Question

i have been trying to figure out ways where i can return more than one value from a function in C.

Say for instance i have the

int compute(int a, int b)
{
int c;
// do some computation on a,b,c
// return a,b,c
}

now i can return these values say in an array, and i was trying to do that and i then i realized that the array needs to be dynamic so it can be referenced in main() as well.

Another way could be that i create a structure and then just return the struct variable.

Is there any simple and robust way of doing this, other than the above work around ? i have to return about 25 different computation values which are independent of each other from a method, and i have a lot structs in my code already.

Thank you.

Answer 1

You can’t easily return more than one variable like, for example, Python with return (a,b,c). However you can pass pointers that are set to the other “return values” or use a struct.

Out-of-place

If you want a function to actually return separate values and not modify its arguments, a simple way is to pass additional pointers (you could also use structs). e.g. this function will compute the sum and difference of a pair of numbers:

int add_sub(int a, int b, int* ret2) {
   if (ret2) *ret2 = a - b;
   return a + b;
}

The if (ret1) allows you to safely pass NULL as the pointer argument, so that you can ignore that return value. You use this like:

int add, sub;
add = add_sub(10, 3, &sub);
// add == 13, sub == 7

int just_add;
just_add = add_sub(15, 5, NULL);
// just_add == 20

(Note, this technique can lead to confusing behaviour if you pass the same argument twice, e.g. a = add_sub(a, b, &b) doesn’t give you what want.)

In-place

If you want to modify the arguments to a function, you can just pass those as pointers. e.g. this function will increment both its arguments and then return their sum:

int increment_and_sum(int* a, int* b) {
     (*a)++;
     (*b)++;
     return a + b
}

Which you use like:

int a = 10, b = 3, sum;
sum = increment_and_sum(&a, &b);
// a == 11, b == 4, sum == 15

(Note that you can’t use literals in increment_and_sum, that is, you can’t do increment_and_sum(3, 4) or increment_and_sum(&3, &4).)