memory access failure

**Artie Gold** · Jul 19 '05, 07:22 PM

Re: memory access failure

Thomas Baier wrote:[color=blue]
> Hi there,
>
> I'v got a little program generating coincidental lists. It crashes with
> memory access failure, but not always at the same time.
>
> If I start the programm using valgrind it always crashes after sorting the
> first list with the following messages:
>
> thomas@linux:~/Info/C++> valgrind list
> /usr/bin/valgrind: line 106: exec: list: not found
> thomas@linux:~/Info/C++> valgrind ./list
> ==7888== Memcheck, a.k.a. Valgrind, a memory error detector for x86-linux.
> ==7888== Copyright (C) 2002, and GNU GPL'd, by Julian Seward.
> ==7888== Using valgrind-1.9.5pre, a program instrumentation system for
> x86-linux.
> ==7888== Copyright (C) 2000-2002, and GNU GPL'd, by Julian Seward.
> ==7888== Estimated CPU clock rate is 1809 MHz
> ==7888== For more details, rerun with: -v
> ==7888==
> liste...ok //Ausgabe
> des Programms
> ==7888== Invalid write of size 4
> ==7888== at 0x8048A28: List<int>::appe nd(int) (lib/list.h:107)
> ==7888== by 0x80488B7: main (list.cpp:28)
> ==7888== by 0x4030D8AD: __libc_start_ma in (in /lib/libc.so.6)
> ==7888== by 0x8048710: (within /home/thomas/Info/C++/list)
> ==7888== Address 0x4112F060 is 0 bytes after a block of size 4 alloc'd
> ==7888== at 0x40162F43: malloc (in /usr/lib/valgrind/valgrind.so)
> ==7888== by 0x40163448: realloc (in /usr/lib/valgrind/valgrind.so)
> ==7888== by 0x80489F5: List<int>::appe nd(int) (lib/list.h:103)
> ==7888== by 0x80488B7: main (list.cpp:28)
> Append...ok
> ==7888==
> ==7888== Invalid read of size 4
> ==7888== at 0x8048A66: List<int>::out( ) (lib/list.h:113)
> ==7888== by 0x80488E1: main (list.cpp:30)
> ==7888== by 0x4030D8AD: __libc_start_ma in (in /lib/libc.so.6)
> ==7888== by 0x8048710: (within /home/thomas/Info/C++/list)
> ==7888== Address 0x4112F060 is 0 bytes after a block of size 4 alloc'd
> ==7888== at 0x40162F43: malloc (in /usr/lib/valgrind/valgrind.so)
> ==7888== by 0x40163448: realloc (in /usr/lib/valgrind/valgrind.so)
> ==7888== by 0x80489F5: List<int>::appe nd(int) (lib/list.h:103)
> ==7888== by 0x80488B7: main (list.cpp:28)
> 98 55 0 10 7 17 97 85 75 87 9 24 4 36 28 76 58 45 37 47 62 60 82 43 62 87 48
> 51 56 50 34 55 7 34 63 12 49 61 95 26 50 6 48 52 42 74 30 98 21 67 46 81 27
> 29 25 89 15 71 39 71 23 74 26 28 7 89 40 57 50 36 81 98 40 28 51 81 4 79 80
> 23 48 26 5 75 53 28 63 69 1 4 39 22 76 65 48 83 54 86 39 3
> 0________
> Speicherzugriff sfehler
>
>
> The methods of the list class is in short:
>
> List <T>::List()
> {
> elements = NULL; //Array for saving the list elements
> length = 0;
>
> }
> void List <T>::append(T newElement)
> {
> if((elements = (T*) realloc(element s, sizeof(T))) == NULL) {[/color]

This is not what you want to do.

First of all, if realloc() returns NULL, you have a memory leak;
you've lost the original value of `elements'.

More to the point, though, is the fact that you've reallocated only
enough memory for one object of type `T' -- not what you want at all
-- hence the memory error.
[color=blue]
> printf("\n!!!Er ror!!!\n");
> //Doesn't appear anytime
>
> }
> elements[length++] = newElement;
> }
>
> The code of my little program is:
>
>
> main()[/color]
There is no implicit `int' in C++, so this should be:

int main()
[color=blue]
> {
> srand((unsigned ) time(NULL));
> for(int i=0;i<100;i++) {
> cout << "liste...";
> List <int> *a = new List<int>;
> cout << "ok\n";
> cout << "Append..." ;
> for(int j=0;j<100;j++)
> a->append(rand()% 99);
> cout << "ok\n";
> a->out();
> cout << "\n" << i << "________\n ";
> delete(a);
> }
> }
>
>[/color]
You're also missing headers and there are neamespace problems, but I
won't get into that here.

In general, it would be much better if you included *real*,
*compilable* code (copied'n'paste d); it would make it easier to
correct whatever other problems you've got.

HTH,
--ag

--
Artie Gold -- Austin, Texas
Oh, for the good old days of regular old SPAM.

**Thomas Baier** · Jul 19 '05, 07:25 PM

Re: memory access failure - add

So here is the compilable code (shorter version):

list.h:

template <typename T> class List
{
private:
int length;

protected:
T* elements;

public:
List();

/************
* Methoden: *
************/

void append(T newelement);
void exchange(int first, int second); //Vertauschen
void out(); //Liste ausgeben
int getLength();

};

/*************** *************** *************** ***********
* Konstruktoren: *
* *
* mehrere Initialisierung svarianten durch Überladen *
*************** *************** *************** ***********/

template <typename T> List <T>::List()
{
elements = NULL;
length = 0;

}

/*************** *********
* Methoden: *
*************** *********/

template <typename T> void List <T>::append(T newElement)
{
if((elements = (T*) realloc(element s, sizeof(T))) == NULL) {
printf("\n!!!Er ror!!!\n");
exit();

}
elements[length++] = newElement;
}

template <typename T> void List <T>::out()
{
for(int i=0; i<length; i++) {
cout << elements[i] << " ";
}

}

template <typename T> void List <T>::exchange(i nt first, int second)
{
int temp = elements[first];
elements[first] = elements[second];
elements[second] = temp;
}

template <typename T> int List <T>::getLength( )
{
return length;
}

_______________ _______________ _______________ _______________ _____________

list.cpp

#include "./lib/list.h"
#include <sys/time.h>
#include <stdlib.h>

main()
{
srand((unsigned ) time(NULL));

for(int i=0;i<100;i++) {
List <int> *a = new List<int>;
for(int j=0;j<100;j++)
a->append(rand()% 99);
}
}

_______________ _______________ _______________ _______________ _______________

I do not understand why

template <typename T> void List <T>::append(T newElement)
{
if((elements = (T*) realloc(element s, sizeof(T))) == NULL) {
printf("\n!!!Er ror!!!\n");
exit();

}
elements[length++] = newElement;
}

should not do what I want. I thought it reallocates memory and returns
"Error" if impossible. If possible the new element is appended to the list.
Where's my mistake?

Thomas

**James Lothian** · Jul 19 '05, 07:25 PM

Re: memory access failure - add

Thomas Baier wrote:[color=blue]
> snip
> I do not understand why
>
> template <typename T> void List <T>::append(T newElement)
> {
> if((elements = (T*) realloc(element s, sizeof(T))) == NULL) {
> printf("\n!!!Er ror!!!\n");
> exit();
>
> }
> elements[length++] = newElement;
> }
>
> should not do what I want. I thought it reallocates memory and returns
> "Error" if impossible. If possible the new element is appended to the list.
> Where's my mistake?
>[/color]
Read the previous reply. You're calling realloc with the wrong arguments
-- you're only allocating enough memory for one element *in total*,
rather
than the current size *plus one*.

James

memory access failure

memory access failure

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