# K&R2, exercise 4-2

Discussion in 'C Programming' started by arnuld, Apr 3, 2008.

1. ### arnuldGuest

STATEMENT: Write the function strindex(s, t), which returns the position of the
rightmost occurence of t in s, or -1 if there is none.

PURPOSE: this program prints the index of the rightmost match on the line.
The match we have to find is a char array named pattern. We also print out
the number of matches we have found. We will take the input from
command-line.

PROBLEM: Segmentation Fault

The programe compiles fine but at run-time it segfaults

here is my solution which is a little modified version of the example
provided in the same section:

/* Exercise # 4.1 */

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

enum MAXSIZE { ARR_SIZE = 1000 };

int getline( char current_line[], int max );
int str_index( char current_line[], char search_for[] );

char pattern[] = "term";

int match_num;

int main( void )
{
char current_line[ARR_SIZE];
int matched_idx, idx;

idx = 0;

while( getline(current_line, ARR_SIZE) > 0 )
{
if( (matched_idx = str_index(current_line, pattern)) >= 0 )
{
printf("\n%d matches, \n%d is the last to match", match_num, matched_idx);
}
}

return 0;

}

/* takes a line as input and returns the length of the line */
int getline( char s[], int max )
{
int c, i;

for( i = 0; ( (c = getchar()) != EOF && (c != '\n') && (--max > 0) ); ++i )
{
s = c;
}

if( c == '\n' )
{
s[i++] = '\n';
}

s = '\0';

return i;
}

/* search for a pattern in the line, will save every index position of
source string where the pattern starts to match. For string the index we
use an array of ints. we then return the last element of array which is the
index of the rightmost match. We also return the number of matches we have
found using an int match_num.
*/
int str_index( char s[], char p[] )
{
int i, j, k;
int idx, last_match;
int saved_pos[ARR_SIZE];
memset( saved_pos, '\0', sizeof( saved_pos ));

idx = 0;
match_num = 0;

for( i = 0; s != '\0'; ++i )
{
for( k = 0, j = i; p[k] != '\0' ; ++k, ++j )
{
if( s[j] != p[k] )
{
break;
}
}

if( (k > 0) && p[k] == '\0' )
{
saved_pos[idx] = i;
++match_num;
}
}

last_match = sizeof(saved_pos) - 2;
/* it checks whether we have any match or not. If we do not have any match
* then we have nothing in array */
if( saved_pos[0] )
{
return saved_pos[last_match];
}
else
{
return -1;
}
}

=========== OUTPUT =============
/home/arnuld/programs/C \$ gcc -ansi -pedantic -Wall -Wextra 4-1.c
/home/arnuld/programs/C \$ ./a.out
terminal
and no one
and this
term
and Xterminal
segmentation fault
/home/arnuld/programs/C \$

arnuld, Apr 3, 2008

2. ### arnuldGuest

> On Thu, 03 Apr 2008 19:14:49 +0530, arnuld wrote:

> last_match = sizeof(saved_pos) - 2;
> /* it checks whether we have any match or not. If we do not have any
> match
> * then we have nothing in array */
> if( saved_pos[0] )
> {
> return saved_pos[last_match];
> }

that was the source of trouble. I changed it to this:

if( saved_pos[0] )
{
return saved_pos[match_num - 1];
}

and I got a new bug:

[arnuld@raj C]\$ gcc -ansi -pedantic -Wall -Wextra 4-1.c
[arnuld@raj C]\$ ./a.out
and no TERM
term
and term

1 matches,
4 is the last to match
nope
the terminal is the xterm and urxvt term

3 matches,
0 is the last to match[arnuld@raj C]\$

whats the problem now ?

arnuld, Apr 3, 2008

3. ### Ben BacarisseGuest

Richard Heathfield <> writes:

> arnuld said:
>
> <snip>
>
>> PROBLEM: Segmentation Fault
>>
>> The programe compiles fine but at run-time it segfaults

>
> <snip>
>
>> int str_index( char s[], char p[] )
>> {
>> int i, j, k;
>> int idx, last_match;
>> int saved_pos[ARR_SIZE];

>
> <snip>
>>
>> last_match = sizeof(saved_pos) - 2;

>
> This doesn't do what you think. sizeof saved_pos gives the size, in bytes,
> of the array. That's fine if ints are 1 byte in size, which they can be,
> but it doesn't seem to be the case on your system.
>
> What you meant to write was:
>
> last_match = sizeof saved_pos / sizeof saved_pos[0] - 2;
>

I am not sure if you are joking. It fixes the problem only for some
very limited meanings of either "fix" or "problem".

--
Ben.

Ben Bacarisse, Apr 3, 2008
4. ### Ben BacarisseGuest

arnuld <> writes:

>> On Thu, 03 Apr 2008 19:14:49 +0530, arnuld wrote:

>
>> last_match = sizeof(saved_pos) - 2;
>> /* it checks whether we have any match or not. If we do not have any
>> match
>> * then we have nothing in array */
>> if( saved_pos[0] )
>> {
>> return saved_pos[last_match];
>> }

>
> that was the source of trouble. I changed it to this:
>
>
> if( saved_pos[0] )
> {
> return saved_pos[match_num - 1];
> }
>
>
>
> and I got a new bug:

You have a few. I'll take only one that I can illustrate from the
fragment above: you can't tell if there is or is not a match by
testing saved_pos[0] because there match might be a first match at
position zero. Now, as it happens, you store all the match positions
in index 0 of this array because you don't increment idx (not shown)
so this bug only shows up when the first and last match are at
position zero.

I would suggest you don't try to store the matches. Try to find a way
that does not involve remembering them all, just to return the
right-most one.

--
Ben.

Ben Bacarisse, Apr 3, 2008
5. ### arnuldGuest

> On Thu, 03 Apr 2008 19:22:29 +0530, Richard Heathfield wrote:

> This doesn't do what you think. sizeof saved_pos gives the size, in
> bytes, of the array. That's fine if ints are 1 byte in size, which they
> can be, but it doesn't seem to be the case on your system.

your way/idea of telling the problem is very clear. I never expected that
from the author of a book like "C Unleashed". NO, I have not read the book
but heard about its toughness and harder ideas.

> What you meant to write was:
>
> last_match = sizeof saved_pos / sizeof saved_pos[0] - 2;
>

I got a new one:

[arnuld@raj C]\$ ./a.out
term
term
Xterm

1 matches,
Position 1 is the last to match

[arnuld@raj C]\$ ./a.out
this terminal

1 matches,
Position 1 is the last to match
[arnuld@raj C]\$

look at the output. program is not reading the position 0 and always saying
that position 1 is th elast to match. I think I have modified the file a
little. you can check the whole source here:

http://pastebin.ca/969121

> And in case anyone's curious, the only use I made of gdb for finding
> this bug was to get a backtrace, from which the problem was immediately
> obvious.

Oh... I never used gdb. I think I need to read the manual to start using
it.

arnuld, Apr 3, 2008
6. ### arnuldGuest

> On Thu, 03 Apr 2008 19:46:34 +0530, Ben Bacarisse wrote:

> You have a few. I'll take only one that I can illustrate from the
> fragment above: you can't tell if there is or is not a match by testing
> saved_pos[0] because there match might be a first match at position
> zero. Now, as it happens, you store all the match positions in index 0
> of this array because you don't increment idx (not shown) so this bug
> only shows up when the first and last match are at position zero.
>
> I would suggest you don't try to store the matches. Try to find a way
> that does not involve remembering them all, just to return the
> right-most one.

any more bugs left:

/* Exercise # 4.1 */

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

enum MAXSIZE { ARR_SIZE = 1000 };

int getline( char current_line[], int max );
int str_index( char current_line[], char search_for[] );

char pattern[] = "term";

int match_num;

int main( void )
{
char current_line[ARR_SIZE];
int matched_idx, idx;

idx = 0;

while( getline(current_line, ARR_SIZE) > 0 )
{
if( (matched_idx = str_index(current_line, pattern)) >= 0 )
{
printf("\n%d matches -- ", match_num);
if( match_num )
{
printf("Position %d is the last to match\n\n", matched_idx);
}
}
}

return 0;

}

/* takes a line as input and returns the length of the line */
int getline( char s[], int max )
{
int c, i;

for( i = 0; ( (c = getchar()) != EOF && (c != '\n') && (--max > 0) ); ++i )
{
s = c;
}

if( c == '\n' )
{
s[i++] = '\n';
}

s = '\0';

return i;
}

/* search for a pattern in the line, will save every index position of source
string where the pattern starts to match. For string the index we use an
array of ints. we then return the last element of array which is the
index of the rightmost match. We also return the number of matches we have
found using an int match_num.
*/
int str_index( char s[], char p[] )
{
int i, j, k;
int idx, match_pos;

idx = 0;
match_num = 0;

for( i = 0; s != '\0'; ++i )
{
for( k = 0, j = i; p[k] != '\0' ; ++k, ++j )
{
if( s[j] != p[k] )
{
break;
}
}

if( (k > 0) && p[k] == '\0' )
{
++match_num;
match_pos = i;
}
}

/* it checks whether we have any match or not. */
if( match_num )
{
return match_pos;
}
else
{
return -1;
}
}

========== OUTPUT ============
[arnuld@raj C]\$ gcc -ansi -pedantic -Wall -Wextra -ggdb 4-1.c
[arnuld@raj C]\$ ./a.out
and this
and that
9843788327#&&\$^\$^\$^^\$TER%%^
TERM
term

1 matches -- Position 0 is the last to match

term and term and term\0

3 matches -- Position 18 is the last to match

[arnuld@raj C]\$

arnuld, Apr 3, 2008
7. ### Ben BacarisseGuest

Richard Heathfield <> writes:

> Ben Bacarisse said:
>
>> Richard Heathfield <> writes:
>>

> <snip>
>>>
>>> What you meant to write was:
>>>
>>> last_match = sizeof saved_pos / sizeof saved_pos[0] - 2;
>>>

>>
>> <snip> It fixes the problem only for some
>> very limited meanings of either "fix" or "problem".

>
> Well, okay, it fixes the segfault, anyway.

Right. I thought that is maybe what you meant.

--
Ben.

Ben Bacarisse, Apr 3, 2008
8. ### Ben BacarisseGuest

arnuld <> writes:

>> On Thu, 03 Apr 2008 19:46:34 +0530, Ben Bacarisse wrote:

<snip>
>> I would suggest you don't try to store the matches. Try to find a way
>> that does not involve remembering them all, just to return the
>> right-most one.

>
> any more bugs left:

The str_index function looks good to me but there is a bug in the input

> /* Exercise # 4.1 */
>
>
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> enum MAXSIZE { ARR_SIZE = 1000 };
>
> int getline( char current_line[], int max );
> int str_index( char current_line[], char search_for[] );
>
> char pattern[] = "term";
>
> int match_num;
>
>
> int main( void )
> {
> char current_line[ARR_SIZE];
> int matched_idx, idx;
>
> idx = 0;
>
>
> while( getline(current_line, ARR_SIZE) > 0 )
> {
> if( (matched_idx = str_index(current_line, pattern)) >= 0 )
> {
> printf("\n%d matches -- ", match_num);

It is more reliable to get into the habit of ending output with
newlines rather than starting it with then. In your case...

> if( match_num )
> {
> printf("Position %d is the last to match\n\n", matched_idx);
> }

.... when there is no match, the output is "left hanging". Output that
does not end with a newline is guaranteed to appear on all systems.

> }
> }
>
> return 0;
>
> }
>
>
> /* takes a line as input and returns the length of the line */
> int getline( char s[], int max )
> {
> int c, i;
>
> for( i = 0; ( (c = getchar()) != EOF && (c != '\n') && (--max > 0) ); ++i )
> {
> s = c;
> }

For simple testing, I prefer to read a whole line, storing only those
characters that fit but this way is fine. Except...

> if( c == '\n' )
> {
> s[i++] = '\n';
> }
>
> s = '\0';

.... I think you have a bug here. The loop above can put max-1 chars
into the buffer (max being what it was originally, of course) but it
can end when c == '\n'. You then try to put the '\n' and the '\0' in
the one remaining space!

> return i;
> }
>
>
> /* search for a pattern in the line, will save every index position of source
> string where the pattern starts to match. For string the index we use an
> array of ints. we then return the last element of array which is the
> index of the rightmost match. We also return the number of matches we have
> found using an int match_num.
> */
> int str_index( char s[], char p[] )
> {
> int i, j, k;
> int idx, match_pos;
>
>
> idx = 0;

No used.

> match_num = 0;

OK, maybe you do this for testing, but a utility function like this
should not use a file-scope (AKA "global") variable like match_num.
Of course, you code works if you just make match_num local to this
function.

> for( i = 0; s != '\0'; ++i )
> {
> for( k = 0, j = i; p[k] != '\0' ; ++k, ++j )
> {
> if( s[j] != p[k] )
> {
> break;
> }
> }
>
> if( (k > 0) && p[k] == '\0' )
> {
> ++match_num;
> match_pos = i;
> }
> }
>
>
>
> /* it checks whether we have any match or not. */
> if( match_num )
> {
> return match_pos;
> }
> else
> {
> return -1;
> }
> }

Looks good to me.

--
Ben.

Ben Bacarisse, Apr 3, 2008
9. ### Keith ThompsonGuest

Ben Bacarisse <> writes:
[...]
> ... when there is no match, the output is "left hanging". Output that
> does not end with a newline is guaranteed to appear on all systems.

[...]

You mean "is not guaranteed".

--
Keith Thompson (The_Other_Keith) <>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

Keith Thompson, Apr 3, 2008
10. ### Default UserGuest

arnuld wrote:

> > On Thu, 03 Apr 2008 19:22:29 +0530, Richard Heathfield wrote:

>
>
> > This doesn't do what you think. sizeof saved_pos gives the size, in
> > bytes, of the array. That's fine if ints are 1 byte in size, which
> > they can be, but it doesn't seem to be the case on your system.

>
> your way/idea of telling the problem is very clear. I never expected
> that from the author of a book like "C Unleashed". NO, I have not
> read the book but heard about its toughness and harder ideas.

Brian

Default User, Apr 3, 2008
11. ### Ben BacarisseGuest

Keith Thompson <> writes:

> Ben Bacarisse <> writes:
> [...]
>> ... when there is no match, the output is "left hanging". Output that
>> does not end with a newline is guaranteed to appear on all systems.

> [...]
>
> You mean "is not guaranteed".

Yes, thanks. No point in my saying I'll proof read more since I've
said that before and it is obviously not working!

--
Ben.

Ben Bacarisse, Apr 3, 2008
12. ### arnuldGuest

> On Thu, 03 Apr 2008 21:17:30 +0530, Ben Bacarisse wrote:

>> arnuld <> writes:

>> if( match_num )
>> {
>> printf("Position %d is the last to match\n\n", matched_idx);
>> }
>> }

> ... when there is no match, the output is "left hanging". Output that
> does not end with a newline is guaranteed to appear on all systems.

I really don't get it at all. If there is no match then condition will
fail and hence othing to do.

> For simple testing, I prefer to read a whole line, storing only those
> characters that fit but this way is fine. Except...

I am at chapter 4 where authors did not start discussing the "whole line"
concept except using a rudimentary function named getline() to do that.

>> if( c == '\n' )
>> {
>> s[i++] = '\n';
>> }
>> }
>> s = '\0';

> ... I think you have a bug here. The loop above can put max-1 chars
> into the buffer (max being what it was originally, of course) but it can
> end when c == '\n'. You then try to put the '\n' and the '\0' in the
> one remaining space!

sorry, this function is just a copy of K&R2 example, as I said earlier.
so, something wrong with K&R2 ?

>> match_num = 0;

> OK, maybe you do this for testing, but a utility function like this
> should not use a file-scope (AKA "global") variable like match_num. Of
> course, you code works if you just make match_num local to this
> function.

I have used match_num in main() function and that is why I made it global.
If it is local then main() is not going to access it.

arnuld, Apr 4, 2008
13. ### Ben BacarisseGuest

arnuld <> writes:

>> On Thu, 03 Apr 2008 21:17:30 +0530, Ben Bacarisse wrote:

>
>>> arnuld <> writes:

>
>>> if( match_num )
>>> {
>>> printf("Position %d is the last to match\n\n", matched_idx);
>>> }
>>> }

>
>> ... when there is no match, the output is "left hanging". Output that
>> does not end with a newline is guaranteed to appear on all systems.

>
> I really don't get it at all. If there is no match then condition will
> fail and hence othing to do.

No, you are right. I based my comment on an old version that
sometimes did this:

./arnuld
a term
^D
1 matches,
0 is the last to match\$

The \$ is my system's prompt. Your new fixed version prints nothing if
there is no match, but it takes a while to reason that out. You
should probably remove the test "if (match_num)" since it suggests
that a partial line can be printed. The test is redundant.

>> For simple testing, I prefer to read a whole line, storing only those
>> characters that fit but this way is fine. Except...

>
>
> I am at chapter 4 where authors did not start discussing the "whole line"
> concept except using a rudimentary function named getline() to do
> that.

I was suggesting an alternative, that is all. You do, essentially,
this:

while (there-is-room && next-char-is-not-newline)
store-that-char;

I often write:

while (next-char-is-not-newline)
if (there-is-room)
store-that-char;

so that a whole line is always read, even when there is not room for
it all.

>>> if( c == '\n' )
>>> {
>>> s[i++] = '\n';
>>> }
>>> }
>>> s = '\0';

>
>> ... I think you have a bug here. The loop above can put max-1 chars
>> into the buffer (max being what it was originally, of course) but it can
>> end when c == '\n'. You then try to put the '\n' and the '\0' in the
>> one remaining space!

>
> sorry, this function is just a copy of K&R2 example, as I said earlier.
> so, something wrong with K&R2 ?

I'd love to be the one to find a bug in K&R2 after all those eyes have
looked at it but, no, K&R is correct. There are two getline functions
in the book (I have K&R first edition so I won't give you page
numbers). The first tests against i < lim-1 and the second has a test
for --lim > 0 *before* reading the next character. Your code is
slightly different and has the bug I described.

The moral is that you can't just change

for (i = 0; --max > 0 && (c = getchar()) != EOF && c != '\n'; ++i)

into

for (i = 0; (c = getchar()) != EOF && c != '\n' && --max > 0; ++i)

with no consequences.

Interestingly, both of K&R's getline functions exhibit undefined
behaviour when called with a buffer size of 1. This is daft enough
not to be a bug as such, but I would have preferred to avoid it in a
teaching text.

>>> match_num = 0;

>
>> OK, maybe you do this for testing, but a utility function like this
>> should not use a file-scope (AKA "global") variable like match_num. Of
>> course, you code works if you just make match_num local to this
>> function.

>
> I have used match_num in main() function and that is why I made it global.
> If it is local then main() is not going to access it.

That is fine for now, but global variables are not a good way to
communicate with general-purpose functions like str_index. When you
have got further though the book, you'll find ways to get more
information out of a function that means you don't need global
variables like that.

--
Ben.

Ben Bacarisse, Apr 4, 2008