Adding integers with pointers in C

Question

Say i have int *a, int *b, int *c and say a and b already point to some integers.

I want to add the integers down a and b and save them to wherever c is pointing to

This:

*c = *a + *b;

does not work. It always spits out "invalid argument of 'unary *'. Why so?

ADDITIONAL INFO: here's how I'm trying to implement it:

int getCoordinates(int argc, char *argv[], FILE *overlay, FILE *base, int *OVx, int *OVy, int *OVendx, int  *OVendy, int *Bx, int *By, int *Bendx, int *Bendy) 
{

     ... // OVx and OVw are assigned here.  I know it works so I won't waste your time with this part.

     // Set overlay image's x and y defaults (0,0).
     *OVx = 0;
     *OVy = 0;
     ...

     OVendx = (*OVx) + (*OVw);
     OVendy = (*OVy) + (*OVh);

It works on my machine. What problem exactly are you having? — Philip Potter
– Philip Potter, Commented Mar 8, 2011 at 0:57
Are you sure you're assigning the values correctly? This example works fine. (Notice you have to dereference the pointer to see the value. You might not be doing this.) — user142162
– user142162, Commented Mar 8, 2011 at 0:58
Have you initialized the pointer c to point to a valid memory location ? — Mahesh
– Mahesh, Commented Mar 8, 2011 at 1:00
It appears you need to dereference your assignment: *OVendx = (*OVx) + (*OVw); Your version is just moving the pointer. — Don Neufeld
– Don Neufeld, Commented Mar 8, 2011 at 1:09
Some of us suspected as much. Again, when posting questions, please don't leave out the critical lines that identify the bug. — Jim Balter
– Jim Balter, Commented Mar 8, 2011 at 1:44

Don Neufeld · Accepted Answer · 2011-03-08 01:13:46Z

2

Here is a working example:

#include <stdio.h>

int main( int argc, const char* argv[] )
{
    int x = 1;
    int y = 2;
    int z = 0;
    int *a = &x;
    int *b = &y;
    int *c = &z;

    *c = *a + *b;

    printf( "%d + %d = %d\n", *a, *b, *c );
    return 1;
}

Running yields:

./a.out 
1 + 2 = 3

Common errors you might have encountered:

Not pointing a, b or c at valid memory. This will result in your program crashing.
Printing the value of the pointer (a) rather than the value it points to (*a). This will result in a very large number being displayed.
Not dereferencing the assignment c = *a + *b rather than *c = *a + *b. In this case, the program will crash when you try to dereference c after the assignment.

edited Mar 8, 2011 at 1:13

answered Mar 8, 2011 at 1:00

Don Neufeld

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Comments

Mahesh · Accepted Answer · 2011-03-08 01:14:23Z

2

If Ovendx, Ovendy are pointing to a valid memory locations, then to assign values to that location, you need to dereference them. So, it should be -

(*OVendx) = (*OVx) + (*OVw);
(*OVendy) = (*OVy) + (*OVh);

You aren't dereferencing in the snippet posted.

answered Mar 8, 2011 at 1:14

Mahesh

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