000, Richard Heathfield wrote:

>

>arnuld said:

>

>Where do these point? (Hint: nowhere in particular.)

>
>
OK, here is the 2nd version:
>
>
/* K&R2, section 7.7, exercise 7.6
*
* write a program to compare 2 files, printing that first line
* where they differ.
*
* version 1.1
*/
>
>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
>
enum MAXSIZE { ARRSIZE=1000 };
>
void compare_files( FILE*, FILE* );
void print_line( const char* );
>
>
int main( int argc, char* argv[] )
{
FILE *pf1, *pf2;
>
if( argc !=3 )
{
fprintf( stderr, "You iDiOT, I expect 2 files as input. \n" );
exit(EXIT_FAILU RE);
}
>
>
/* open files */
pf1 = fopen( *++argv, "r" );
pf2 = fopen( *++argv, "r" );
>
/* error check */
if( pf1 == NULL || pf2 == NULL )
{
fprintf( stderr, "error opening files\n");
exit(EXIT_FAILU RE);
}
else
{
compare_files( pf1,pf2 );
}
>
>
>
/* don't forget to close the files */
fclose( pf1 );
fclose( pf2 );
>
>
return 0;
}
>
>
>
/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
int match;
char c1, c2;
char line1[ARRSIZE];
char line2[ARRSIZE];
char *begin_line1, *begin_line2, *p1, *p2;
>
>
match = 1;
>
while( (fgets( line1, ARRSIZE, pf1 ) != NULL) ||
(fgets( line2, ARRSIZE, pf2 ) != NULL))
{
begin_line1 = line1;
begin_line2 = line2;
>
p1 = line1;
p2 = line2;
for( c1 = *p1, c2 = *p2; c1 != '\0' && c2 != '\0'; ++p1, ++p2 )
{
if ( c1 != c2 )
{
match = 0;
print_line( begin_line1 );
printf("\n-----------------------\n"); print_line( begin_line2
); }
}
}
}
>
>
>
void print_line( const char* line )
{
while( *line != '\0')
{
puts(*line++);
}
}
>
=============== = OUTPUT =============== ========
>
[arnuld@raj C]$ gcc -ansi -pedantic -Wall -Wextra 7-6.c
7-6.c: In function `print_line':
7-6.c:96: warning: passing arg 1 of `puts' makes pointer from integer
without a cast

What does that warning tell you?

[arnuld@raj C]$ ./a.out 7-6.c replace-blanks.c Segmentationfau lt
[arnuld@raj C]$

You lied to your compiler (and he spotted it and warned you about it) and he
got his revenge...

Bye, Jojo

[arnuld]

Re: K&amp;R2 , exercise 7.6

On Mon, 21 Apr 2008 11:35:00 +0100, Chris Dollin wrote:

Ask yourself: what does `puts` accept as an parameter? What is
the type of the expression `*line++`?

K&R2 Appendix B, page 247:

"int puts( const char *s )

puts writes the string s and the newline to stdout. It returns EOF
if an error occurs, non-negative otherwise"


Now there are no strings in C. We have arrays of characters. It is same
like printf() which accepts %s as an array of chars. So <lineis a single
line of input form file stored into an array of chars.

Are C characters a kind
of integer? Do you really expect an integer to be freely and
automatically converted to an integer?

that was my mistake, my typo(s), I think I am getting sleepy. I just
recently shifted from North-India to South-India and I had to change my
food-habits. I am not able to get any food I used to eat from last 27
years (since I was born). So I have lots of difficulties in living a
normal daily routine here.




--

http://lispmachine.wordpress.com/

my email ID is at the above address

[Richard Heathfield]

Re: K&amp;R2 , exercise 7.6

arnuld said:

>On Mon, 21 Apr 2008 11:35:00 +0100, Chris Dollin wrote:

>
>

>Ask yourself: what does `puts` accept as an parameter? What is
>the type of the expression `*line++`?

>
>
K&R2 Appendix B, page 247:
>
"int puts( const char *s )
>
puts writes the string s and the newline to stdout. It returns
EOF
if an error occurs, non-negative otherwise"

I don't know whether you noticed, but I gave you a working version of
print_line - and you appear to have broken it again.

>
>
Now there are no strings in C.

Wrong. In C, a string is a contiguous sequence of characters, terminated by
the first null character.

We have arrays of characters. It is same
like printf() which accepts %s as an array of chars.

Wrong. printf interprets %s as meaning "the parameter matching this format
specifier is a ##pointer## to the first character in a contiguous sequence
of characters that is terminated by a null character" - in other words, %s
means "I'm giving you a string".

<snip>

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[arnuld]

Re: K&amp;R2 , exercise 7.6

On Mon, 21 Apr 2008 11:57:59 +0000, Richard Heathfield wrote:

I don't know whether you noticed, but I gave you a working version of
print_line - and you appear to have broken it again.

yes. I fixed it again but I am using putchar() now.

Wrong. In C, a string is a contiguous sequence of characters, terminated
by the first null character.

but these 2 are different:

char arr[] = "Richard";
char* pc = "Richard";

both are contiguous sequence of characters ended by '\0'.

Wrong. printf interprets %s as meaning "the parameter matching this
format specifier is a ##pointer## to the first character in a contiguous
sequence of characters that is terminated by a null character" - in
other words, %s means "I'm giving you a string".

you can never use the pc for %s in printf(). Thats what I meant, there
are no strings, only arrays of chars. I tried to print the pc but all I
got was:


^A^M^D @#





--

http://lispmachine.wordpress.com/

my email ID is at the above address

[arnuld]

Re: K&amp;R2 , exercise 7.6

On Mon, 21 Apr 2008 10:02:16 +0000, Richard Heathfield wrote:

>arnuld wrote:
> int c1, c2, match;
> char *line1, *line2;

Where do these point? (Hint: nowhere in particular.)

okay, here is the 3rd version of the program. It compiles and runs, the
only thing is semantic-bug, it always does one thing, no matter whether 2
files are same or different:



/* K&R2, section 7.7, exercise 7.6
*
* write a program to compare 2 files, printing that first line
* where they differ.
*
* version 1.2
*/


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

enum MAXSIZE { ARRSIZE=1000 };

void compare_files( FILE*, FILE* );
void print_line( const char* );


int main( int argc, char* argv[] )
{
FILE *pf1, *pf2;

if( argc !=3 )
{
fprintf( stderr, "You iDiOT, I expect 2 files as input. \n" );
exit(EXIT_FAILU RE);
}


/* open files */
pf1 = fopen( *++argv, "r" );
pf2 = fopen( *++argv, "r" );

/* error check */
if( pf1 == NULL || pf2 == NULL )
{
fprintf( stderr, "error opening files\n");
exit(EXIT_FAILU RE);
}
else
{
compare_files( pf1,pf2 );
}



/* don't forget to close the files */
fclose( pf1 );
fclose( pf2 );


return 0;
}



/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
int match;
char c1, c2;
char line1[ARRSIZE];
char line2[ARRSIZE];
const char *begin_line1, *begin_line2, *p1, *p2;


match = 1;

while( (fgets( line1, ARRSIZE, pf1 ) != NULL) || (fgets( line2, ARRSIZE, pf2 ) != NULL))
{
begin_line1 = line1;
begin_line2 = line2;

p1 = line1;
p2 = line2;
for( c1 = *p1, c2 = *p2; c1 != '\0' && c2 != '\0'; ++p1, ++p2 )
{
if ( c1 != c2 )
{
match = 0;
print_line( begin_line1 );
printf("\n-----------------------\n");
print_line( begin_line2 );
return;
}
}
}
}



void print_line( const char* line )
{
while( *line != '\0')
{
putchar(*line++ );
}
}





--

http://lispmachine.wordpress.com/

my email ID is at the above address

[Richard Tobin]

Re: K&amp;R2 , exercise 7.6

In article <pan.2008.04.21 .17.39.57.67062 6@NoPain.Rome>,
arnuld <NoSpam@NoPain. Romewrote:

>Wrong. In C, a string is a contiguous sequence of characters, terminated
>by the first null character.

>but these 2 are different:
>
char arr[] = "Richard";
char* pc = "Richard";
>
>both are contiguous sequence of characters ended by '\0'.

And they are both strings.

>Wrong. printf interprets %s as meaning "the parameter matching this
>format specifier is a ##pointer## to the first character in a contiguous
>sequence of characters that is terminated by a null character" - in
>other words, %s means "I'm giving you a string".

>you can never use the pc for %s in printf().

What do you mean by "the pc"? Do you mean the variable pc declared
above? If so, you can certainly print it with %s.

-- Richard
--
:wq

[Joachim Schmitz]

Re: K&amp;R2 , exercise 7.6

arnuld wrote:

>On Mon, 21 Apr 2008 11:57:59 +0000, Richard Heathfield wrote:

>Wrong. In C, a string is a contiguous sequence of characters,
>terminated by the first null character.

>
but these 2 are different:
>
char arr[] = "Richard";
char* pc = "Richard";
>
both are contiguous sequence of characters ended by '\0'.

pc is a pointer to a (read-only) string
arr is an array containing a (modifyable) string

>Wrong. printf interprets %s as meaning "the parameter matching this
>format specifier is a ##pointer## to the first character in a
>contiguous sequence of characters that is terminated by a null
>character" - in other words, %s means "I'm giving you a string".

>
>
you can never use the pc for %s in printf().

Nonsense. In both cases printf only sees a pointer to char and prints that
up to the terminating \0.

Thats what I meant, there
are no strings, only arrays of chars. I tried to print the pc but all
I got was:
>
>
^A^M^D @#

They you must have done something severly wrong.

Bye, Jojo

[Ben Bacarisse]

Re: K&amp;R2 , exercise 7.6

arnuld <NoSpam@NoPain. Romewrites:

okay, here is the 3rd version of the program. It compiles and runs, the
only thing is semantic-bug, it always does one thing, no matter whether 2
files are same or different:

<snip>

/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
int match;
char c1, c2;
char line1[ARRSIZE];
char line2[ARRSIZE];
const char *begin_line1, *begin_line2, *p1, *p2;
>
>
match = 1;

What is this for?

while( (fgets( line1, ARRSIZE, pf1 ) != NULL) || (fgets( line2,
ARRSIZE, pf2 ) != NULL))

The logic here is wrong. || is a "a short-circuit" operator. Once
the first half gets a line the second half will not be executed. You
must read pairs of lines and I'd stop when one of the files runs out.

{
begin_line1 = line1;
begin_line2 = line2;
>
p1 = line1;
p2 = line2;
for( c1 = *p1, c2 = *p2; c1 != '\0' && c2 != '\0'; ++p1, ++p2 )
{
if ( c1 != c2 )

Why so many variables? What is wrong with strcmp?

{
match = 0;
print_line( begin_line1 );
printf("\n-----------------------\n");
print_line( begin_line2 );
return;
}
}
}
}
>
>
>
void print_line( const char* line )
{
while( *line != '\0')
{
putchar(*line++ );
}
}

What is the point of this function? It is just fputs, in effect.

--
Ben.

[arnuld]

Re: K&amp;R2 , exercise 7.6

On Mon, 21 Apr 2008 13:56:44 +0100, Ben Bacarisse wrote:

>arnuld wrote:

What is this for?

:-\

> while( (fgets( line1, ARRSIZE, pf1 ) != NULL) || (fgets( line2,
> ARRSIZE, pf2 ) != NULL))

The logic here is wrong. || is a "a short-circuit" operator. Once
the first half gets a line the second half will not be executed. You
must read pairs of lines and I'd stop when one of the files runs out.

As per my slowly developing C knowledge :P, If one line ends before ther
other then of course, the lines are not equal. This is true when either
both or any one of them runs out.

may be I did not understand your logic here but right now this is the
limit of my thinking.

What is the point of this function? It is just fputs, in effect.

K&R2 never talked about using it. So I never came across it.



Here is the 4th revision of the program. As usual output is not what I
wanted :(



/* K&R2, section 7.7, exercise 7.6
*
* write a program to compare 2 files, printing that first line
* where they differ.
*
* version 1.3
*/


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

enum MAXSIZE { ARRSIZE=1000 };

void compare_files( FILE*, FILE* );


int main( int argc, char* argv[] )
{
FILE *pf1, *pf2;

if( argc !=3 )
{
fprintf( stderr, "You iDiOT, I expect 2 files as input. \n" );
exit(EXIT_FAILU RE);
}


/* open files */
pf1 = fopen( *++argv, "r" );
pf2 = fopen( *++argv, "r" );

/* error check */
if( pf1 == NULL || pf2 == NULL )
{
fprintf( stderr, "error opening files\n");
exit(EXIT_FAILU RE);
}
else
{
compare_files( pf1,pf2 );
}



/* don't forget to close the files */
fclose( pf1 );
fclose( pf2 );


return 0;
}



/* compare 2 files */
void compare_files( FILE* pf1, FILE* pf2 )
{
char line1[ARRSIZE];
char line2[ARRSIZE];
const char *begin_line1, *begin_line2, *p1, *p2;


while( (fgets( line1, ARRSIZE, pf1 ) != NULL) ||
(fgets( line2, ARRSIZE, pf2 ) != NULL))
{
begin_line1 = p1 = line1;
begin_line2 = p2 = line2;

if( !strcmp( p1, p2 ) )
{
puts( begin_line1 );
printf("-------------- Lines differ here ---------------\n");
puts( begin_line2 );
return;
}
}
}
============= OUTPUT =============== ==

[arnuld@raj C]$ gcc -ansi -pedantic -Wall -Wextra test.c
[arnuld@raj C]$ ./a.out file1.txt file2.txt




-------------- Lines differ here ---------------




[arnuld@raj C]$













--

http://lispmachine.wordpress.com/

my email ID is at the above address

[Chris Dollin]

Re: K&amp;R2 , exercise 7.6

arnuld wrote:

>On Mon, 21 Apr 2008 11:35:00 +0100, Chris Dollin wrote:

>
>

>Ask yourself: what does `puts` accept as an parameter? What is
>the type of the expression `*line++`?

>
>
K&R2 Appendix B, page 247:
>
"int puts( const char *s )
>
puts writes the string s and the newline to stdout. It returns EOF
if an error occurs, non-negative otherwise"
>
Now there are no strings in C.

What makes you think that?

We have arrays of characters. It is same
like printf() which accepts %s as an array of chars. So <lineis a single
line of input form file stored into an array of chars.

`line` needn't have come from any of the program's input files:

puts( "I do not come from from this program's input files." );

--
"Thereafter , events may roll unheeded." /Foundation/

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

[edwin ng]

Re: K&amp;R2 , exercise 7.6

arnuld schreef:

>On Mon, 21 Apr 2008 13:56:44 +0100, Ben Bacarisse wrote:

>

>>arnuld wrote:

>
>
>

>> while( (fgets( line1, ARRSIZE, pf1 ) != NULL) || (fgets( line2,
>> ARRSIZE, pf2 ) != NULL))

>

>The logic here is wrong. || is a "a short-circuit" operator. Once
>the first half gets a line the second half will not be executed. You
>must read pairs of lines and I'd stop when one of the files runs out.

[snip]

>
while( (fgets( line1, ARRSIZE, pf1 ) != NULL) ||
(fgets( line2, ARRSIZE, pf2 ) != NULL))

You are still not seeing it ?
If you read line1 with success you will NOT read line2.

|| means OR, so short circuit will prevent you from evaluating the second part (after the || ) if the first is true
because the answer is already known.

You could try && instead and see if that fixes it.
Good luck,
Edwin