Dynamic memory allocation by a function within another function

Hi All,

I have been using C for a while, still haven't got around with pointers :(

I am defining a pointer (to a pointer) **v; in main.
Then **v is passed to a function Initialise() to assign values.
Initialise() calls dmatrix() to allocates memory to v.
Finally, the function finalise(v) frees the memory.

PROBLEM: Even though v is passed as a reference (via pointer), its values are not accessible to the main function.The program fails when tries to free the memory.

I have multiple variables that need to be initialised in this fashion.
Can anyone offer any clues or workarounds.
Thanks very much in advance for your help.

Code:

---

#include <stdio.h>
#include <stdlib.h>
void Initialise(double **v, int row, int column, int value);
void finalise(double **v);
double **dmatrix(int row, int column);
void freedmatrix(double **a);

main(){
        double **v;
        Initialise(v,10,10,3);
        printf("v from main %f\n",v[1][1]);
        finalise(v);
        }

void Initialise(double **v, int row, int column, int value){
        int i,j;
        v=dmatrix(row+1,column+1);

        for (i=1;i<=row;i++){
        for (j=1;j<=column;j++){
                v[i][j]=value;}}
        printf("v from function Initialise %f\n",v[1][1]);
}

void finalise(double **v){
  freedmatrix(v);}

double **dmatrix(int row, int column){
        int i;
        double **a;
        a=(double **) malloc((row)*sizeof(double *));
        a[0]= (double *) malloc((row)*(column)*sizeof(double));
       
        for(i=1;i<=(row-1);i++){
                a[i]=a[i-1]+column;
        }
        return(a);
}

void freedmatrix(double **a){
  free(a[0]);
  free(a);
}

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But you're not passing a double ** var by reference, you're passing it by value. Passing it as you do means that any changed to v never get propagated back to the calling function since the callee is just modifying a copy of v. Just like an int - the rules don't change because your variable is a pointer.

If you want to pass a double ** by reference, the parameter type needs to be double ***, it has to be called as Initialize(&v,...), and modified in the called function using *v = dmatrix(...) and so on.

Thanks very much- that makes sense from the results. (I always thought if we declare double **v, then v=&v[0][0]).

Anyway, I resolved the issue (for now) by declaring v as a global variable.

Quote:

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Originally Posted by saj

Anyway, I resolved the issue (for now) by declaring v as a global variable.

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That's not resolving an issue, that's creating future issues.

Quote:

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Originally Posted by KCfromNC

But you're not passing a double ** var by reference, you're passing it by value. Passing it as you do means that any changed to v never get propagated back to the calling function since the callee is just modifying a copy of v. Just like an int - the rules don't change because your variable is a pointer.

If you want to pass a double ** by reference, the parameter type needs to be double ***, it has to be called as Initialize(&v,...), and modified in the called function using *v = dmatrix(...) and so on.

---

Just a bit of nit picking really, what you are saying makes sense, but note that there is no passing by reference in C. Everything is passed by value. The fact that a pointer is the address of something else is a completely different subject altogether; the address pointed to by the pointer (i.e. its value) is what gets passed to the function. As far as the compiler is concerned that address is a value just like 42 is.

C++ on the other hand does support passing by reference, using, well... references. They follow sensibly different rules than pointers though.