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Program repeats itself, pointer trouble I suspect.

 
 
Neil
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Posts: n/a
 
      02-15-2007
Hello all!


I wrote program with a array of pointers, and I suspect they are
pointing at each other in the Do ...While loop.
Something is messed up with the increment variable word. A program
clip of what I'm talking about.

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

int main(void)
{
char string[50] = {"Have a nice day folks"};
char *line_ptr;
char *list[20] = { '\0' }; //Initialize the array of pointers to
NULL.
int word = 0;

line_ptr = strtok(string, " ");

do
{
list[word] = line_ptr;
word++;
line_ptr = strtok(NULL," ");
} while (line_ptr != NULL);

return 0;
}
It's kinda weird, cause the program repeats itself when I run it. I
left out the output section, cause I know it works fine. In the
debugger it works OK. Is there a memory leak? Do you need to assign
pointers to NULL after you use them? Should I free up the memory from
the array of pointers before the program terminates? What is the best
way handle pointers after your done with them?

Thanks for all your help
Neil

 
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Old Wolf
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      02-15-2007
On Feb 15, 1:50 pm, "Neil" <(E-Mail Removed)> wrote:
> #include <stdio.h>
> #include <string.h>
>
> int main(void)
> {
> char string[50] = {"Have a nice day folks"};
> char *line_ptr;
> char *list[20] = { '\0' };


{ 0 } does the same thing and makes a bit more sense.

> int word = 0;
>
> line_ptr = strtok(string, " ");
>
> do
> {
> list[word] = line_ptr;
> word++;
> line_ptr = strtok(NULL," ");
> } while (line_ptr != NULL);
>
> return 0;}
>
> It's kinda weird, cause the program repeats itself when I run it.


This program is fine. What do you mean by 'repeats itself' ?
This program generates no output.

> In the debugger it works OK. Is there a memory leak?


No

>Do you need to assign pointers to NULL after you use them?


No

> Should I free up the memory from the array of pointers before the
> program terminates?


No

> What is the best way handle pointers after your done with them?


Take no special action.

> I left out the output section, cause I know it works fine.


Apparently not...

 
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Barry Schwarz
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      02-15-2007
On 14 Feb 2007 16:50:11 -0800, "Neil" <(E-Mail Removed)> wrote:

>Hello all!
>
>
>I wrote program with a array of pointers, and I suspect they are
>pointing at each other in the Do ...While loop.
>Something is messed up with the increment variable word. A program
>clip of what I'm talking about.
>
>#include <stdio.h>
>#include <string.h>
>
>int main(void)
>{
>char string[50] = {"Have a nice day folks"};
>char *line_ptr;
>char *list[20] = { '\0' }; //Initialize the array of pointers to
>NULL.


This is why you should not use // style comments in usenet.

If you want assign each of the 20 pointer in the array the NULL value,
use NULL. While 0 and '\0' will both work, they are visually
misleading. Someone might be tempted to think that the pointers point
to a char containing '\0'.

>int word = 0;
>
>line_ptr = strtok(string, " ");
>
>do
> {
> list[word] = line_ptr;
> word++;
> line_ptr = strtok(NULL," ");
> } while (line_ptr != NULL);
>
>return 0;
>}
>It's kinda weird, cause the program repeats itself when I run it. I
>left out the output section, cause I know it works fine. In the


Define repeat.

The archives are full of messages where the error was in the section
omitted by the poster because "it works." Post a compilable example
that demonstrates the behavior in question and let us help you find
the problem.

>debugger it works OK. Is there a memory leak? Do you need to assign


You cannot have a memory leak without dynamic allocation.

>pointers to NULL after you use them? Should I free up the memory from


Unless you test a pointer for NULL, you never need to reset it to
NULL.

>the array of pointers before the program terminates? What is the best


Any attempt to free memory that you did not allocate will invoke
undefined behavior.

>way handle pointers after your done with them?


The same way you handle an object of any other type when you are done
with it. In most cases, it is sufficient to simply not use it in any
subsequent code.


Remove del for email
 
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Beej
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      02-15-2007
On Feb 14, 4:50 pm, "Neil" <(E-Mail Removed)> wrote:
> line_ptr = strtok(string, " ");


Unless you're sure you're always going to have a token, you should
probably check line_ptr for NULL here, too.

Here's a compact form of that, if you don't mind assignments in your
expressions:

if ((p = strtok(string, " ")) != NULL) {
do {
printf("Token: %s\n", p);
} while ((p = strtok(NULL, " ")) != NULL);
}

> do
> {
> list[word] = line_ptr;
> word++;
> line_ptr = strtok(NULL," ");
> } while (line_ptr != NULL);


Here is my output when I ran it.

'Have'
'a'
'nice'
'day'
'folks'

Looks fine to me. What's your output?

-Beej

 
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Christopher Layne
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      02-15-2007
Neil wrote:

> #include <stdio.h>
> #include <string.h>
>
> int main(void)
> {
> char string[50] = {"Have a nice day folks"};
> char *line_ptr;
> char *list[20] = { '\0' }; *//Initialize the array of pointers to


Cleaner version:

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

enum constants {
STRING_MAX = 50,
WORD_MAX = 20
};

int main(void)
{
char string[STRING_MAX] = "Have a nice day folks";
char *word[WORD_MAX];
int i;

for (i = 0; i < WORD_MAX; i++) {
if ((word[i] = strtok(i ? NULL : string, " ")) == NULL)
break;
fprintf(stdout, "word[%d] = %s\n", i, word[i]);
}

return 0;
}

 
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Richard Heathfield
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Posts: n/a
 
      02-15-2007
Old Wolf said:

> On Feb 15, 1:50 pm, "Neil" <(E-Mail Removed)> wrote:
>>
>> do
>> {
>> list[word] = line_ptr;
>> word++;
>> line_ptr = strtok(NULL," ");
>> } while (line_ptr != NULL);
>>
>> return 0;}
>>
>> It's kinda weird, cause the program repeats itself when I run it.

>
> This program is fine.


You think so? I don't think you read it carefully enough.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.
 
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Ben Bacarisse
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      02-15-2007
"Neil" <(E-Mail Removed)> writes:

> #include <stdio.h>
> #include <string.h>
>
> int main(void)
> {
> char string[50] = {"Have a nice day folks"};
> char *line_ptr;
> char *list[20] = { '\0' }; /* Initialize the array of pointers to NULL.*/
> int word = 0;
>
> line_ptr = strtok(string, " ");
>
> do
> {
> list[word] = line_ptr; /* MARK */
> word++;
> line_ptr = strtok(NULL," ");
> } while (line_ptr != NULL);


Other than and hint in a reply to another reply, no one has pointed
out that you are in danger of accessing outside the bounds of your
array "list".

If it reasonable (is it ever?) to simply discard tokens that don't
fit, you can write:

if (word < 20) list[word] = line_ptr;

where I put /* MARK */ in your code.

I don't know of this is the source of your problem, because the
description is rather vague.

--
Ben.
 
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Old Wolf
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      02-15-2007
On Feb 15, 7:19 pm, Richard Heathfield <(E-Mail Removed)> wrote:
> Old Wolf said:
> > On Feb 15, 1:50 pm, "Neil" <(E-Mail Removed)> wrote:
> >> do
> >> {
> >> list[word] = line_ptr;
> >> word++;
> >> line_ptr = strtok(NULL," ");
> >> } while (line_ptr != NULL);

>
> >> return 0;}

>
> >> It's kinda weird, cause the program repeats itself when I run it.

>
> > This program is fine.

>
> You think so? I don't think you read it carefully enough.


By 'fine' I mean that the OP's code doesn't contain any bugs.
Of course I would not pedantic about the use of C99 features,
or bugs introduced by wordwrapping during the posting process.

Are you perhaps referring to the fact that the program might
break if its source is modified to introduce a bug, as
suggested by Ben Bacarisse?
If not, then please be more explicit.

 
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Ben Bacarisse
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      02-15-2007
"Old Wolf" <(E-Mail Removed)> writes:

> Are you perhaps referring to the fact that the program might
> break if its source is modified to introduce a bug, as
> suggested by Ben Bacarisse?


I'm not seeing it. Can you post a correction to my message (or
explain it to me and I'll post a correction)?

--
Ben.
 
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Old Wolf
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      02-16-2007
On Feb 16, 12:46 pm, Ben Bacarisse <(E-Mail Removed)> wrote:
> "Old Wolf" <(E-Mail Removed)> writes:
> > Are you perhaps referring to the fact that the program might
> > break if its source is modified to introduce a bug, as
> > suggested by Ben Bacarisse?

>
> I'm not seeing it. Can you post a correction to my message (or
> explain it to me and I'll post a correction)?


Your message appears to be saying that the program could
break if the input string were modified to have more than
20 words in it, which would be a bug. But the original
post only had 4 words in the string, so there is no problem.

(Of course it is not a bad idea to add in checking, as
you suggested).

 
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