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problem with freeing data

P: n/a
I will past only two segments from the code it should be enough to see
what I did wrong, I think I know there I made a mistake, but how to
fix it I can not tell. This why I need help from you all.

Main code:
----------------
/* duomenu rasymas i faila */
dst = fopen(argv[2], "w");
if (dst != NULL) {
wordDBSize = sizeStack(wordDB);
for (tmp = 0; tmp < wordDBSize; tmp++) {
word = NULL;
word = popStack(wordDB);
if (fprintf(dst, "%d: %s\n", tmp + 1, word) < 0)
show_err("(antra)[main] Can not write to file. Please
check <destination_filepermissions. Exiting.");
/* FIXME - PROBLEM HERE - (line below) */
free(word);
}
charAverage = wordCountAll / (float)wordCount;
fprintf(dst, "Average (Words: %d, Total chars: %d) : %.2f\n",
wordCount, wordCountAll, charAverage);
fclose(dst);
destroyStack(&wordDB);
} else
show_err("(antra)[main] Can not create <destination_file>
file.");

Stack code:
-----------------
char* popStack(stack *item) {
child *tmp;
char *value;

if (item->root == NULL)
return NULL;
value = item->root->value;
tmp = item->root;
item->root = item->root->next;
free(item->root);
item->size--;

return value;
}

I highlighted in the code there I am have problem. As you can see when
I call popStack it return pointer to the string, which is in my linked
list element and then I free memory of the element. free(word);
returns error:

Macbook:antra_new marius$ ./antra src.txt dst.txt sep.txt
antra(882) malloc: *** error for object 0xc0000003: Non-aligned
pointer being freed
*** set a breakpoint in malloc_error_break to debug
Segmentation fault

So, I think that freeing the element of my list frees my string, which
pointer is located in that element. Removing "free(item-
>root);" (stack code) fix this problem and now I can free my string in
the main code, but as you can see in this situation I am leaving
garbage in the memory and I don not want to do that.

Firstly I would like to know, does freeing my element from linked list
frees the string which pointer is located in that element. And finally
how could I fix that?

P.S. My stack holds the pointers in char arrays.
Mar 4 '08 #1
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9 Replies


P: n/a
P.S. My stack holds the pointers to(in) char arrays
Mar 4 '08 #2

P: n/a
david wrote:
[Snip]
char* popStack(stack *item) {
child *tmp;
char *value;

if (item->root == NULL)
return NULL;
value = item->root->value;
tmp = item->root;
item->root = item->root->next;
free(item->root);
Are you _sure_ you meant to do this, rather than free(tmp)?
Mar 4 '08 #3

P: n/a
david wrote:
I will past only two segments from the code it should be enough to see
what I did wrong,
No. It isn't enough.

As you don't know what's wrong, I don't see how you think you can decide
how much we need to know...

In future, please post something complete enough for us to see what's
happening - ideally produce the smallest possible self-contained
testcase.

[snip]
free(word);
Was "word" allocated with malloc/calloc/realloc? We don't know, because
you haven't shown us....

[Snip]
Firstly I would like to know, does freeing my element from linked list
frees the string which pointer is located in that element. And finally
how could I fix that?
free(pointer) frees what the pointer points to, no more and no less.
(Obviously "pointer" must point to space allocated by one of the
malloc family of functions).
P.S. My stack holds the pointers in char arrays.
What is that supposed to mean?
Mar 4 '08 #4

P: n/a
david <Da*****************@gmail.comwrote:
word = popStack(wordDB);
/* FIXME - PROBLEM HERE - (line below) */
free(word);
char* popStack(stack *item) {
value = item->root->value;
tmp = item->root;
item->root = item->root->next;
free(item->root);
This is wrong; you want free(tmp); here. But that's probably not what's
causing your problem.
return value;
}
antra(882) malloc: *** error for object 0xc0000003: Non-aligned
pointer being freed
So, I think that freeing the element of my list frees my string,
That depends. If item->root->value is a char *, it doesn't. If it's a
char[], it does. But you have failed to show us two critical parts of
your code:
- the definition of a stack, and that of stack's member root;
- the code where you put your words _in_ your stack, in particular, how
you declare the memory for each root's value string.

If you malloc() memory for each word separately before you push them
onto your stack, and don't manipulate that address before you push it on
(or, say, free() it in between), you should be able to free() it. OTOH,
if you get the memory for each word from part of a single malloc(), or
from an automatic ("local") variable, or any number of other
possibilities, you not only do not need to free() it, but shouldn't.

What the solution to your problem is depends on the two bits of code I
mentioned above.

Richard
Mar 4 '08 #5

P: n/a
The is not that small and I will post in the one of pastebin websites.

stack.h - http://www.paste.lt/paste/6aa50aa1d9...56c8d24e3f1f16
stack.c - http://www.paste.lt/paste/684c6fa2ef...2d4be11c65e796
antra.c (main) - http://www.paste.lt/paste/0f2dd328b2...0383ad9fb41cdf

antra_lib.c - http://www.paste.lt/paste/83ec85cfac...55f7acc4d106f0
antra_lib.h - http://www.paste.lt/paste/cc88b47394...edd6edf9487b4c

I am sorry that I am not using buffered reading in this program. This
is the hole program and you should be able to compile it.
../antra <source_file<destination_file<separators_list_file >
should put words in destination file whose length is even (sorry if
wrong word, my native language is not English) and word does not
contains any number.

I use realloc to allocate memory and grow my string until I get to
separator and when I push to stack the pointer of this string and then
I NULL the pointer and reseting the length of string back to zero,
this should protect the later string I created. And again I reallocate
a new one and etc. till the end.

Stack struct has pointer "root" which points to the top of my stack.
child struct contains of next pointer (next item in the linked list)
and char pointer there it holds the location of the string I pushed.

I again read how works realloc and I can see that I should be using it
correctly, if pointer is NULL when realloc works just like malloc.

I am very interested why I can not free that string later after poping
it from stack.
Thanks everyone for helping.
Mar 4 '08 #6

P: n/a
david <Da*****************@gmail.comwrites:
The is not that small and I will post in the one of pastebin websites.

stack.h - http://www.paste.lt/paste/6aa50aa1d9...56c8d24e3f1f16
stack.c - http://www.paste.lt/paste/684c6fa2ef...2d4be11c65e796
antra.c (main) - http://www.paste.lt/paste/0f2dd328b2...0383ad9fb41cdf

antra_lib.c - http://www.paste.lt/paste/83ec85cfac...55f7acc4d106f0
antra_lib.h -
http://www.paste.lt/paste/cc88b47394...edd6edf9487b4c
This looks like the code you posted earlier. The popStack function
still has the error that I saw being pointer out so I don't see any
point in taking time to look at the rest. You are freeing the wrong
thing and you need to fix that before anything else.

--
Ben.
Mar 4 '08 #7

P: n/a
In this code:
1 for (tmp = 0; tmp < wordDBSize; tmp++) {
2 word = NULL;
3 word = popStack(wordDB);
4 if (fprintf(dst, "%d: %s\n", tmp + 1, word) < 0)
5 show_err("(antra)[main] Can not write to file. Please
check <destination_filepermissions. Exiting.");
6 /* FIXME - PROBLEM HERE - (line below) */
7 free(word);
8 }

In line 3, you get "word" from popStack(). What if word was NULL?
popStack() do return NULL on one case. You can't free NULL
david wrote:
I will past only two segments from the code it should be enough to see
what I did wrong, I think I know there I made a mistake, but how to
fix it I can not tell. This why I need help from you all.

Main code:
----------------
/* duomenu rasymas i faila */
dst = fopen(argv[2], "w");
if (dst != NULL) {
wordDBSize = sizeStack(wordDB);
for (tmp = 0; tmp < wordDBSize; tmp++) {
word = NULL;
word = popStack(wordDB);
if (fprintf(dst, "%d: %s\n", tmp + 1, word) < 0)
show_err("(antra)[main] Can not write to file. Please
check <destination_filepermissions. Exiting.");
/* FIXME - PROBLEM HERE - (line below) */
free(word);
}
charAverage = wordCountAll / (float)wordCount;
fprintf(dst, "Average (Words: %d, Total chars: %d) : %.2f\n",
wordCount, wordCountAll, charAverage);
fclose(dst);
destroyStack(&wordDB);
} else
show_err("(antra)[main] Can not create <destination_file>
file.");

Stack code:
-----------------
char* popStack(stack *item) {
child *tmp;
char *value;

if (item->root == NULL)
return NULL;
value = item->root->value;
tmp = item->root;
item->root = item->root->next;
free(item->root);
item->size--;

return value;
}

I highlighted in the code there I am have problem. As you can see when
I call popStack it return pointer to the string, which is in my linked
list element and then I free memory of the element. free(word);
returns error:

Macbook:antra_new marius$ ./antra src.txt dst.txt sep.txt
antra(882) malloc: *** error for object 0xc0000003: Non-aligned
pointer being freed
*** set a breakpoint in malloc_error_break to debug
Segmentation fault

So, I think that freeing the element of my list frees my string, which
pointer is located in that element. Removing "free(item-
root);" (stack code) fix this problem and now I can free my string in
the main code, but as you can see in this situation I am leaving
garbage in the memory and I don not want to do that.

Firstly I would like to know, does freeing my element from linked list
frees the string which pointer is located in that element. And finally
how could I fix that?

P.S. My stack holds the pointers in char arrays.
Mar 5 '08 #8

P: n/a
Thought about that and checked some time ago, it does not return NULL;
It does return exact the same amount of strings (char pointers) I
pushed in the code above, the only problem it does not show the words
and some garbage.

Ben:
Sorry, only after your second post I noticed the mistake. I will try
to recompile code when I have a chance.
Mar 5 '08 #9

P: n/a
ru*********@gmail.com writes:
[...]
In line 3, you get "word" from popStack(). What if word was NULL?
popStack() do return NULL on one case. You can't free NULL
[...]

Yes, you can. free(NULL) does nothing. (Doing so may well be an
error; I haven't looked closely at the code.)

--
Keith Thompson (The_Other_Keith) <ks***@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Mar 5 '08 #10

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