Thread: Odd Pointer Problem

Ok i am trying to return an int from a void pointer, that points to a bitmap file, and this is my function


note: the file is in little endian and the computers we are using are in big endian so this function also converts it to big endian

Code:

int return_int(void *vp)
{
	int num;
	void *big_endian;
	int i;
	vp = vp+3;
	
	for(i = 0; i < 4; i++)
	{
		*((char*)big_endian+i) = *((char*)vp-i);
	}
	vp = vp+1;
	
	num = *((int*)big_endian);
	return num;
}

so i increment vp by 3 so that i am pointing to the last byte in the bitmap int chunck and use big_endian for scratch space
as the counter moves they switch the byte order
then i increment vp once more so that i will be pointing to the first byte of the next int

the switch from little endian to big endian works fine and the number returns fine but for some crazy reason when i call this function twice like so

Code:

data_size = return_int(file);
height = return_int(file);

data_size is 196662, what it is supposed to be
height is also 196662, not what it is supposed to be but the same as data_size

so when i do this

Code:

data_size = return_int(file);
file=file+4;
height = return_int(file);

data_size is 196662, what it is supposed to be
height is also 256, what it is supposed to be

for some insane reason the pointer arrithmetic happening in the function is localized, its been a while since i did pointer arrithmetic but this to be makes no sence to me, help please.

This is nothing to do with pointer arithmetic.

This is how C transferres parameters to the functions - by value. All changes made to the parameter variables inside the function are hidden from the calling function.

If you want that your file pointer will be updated inside the function - you should past the pointer to it into the function. In this case its value can be modified inside the function...

do you mean like this:
int return_int(void **vp);

instead of that i already have:
int return_int(void *vp)