allocating memory for structure

This is a discussion on allocating memory for structure within the C Programming forums, part of the General Programming Boards category; hi, i have a simple question and would apprichiate if someone could give me a detailed explanation. so what i ...

  1. #1
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    allocating memory for structure

    hi, i have a simple question and would apprichiate if someone could give me a detailed explanation. so what i have is a structure like this:

    Code:
    
    typedef struct tuple{
    	int x;
    	struct tuple *childTuple;
    }Tuple;
    and the code that follows look like this:
    Code:
    int ntop = -1;
    
    void fpush (Tuple *newTuple, Tuple **top,int topSize){
    	if ( ntop== topSize){
    		printf( "stack overflow");
    		return;
    	}
    	ntop++;
    	top[ntop] = newTuple;
    }
    
    int main(){
            Tuple *lastInterval = (Tuple *)malloc(100*sizeof(Tuple));
    	Tuple **top = (Tuple **)malloc(sizeof(Tuple *));
    	top[0] =(Tuple *)malloc(sizeof(Tuple));
    	top[0]->childTuple = (Tuple *)malloc(100*sizeof(Tuple));
    	lastInterval = NULL;
    	top[0] = NULL;
    
    	for (i=0;i<=100;i++){
    		lastInterval = fcreateTup(i,i+1,i+2,lastInterval);
    		fpush(lastInterval,top,200);
    	}
    
    }
    so my question now is : do i need to allocate 100 times Tuple for:

    Code:
    Tuple **top = (Tuple **)malloc(sizeof(Tuple *));
    top[0] =(Tuple *)malloc(sizeof(Tuple));
    or not , because it seams to work both ways. and why does it work if it works and it is not just my imagination ?

    cheers

    baxy

  2. #2
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    I'm not really clear on what your code is trying to do, and you don't show us fcreateTup. I'm guessing your after something like a 2-d array, but dynamically allocated. See if this link helps: Question 6.16. Otherwise, you will have to do lots more explaining.

  3. #3
    and the hat of int overfl Salem's Avatar
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    > Tuple **top = (Tuple **)malloc(sizeof(Tuple *));
    You should avoid casting the return result of malloc (see the FAQ).

    Also, if you write malloc calls in this form, there is minimal chance of screwing things up.
    p = malloc( numNeeded * sizeof(*p) );

    So for example
    Tuple **top = malloc(sizeof(*top));



    Code:
    	top[0] =(Tuple *)malloc(sizeof(Tuple));
    	top[0]->childTuple = (Tuple *)malloc(100*sizeof(Tuple));
    	lastInterval = NULL;
    	top[0] = NULL;
    This just leaked some memory.
    It'll also cause a segfault if you try and use top[0] now for something.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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