C : double pointer passed as parameters comes back empty [duplicate] - c

Question

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malloc-ating multidimensional array in function
(7 answers)
Closed 7 years ago.
At first, it may sound normal if I free the allocation within the function, but it's not the case. As I'm writting these lines, I've found a work around, but I would like to keep a certain homogeneity in my code and would prefer to keep it the way it ways but you know working properly, so is there another solutions or my workaround is the only option?
The main function:
void main(void)
{
SHead head; // Custom struct
unsigned char **array = NULL; // pointer to 2D array
allocArray2D(&head, array) // the function signature: (SHead*, unsigned char**)
// here, the array pointer is still NULL (0x0)
//...
return EXIT_SUCCESS;
}
The allocation function malloc a very small amount of memory around 21 unsigned char* and for each simple pointer 21 unsigned char.
Within the function, the pointer is fine and points to a proper address.
So my work around was modify the function from:
void allocArray(SHead* h, unsigned char** arr)
{
int x, y, i;
getsize(head, *x, *y);
arr = (unsigned char**)malloc(sizeof(unsigned char*)*y);
if(arr)
printf(">> Erro allocating memory\n"), return;
for(i =0; i<y; i++)
{
arr[i] = (unsigned char)malloc(sizeof(unsigned char)*x);
}
}
to the following:
unsigned char** allocArray(SHead*)
{
int x, y, i;
unsigned char **arr;
getsize(head, *x, *y);
arr = (unsigned char**)malloc(sizeof(unsigned char*)*y);
if(arr)
printf(">> Erro allocating memory\n"), return;
for(i =0; i<y; i++)
{
arr[i] = (unsigned char)malloc(sizeof(unsigned char)*x);
}
return arr; // returning the address
}
As I said earlier, I'd like to keep homogeneity within my code and would prefer to keep a function signature similar to other function I have. My workaround work properly. And I'm wondering if it is the only solution, or perhaps I am missing something.
EDIT: Following the comments I added more code.
Thank you,
Alex.

Answer 1

You have to pass a pointer to the 2dimensional array to your function, to write in the function to the value behind the pointer:
SHead head; // Custom struct
unsigned char **array = NULL; // pointer to 2D array
allocArray2D(*head, &array)
// ^ address of array
-
void allocArray(SHead* head, unsigned char*** pArray)
// ^ pointer to char** because its an output parameter
{
int x, y, i;
getsize( head, &x, &y );
*pArray = malloc( y * sizeof( unsigned char * );
// ^ assigne somtething to the variable array refered by the pointer pArray
if( *pArray == NULL )
{
printf(">> Erro allocating memory\n")
return;
}
for ( i = 0; i < y; i ++ )
(*pArray)[i] = malloc( x * sizeof( unsigned char ) );
}
Note, what you did was to pass a NULL-pointe to the function allocArray.
An other solution would be to return the allocated memory by the return value of function allocArray:
SHead head; // Custom struct
unsigned char **array = NULL;
array = allocArray( &head );
-
unsigned char** allocArray( SHead* head )
{
int x, y, i;
getsize( head, &x, &y );
unsigned char** arr = malloc( y * sizeof( unsigned char * );
if( arr == NULL )
{
printf(">> Erro allocating memory\n")
return;
}
for ( int i = 0; i < y; i ++ )
arr[i] = malloc( x * sizeof( unsigned char ) );
return arr;
}

Answer 2

If I have understood you correctly what you need is something like the following
For simplicity I excluded the parameter that refers to the structure.
void allocArray2D( unsigned char ***a, size_t n )
{
*a = malloc( n * sizeof( unsigned char * ) );
for ( size_t i = 0; i < n; i++ ) ( *a )[i] = malloc( n * sizeof( unsigned char ) );
}
//...
unsigned char **array = NULL; // pointer to 2D array
allocArray2D( &array, 21 );
If you want that the original object would be changed after passing it to a function then you have to pass it by reference.

Answer 3

Your call looks very strange.
First, you are passing *head. head seems to be an uninitialised variable, so passing *head seems badly wrong.
Second, the function called has no way of seeing array. Your call is equivalent to allocArray2D (*head, NULL) and not having the array variable at all. This also seems very wrong.