Pointers and references

[lilltroll]

So I have updated my memory on pointers in C.

What is the "correct" way to transfer somthing larger than a variable - say an array from

main.xc -> subroutine.c -> main.xc without doing a memory copy ?
Suppose that I would like to calculate the 1000*sin(A), thus I need to use the math.h



main.xc
main(){

int A[4]={1,2,3,4};
??=calcsin(A)

}

calcsin.c
? calcsin(int A[]){

for(k=...
...round(1000*sin(A[k]));

return ?;
}

[Andy]

Do you need to preserve A? If not you can save some space by writing the value back into the A array.

main.xc:

Code: Select all

int main(void)
{
	int A[4] = {1,2,3,4};
	
	calculate(A);
...

caculate.c:

Code: Select all

void calculate(int array[])
{
	int i; 
	
	for (i=0; i < 4; i++) // No bounds checking!! 4 is a magic number
	{
		array[i] = array[i] * 2; // Or whatever operation on the value
	}
}

This is essentially the same as passing a pointer to the A array (but XC doesn't support pointers anyway, so passing arrays is equivalent).

[Andy]

I should point out that the calculate.c function could be written exactly the same in XC. It's also not good practice to operate on an array without doing some sort of bounds check (the size of the array could change). You'd need to pass the size as a parameter to the calculate() function using something like sizeof(A)/sizeof(A[0]) to get the number of elements.

[lilltroll]

Haha I tried with several combinations of & * int* , but not the one without any. Started to try to cast pointers to variables.

Can the arraysize be dynamically changed without using malloc() ?

[paul]

Not in XC. XC is currently designed to be a 'safe' language - hence fixed sized arrays and it will inserts bounds checking by default unless explicitly turn it off (#pragma unsafe arrays) - hence also no pointers to help avoid shared memory etc.

To pass by reference to/from C (i.e. give it a pointer) you can use the &var form. Example below:

C code:

Code: Select all

void swap( int *a, int *b )
{
int temp;

temp = *a;
*a = *b;
*b = temp;
}

C header file (swap.h):

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#ifdef __XC__
void swap( int &a, int &b );
#else
void swap( int *a, int *b );
#endif

XC file:

Code: Select all

#include "swap.h"

// ...

void func()
{
int a, b;

a = 1; b = 2;
swap( a, b);
}

Hope that helps, cheers,

[N8B]

main.xc:

Code: Select all

int main(void)
{
	int A[4] = {1,2,3,4};
	
	calculate(A);
...

caculate.c:

Code: Select all

void calculate(int array[])
{
	int i; 
	
	for (i=0; i < 4; i++) // No bounds checking!! 4 is a magic number
	{
		array[i] = array[i] * 2; // Or whatever operation on the value
	}
}

This is essentially the same as passing a pointer to the A array (but XC doesn't support pointers anyway, so passing arrays is equivalent).

So, in this example code, the entire contents of array A are not getting pushed onto the stack, correct? In other words, when the calculate(A) function call is made in main.xc it is not pushing four longs onto the stack, it is pushing a pointer, right?

[Andy]

So, in this example code, the entire contents of array A are not getting pushed onto the stack, correct? In other words, when the calculate(A) function call is made in main.xc it is not pushing four longs onto the stack, it is pushing a pointer, right?

That's right. The assembly of the calculate() loop looks something like:

ldw r1, r0[r11]
mul r1, r1, r3
stw r1, r0[r11]
add r11, r11, 0x1