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Dinamic Array Allocation problems

14 New Member
hi, I'm writing a program a kind of calculator that should work with very big numbers (thousand of digits). So, numbers are rappresented by a struct that contains a pointer to integer, e 2 int for dimension of the array e number of digits.
the pointer is than used for the malloc of the array...
my 2 problems are:

1)some time, when I use my exp function I get the error:

*** glibc detected *** ./a.out: free(): invalid size: 0x0975a328 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6[0xb7e8b604]
/lib/tls/i686/cmov/libc.so.6[0xb7e8eabc]
/lib/tls/i686/cmov/libc.so.6(__lib c_malloc+0x95)[0xb7e8f9c5]
./a.out[0x804a8ab]
./a.out[0x804bb52]
./a.out[0x804ba3d]
./a.out[0x804ba3d]
./a.out[0x804ba3d]
./a.out[0x804ba3d]
./a.out[0x804ba3d]
./a.out[0x804ba3d]
./a.out[0x8049e32]
./a.out[0x8048892]
/lib/tls/i686/cmov/libc.so.6(__lib c_start_main+0x e5)[0xb7e32775]
./a.out[0x8048401]

and other stuff... can It be becouse it is a recursive function?

the second and biggest problem is that when the number becomes too large (something like 1000 digits) I get a segmentation fault... there are strange problems with arrays of this dimension?

the code is very big but you can find it at:
http://www.speedyshare.com/403525688.html

ps: I've deleted the free() functions... It caused too many errors...

thanks for you help, and sorry for my eng...
Jun 28 '09 #1
25 6484
donbock
2,426 Recognized Expert Top Contributor
Are you using malloc (or calloc) to dynamically allocate memory; and free to release dynamically allocated memory?

If so, are you trapping the exceptional case when malloc/calloc return an error because there is no available memory to allocate? If not, then you need to do so.

The "free(): invalid size: 0x0975a328" message sounds like you are trying to free a block of memory that wasn't dynamically allocated. This could happen if you free the same block twice; or if the pointer argument gets corrupted somewhere between being returned by malloc and being passed back to free.

Dynamic memory allocation works fine with recursive functions ... provided that you don't run out of memory. You want to be careful to free memory as soon as it isn't needed.

A general problem is memory fragmentation -- when the pattern of free's doesn't match the pattern of malloc's, so small freed blocks are not contiguous, leading eventually to the circumstance where malloc for a large block fails (despite there being more than enough memory) because no portion of free heap space is large enough to satisfy the request.
Jun 29 '09 #2
stararic
14 New Member
thanks for the reply, I don't use free and I'm not trapping that case... how to do it? something like:
a=malloc(d*size of(int));
if(a==NULL) {printf("error" ); exit(1);}

does it works? I will try this afternoon...
Jun 29 '09 #3
stararic
14 New Member
it does't work.... and for the segmentation fault problem?
Jun 29 '09 #4
donbock
2,426 Recognized Expert Top Contributor
So ... I gather from your terse response that you added the error traps to all malloc/calloc/realloc function calls; and that none of the traps were taken; and that the program continues to fail as it did before -- with a run-time segmentation fault.

"Segmentati on fault" means that you tried to access nonexistent memory. The most common ways to do this include:
  • dereferencing a NULL pointer;
  • dereferencing a pointer into dynamic memory after the memory has been freed;
  • dereferencing a corrupted pointer;
  • accessing past the end of a dynamic memory block, typically by using a too-large array index.

1. Examine the segmentation fault error message. It should report the address where the fault occurred. Compare that address to your link map to see which function contains the offending instruction. Presumably the offending instruction dereferences a pointer into one of your allocated memory blocks. Look carefully to make sure that pointer does indeed point into a valid memory block. Also look carefully to make sure you're not accessing past the end of that block (for instance, with a too-large array index).

2. I notice your backtrace was almost all raw hexadecimal addresses. Temporarily make all of your functions global (by removing "static" keyword). See if that induces the backtrace to report function names.

I will try to look at your source code, but it may be quite awhile before I have time to work through such a large source file. It would help if you provided the segmentation fault backtrace, decoding the hex addresses into function names.
Jun 29 '09 #5
stararic
14 New Member
sorry for newbie question... but when you say "link map" what does it means??
and I don't know what is a global function and what is not, so maybe are all global... I will post the code again with the add you said.
however, I've 2 kind of errors... this one:
free(): invalid size: 0x0975a328
caused by the exp function (espone in the code), and the second one, that say just "segmentati on fault" when the number of digits is >1000...
Jun 29 '09 #6
JosAH
11,448 Recognized Expert MVP
@stararic
The 'invalid size' error most likely originates from corrupted memory or a pointer value you're trying to free() that you haven't malloc()'d. A segmentation fault most likely originates from corrupted memory or buffer overflows.

kind regards,

Jos
Jun 29 '09 #7
donbock
2,426 Recognized Expert Top Contributor
Expand|Select|Wrap|Line Numbers
  1. int *VerificaDimezzamento(int array[], int n, int *d)
  2.    {
  3.    int *b, i;
  4.    if(n<=(*d)/4)
  5.       {
  6.       b=malloc(((*d)/4)*sizeof(int));
  7.       for(i=0; i<n;i++) 
  8.          b[i]=array[i];
  9.       for(;i<((*d)/4);i++)
  10.          b[i]=0;
  11.       *d=2*(*d);
  12.       return b;
  13.       }
  14.    return array;
  15.    }
I don't understand what this function is trying to do; but I notice that the size of the memory allocation is (*d)/4 times sizeof(int), but it sets *d to 2*(*d). That difference between the size of the allocation and the new value of *d contrasts with VerificaRaddopp io.


Expand|Select|Wrap|Line Numbers
  1. typedef struct el {int *array; int n; int d; struct el *succ;} elemento;
  2. ...
  3. elemento pop(pile *pila)
  4.    {
  5.    elemento a;
  6.    pile tmp;
  7.    if(pila!=NULL)
  8.       {
  9.       a=**pila;
  10.       tmp=*pila;
  11.       *pila=(*pila)->succ;
  12.       return a;
  13.       }
  14.    else 
  15.       {
  16.       printf("pila vuota!"); 
  17.       return a;
  18.       }
  19. }
Are you sure that *pila is never NULL (like if the pile is empty)? If it is ever NULL then the line 'a=**pila' will provoke a segmentation fault.

If pila is NULL then this function returns an uninitialized elemento without any warning to the caller. In that case, there will probably be a segmentation fault if the caller tries to dereference the array or succ fields of the uninitialized elemento.


I haven't gone very far into this source file yet.

In general, this code makes little attempt to verify that pointers aren't NULL before dereferencing them. It is good practice to get in the habit of always checking pointers. This may result in some unnecessary checks, but it may save you many months of unnecessary debugging over the extent of your programming career.

By the way, you would be doing yourself a favor by adding some comments to this program.

Forget about my earlier suggestion to make the functions global.
Jun 29 '09 #8
stararic
14 New Member
thaks, for first one "VerificaDimezz amento", I never use it, so I I have removed it. the second one "pop" is now:
elemento pop(pile *pila)
{elemento a;pile tmp;
if(*pila!=NULL) {
a=**pila;
tmp=*pila;
*pila=(*pila)->succ;
return a;}
else {printf("pila vuota!"); exit(1);}
}

I'm adding some things to the code, I will post it soon. I know about the comments:D
Jun 29 '09 #9
donbock
2,426 Recognized Expert Top Contributor
@stararic
Please use CODE tags. The following code snippet is much easier to read:
Expand|Select|Wrap|Line Numbers
  1. elemento pop(pile *pila)
  2.    {
  3.    elemento a;
  4.    pile tmp;
  5.    if(*pila!=NULL)
  6.       {
  7.       a=**pila;
  8.       tmp=*pila;
  9.       *pila=(*pila)->succ;
  10.       return a;
  11.       }
  12.    else 
  13.       {
  14.       printf("pila vuota!"); 
  15.       exit(1);
  16.       }
  17.    }
By the way, you should still confirm that pila is not NULL before dereferencing it. Why is tmp there if you don't use it? Now that you have removed elemento 'a' from the pile, you don't want anybody to use a.succ to peek into the pile -- it is good linked-list hygiene to set a.succ to NULL before returning 'a'.

By the way, with all the 'free' calls removed this program is one huge memory leak -- especially the recursive functions. That isn't causing your current problems (as long as malloc returns successfully), but should be fixed before you consider your program done.
Jun 29 '09 #10

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