Propeller C Multiple return

MJF

Can anyone help with multiple returns from a function in C.
I am looking to pass one piece of data into a function and get two pieces of data returned.

Mike Green

What sorts of values do you want to return? Any version of C can only return one value, but you can pack several values into a 32-bit long or 16-bit word and return that. You can also the address of an array to the function and return values by storing them into the array. You can use the dynamic memory allocation routines to provide the address of a small area of memory and return that address from the function ... all sorts of techniques.

MJF

Mike

it would be to integer values a=1234 b=4567.

Mike Green

Given the values you listed, two of them would fit into a 32-bit long. If x and y are those values, you could declare the function as one that returns a long and do a return(x<<16+y). You'd store this returned value in a temporary variable z and then you could write z>>16 to get x and z&0xFFFF to get y. This would work for x and y values from 0 to 32767.

MJF

Thanks Mike,

Don't completely understand , but I will give it a try.

jazzed

A C function can:

1. Return a pointer which could be a pointer to a struct with multiple values.
2. Take a struct parameter and fill multiple values in that.
3. Take multiple pointer parameters and fill values for them.
4. Variations of 1 through 3.

Duane Degn

Another option would be to use a couple global variables. (I'm not sure if that's what jazzed #2 does.)

I haven't used C much lately, but I thought this was something C and Spin had in common.

SRLM

A google search for "C return multiple values" has some good results. From the top of the list: http://stackoverflow.com/questions/2620146/how-do-i-return-multiple-values-from-a-function-in-c

Since Propeller GCC is just C targeted to the Propeller all of the generic C aspects translate. This is one such case.

David Betz

jazzed wrote: »

A C function can:

1. Return a pointer which could be a pointer to a struct with multiple values.
2. Take a struct parameter and fill multiple values in that.
3. Take multiple pointer parameters and fill values for them.
4. Variations of 1 through 3.

You can also return a structure that contains the two values.

This works fine:

#include <stdio.h>

typedef struct {
   int value1;
   int value2;
} ValuePair;

ValuePair foo(int a, int b)
{
    ValuePair result;

    result.value1 = a;
    result.value2 = b;

    return result;
}

int main(void)
{
    ValuePair x;

    x = foo(1, 2);

    printf("value1 %d\n", x.value1);
    printf("value2 %d\n", x.value2);

    return 0;
}

I wouldn't recommend using this technique to return large structures but it is certainly okay for small ones like in this example.

photomankc

David Betz wrote: »

You can also return a structure that contains the two values.
....
I wouldn't recommend using this technique to return large structures but it is certainly okay for small ones like in this example.

That would be my suggestion. Anytime I need to pass around multiple related values a structure pops straight to mind. Mike's packing method will work too and is compact but the structure is more clear in my opinion.

Dave Hein

The easiest method would be to use jazzed's option #3 -- just pass pointers to two variables that will be filled in by the routine. Here's an example of option #3:

#include <stdio.h>

void TestSub(int parm, int *retval1, int *retval2)
{
    *retval1 = parm + 1234;
    *retval2 = parm + 5678;
}

int main(void)
{
    int a, b;

    TestSub(0, &a, &b);
    printf("a = %d, b = %d\n", a, b);
    return 0;
}