Thread: Accessing variables with memory address

Now I want to call a function pointer using the same method I used on the int and float, here's the relevant code:

Code:

void printer()
{
	printf("in printer\n");
}
...
void *test_funcptr = malloc(8);
*(int*)((int*)test_funcptr+1) = (int)&printer;
*((*what_to_put_here)()((int*)test+1))

I don't know what to put at the what_to_put_here on the last line.

Could you explain how I could use it?

Can I use that even though I ditched the structs? Regarding the function pointer problem, I have this code

*(*printer)()((int*)test+1);

where printer refers to a function that takes no arguments and this is the error message I get when I compile

error C2064: term does not evaluate to a function taking 1 arguments

Any ideas?

((int*)test+1) gets the memory address where the function pointer is stored

EDIT:

BTW I'm trying to call printer.

After some quick changes it does. New code

(*((int*)test+1))();

Now I get

error C2064: term does not evaluate to a function taking 0 arguments

Closer, I still don't get what's up with it though

test is an int*. Dereferencing it gives, not a pointer to a function, but an int.

Edit: trying to process the bits and pieces of your code, I think you're trying to do something like this:

Code:

void print_int(void * int_ptr) { 
    printf("%d\n", *((int *)int_ptr);
}
void print_float(void *float_ptr) { ... }
.
.
.
void (*printer)(void*);
printer = print_int;
printer(int_ptr);
printer = print_float;
printer(float_ptr);

but I don't think any of us are very sure about what you want.

Here's the whole code with the relevant code in red

Code:

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

void printer()
{
	printf("in printer\n");
}

int main()
{
	int test;
	void *test_int = malloc(8);
	void *test_float = malloc(8);
	void *test_funcptr = malloc(8);

	*(int*)test_int = 0;
	*(int*)((int*)test_int+1) = 1;
	*(int*)test_float = 1;
	*(float*)((int*)test_float+1) = 2;
	*(int*)test_funcptr = 2;
	*(int*)((int*)test_funcptr+1) = (int)&printer;

	test = (int)test_funcptr;

	printf("trying to call printer from test_funcptr\n");
	(*((int*)test+1))();

	printf("assigning test_int to test\n\n");
	test = (int)test_int;
	printf("the value of test = %i, the type of test = %i\n", *((int*)test+1), *(int*)test);
	printf("adding 762 to test\n");
	*((int*)test+1) += 762;
	printf("the value of test = %i, the type of test = %i\n", *((int*)test+1), *((int*)test));
	printf("\nassigning test_float to test\n\n");
	test = (int)test_float;
	printf("the value of test = %f, the type of test = %i\n", *(float*)((int*)test+1), *((int*)test));
	printf("adding 1.234 to test\n");
	*(float*)((int*)test+1) += 1.234;
	printf("the value of test = %f, the type of test = %i\n\n", *(float*)((int*)test+1), *((int*)test));

	free(test_int);
	free(test_float);
	free(test_funcptr);
	return 0;
}

test holds the memory address of the function pointer as an int. I want to call that function using only test and casting it to the function pointer type.

Notice the (int*) in ((int*)test+1) casts test to an int to allow the +1 to move the memory address down 32 bits. That (int*) part is just to get the right memory address. Once I get that memory address I need to dereference it and cast to the function pointer right? That's where I'm stuck.

It works! Working code snippet without the typedef ((void(*)())*((int*)test+1))(); thank you!