Memory allocation/reallocation

**magda3227** · 07-04-2008

I'm still learning about pointers and using malloc, so please bare with me.

I was wondering if it is possible to allocate memory for a pointer in a main function, send that pointer to another function, and then reallocate memory for that same pointer within the secondary function. I assume that this would make no difference to reallocating memory in the main function, correct?

I'm having trouble reallocating memory for a few pointers that I am sending to a secondary function. I am basically collecting user input in the form of arrays (pointers, actually) and then planning to use these pointers to do some calculations in other functions. The problem is that the values are not storing correctly. I get no compiler error, but when i get to the memory reallocation line in the program, I get a segmentation fault and my compiler goes crazy. Obviously, I'm not using malloc and/or realloc correctly, even though I thought I had the right format, based on looking at other code snippets and what not.

Here's my code...sorry for the complex variables, this project I'm working on is a MatLab simulation program conversion.

main function:

Code:

#include<stdio.h>
#include<stdlib.h>
#include"untitledproto.h"

#include"untitled2.c"


int main()
{

int M, i, j, PAS_type, cluster_number;
double *d_norm, *delta_phi_deg, *amplitude_cluster, *phi_deg, *AS_deg;


d_norm = malloc(sizeof(d_norm));
delta_phi_deg = malloc(sizeof(delta_phi_deg));
amplitude_cluster = malloc(sizeof(amplitude_cluster));
phi_deg = malloc(sizeof(phi_deg));
AS_deg = malloc(sizeof(AS_deg));


if (d_norm==NULL)    /*checking for null pointers*/
printf("OUT OF MEMORY!");

if (delta_phi_deg==NULL)
printf("OUT OF MEMORY!");

if (amplitude_cluster==NULL)
printf("OUT OF MEMORY!");

if (phi_deg==NULL)
printf("OUT OF MEMORY!");

if (AS_deg==NULL)
printf("OUT OF MEMORY!");


---------------------------------------------------------------------------------
int data;

data=dialog(d_norm, delta_phi_deg, amplitude_cluster, phi_deg, AS_deg);

cluster_number = (data&#37;100)%10;
M=((data-cluster_number)%100)/10;
PAS_type=(data-(M+cluster_number))/100;



return 0;
}

secondary function...supposed to gather user input and store them as arrays (basically).
Returns one value that is basically a mathematical combination of the array sizes and another variable. This value is then split back apart into 3 separate variable in the main function. (I needed a multiple return).

Code:

#include<stdio.h>
#include<stdlib.h>


double dialog(double *d_norm, double *delta_phi_deg, double *amplitude_cluster, double *phi_deg, double *AS_deg)
{
	
	double spacing=0;
	int M=0, PAS_type=0, cluster_number=0; 
	int i,j;

	while(M<=1)
	{
	
		printf("\nNumber of elements?\t");
		scanf("%d", &M);
	}

	
	d_norm=realloc(d_norm, M*sizeof(d_norm));


	if(M>1)
	{
	
		while(spacing<=0)
		{

			printf("\nNormalized spacing between elements (in wavelengths)?\t");
			scanf("%lf", &spacing);

	

			/*---------Computes Linearly Spaced Vector-------*/
			

			for(i=0; i<M; i++)
			{
				d_norm[i] = (i)*spacing;
			}
			

	
			while(cluster_number<1)
			{

				printf("\nNumber of impinging clusters of waves?\t");
				scanf("%d", &cluster_number);
		
			}

			delta_phi_deg = realloc(delta_phi_deg, cluster_number*sizeof(delta_phi_deg));
				amplitude_cluster = realloc(amplitude_cluster, cluster_number*sizeof(cluster_number));
				phi_deg = realloc(phi_deg, cluster_number*sizeof(phi_deg));
				AS_deg = realloc(AS_deg, cluster_number*sizeof(AS_deg));

			/*----------------^^^ Crashes here ^^^--------------------*/
			PAS_type=0;
	
			while(PAS_type<1 || PAS_type>3)
			{
	
			printf("\nPAS type (1 = Uniform, 2 = Gaussian, 3 = Laplacian)?\t");
			scanf("%d", &PAS_type);

			}

			if(cluster_number==1)
			{
				if(PAS_type!=1)
				{
					delta_phi_deg[0]=-1;
					while(delta_phi_deg[0]<=0)
					{
						printf("\nHalf Domain Definition (in degrees)?\t");
						scanf("%lf", &delta_phi_deg[0]);
				
					}
				}
				else
				{
					delta_phi_deg[0]= 90;
				}

				amplitude_cluster[0]=1;

				printf("\nMean Angle of Incidence at Node B (in degrees)?\t");
				scanf("%lf", &phi_deg[0]);

				AS_deg[0]=-1;
				while(AS_deg[0]<0)
				{
					printf("\nAzimuth Spread (in degrees)?\t");
					scanf("%lf", &AS_deg[0]);
				}
			}
			else
			{
		
				for(j=0;j<cluster_number;j++)
				{
					if(PAS_type!=1)
					{
					
						delta_phi_deg[j]=-1;
						while(delta_phi_deg[j]<0)
						{
							printf("\nCluster %d/%d: Half Domain Definition (in degrees)?\t", j+1, cluster_number);
							scanf("%lf", &delta_phi_deg[j]);
						}

					}

					else
					{
						delta_phi_deg[j]= 90;
					}


					amplitude_cluster[j]=-1;
					while(amplitude_cluster[j]<0)
					{
						printf("\nCluster %d/%d: Linear Power?\t", j+1, cluster_number);
						scanf("%lf", &amplitude_cluster[j]);
	
					}
		
			
					printf("\nCluster %d/%d: Mean Angle of Incidence at Node B (in degrees)?\t", j+1, cluster_number);
					scanf("%lf", &phi_deg[j]);

			
					AS_deg[j]=-1;
					while(AS_deg[j]<0)
					{
						printf("\nCluster %d/%d: Azimuth Spread (in degrees)?\t", j+1, cluster_number);
						scanf("%lf", &AS_deg[j]);
					}	
				}
			}		
		}
	}



int data=(M*10)+(PAS_type*100)+cluster_number;

return data;
}

Thank you in advance, as well as sorry for my poor programming. I'm still learning...

**Codeplug** · 07-04-2008

All the parameters passed to dialog() or passed by-value. This means that any changes made to these variables inside dialog() will not be seen by main(). For example:
>> d_norm=realloc(d_norm, M*sizeof(d_norm));
This may change the value of d_norm inside dialog() - but the "d_norm" variable in main() is still pointing to the original (now invalid) address. realloc() is free to return a brand new pointer.

So you'll need to "pass by reference" any parameters whose values change, and those changes need to be seen by the caller.

gg

**anon** · 07-04-2008

What happens is that realloc may move the memory contents to some other place in memory but main won't know that. If this happens the pointer in main becomes a wild or dangling pointer.

The function should therefore take a pointer to pointer so that the pointer in the calling code would be updated.

Code:

void foo(int** pp)
{
    //realloc
}

int main(void)
{
    int* p = malloc(N);
    foo(&p); //foo may change where p points
}

Another issue is that if realloc fails to allocate more data it returns NULL but doesn't change the original memory.

Code:

d_norm=realloc(d_norm, M*sizeof(d_norm));

If that should happen you have just lost a reference to the original memory.

Instead store the result in a different pointer and assign it back only if it is not NULL

Code:

double* sizer = realloc(dnorm, M);
if (sizer == NULL) {
    //realloc failed
}
else {
    dnorm = sizer;
}

And the third issue

Code:

d_norm = malloc(sizeof(d_norm));
//and elsewhere

Did you want to allocate a double's worth of memory (sizeof(*d_norm)) or a pointer-to-double's worth of memory (which might not be enough to store a double)?

**magda3227** · 07-04-2008

Originally Posted by Codeplug

So you'll need to "pass by reference" any parameters whose values change, and those changes need to be seen by the caller.

gg

My compiler gives me an error when I try to send the addresses of the pointers. It says that I'm passing from incompatible pointer types.

That's what you meant when you said passing by reference, correct?

**MacGyver** · 07-04-2008

You have to change the function prototype/definition as well.

**dwks** · 07-04-2008

Originally Posted by magda3227

My compiler gives me an error when I try to send the addresses of the pointers. It says that I'm passing from incompatible pointer types.

That's what you meant when you said passing by reference, correct?

When you "send the addresses of pointers", you need to make the function you're calling expect to receive pointers-to-pointers, as anon outlined.

**magda3227** · 07-04-2008

I made the changes so that the second function accepts a pointer to a pointer. But how does this change the way I store the values? I'll have to change the notation for incrementing the pointer and what not, won't I? How would I get these values back to the original pointer in the main function?

Sorry if I'm asking too many questions, but no books or sources I can find cover this...

**dwks** · 07-04-2008

anon has explained it rather well. Basically, you pass a pointer-to-pointer to the function that needs to modify the value (i.e., use realloc()). Everywhere in that function that you need to use the pointer, you use *p instead of p. [1]

The changes are automatically reflected in the calling function (main), because by using *p, you're actually modifying the variable in main. Strange, but true.

[1] Note: *p++ doesn't work. You need to use (*p)++ or else *p+=1.

**MacGyver** · 07-04-2008

Example without testing the code:

Code:

#include <stdio.h>
#include <stdlib.h>

void allocdouble(double **d, double v);

void allocdouble(double **d, double v)
{
    *d = malloc(sizeof(**d)); /* Corrected error of allocating the wrong amount */
    if(*d)
    {
        **d = v;
    }
    else *d = NULL;
}

int main(void)
{
    double *d;

    allocdouble(&d, 5.0);
    if(d != NULL)
    {
        printf("*d = &#37;f\n", *d);
        free(d);
    }
    else printf("Allocation failed\n");

    return 0;
}

**anon** · 07-04-2008

It's just the same as with normal variables. If you want the function to modify them so that the modifications are visible to the caller you pass a pointer to the variable.

With the pointers you simply add +1 level of indirection. There are two things that can be modified: the data that the pointer points to, and where the pointer itself points to.

With variables

Code:

#include <stdio.h>

void by_value(int n)
{
    ++n;
}

void by_pointer(int* n)
{
    ++*n;
}

int main(void)
{
    int n = 1;
    printf("&#37;d ", n); /* n == 1 */
    by_value(n);
    printf("%d ", n); /* still n == 1 */
    by_pointer(&n);
    printf("%d\n", n); /* now n == 2 */
    return 0;
}

With pointers

Code:

#include <stdio.h>

void by_value(char* n)
{
    ++n;
}

void by_pointer( char** n)
{
    ++*n;
}

int main(void)
{
    char str[] = "ABC";
    char* p = str;
    printf("%c ", *p); /* points to 'A' */
    by_value(p);
    printf("%c ", *p);  /* still points to 'A' */
    by_pointer(&p);
    printf("%c\n", *p); /* now points to 'B' */
    return 0;
}

**magda3227** · 07-04-2008

Thank you to all who responded.

Your explanations were very helpful, especially the examples you provided. I learn visually, and they really helped me to understand what's going on and the differences between the two types of increments.

Pointers still seem very complex to me, but they're definitely really useful if you can master them.

Thank you once again.

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