Why and how did the program give the output?

#include <stdio.h>
int sum(int a ,int b , int c ){
    return a , b ,c;
}
int main() {
    int a=10, b=100 , c=1000 ;
    int x = sum(a , b ,c);
    printf("%d %d %d" ,x);
    
    return 0;
}

Output : 1000 1000 100

CodePudding user response：

Wow. This is a spectacularly misleading result.

No, you can not return multiple values like this in C. A function can have at most one single return value. You can't use a comma to return multiple values like this.

But, if you can't do it, then why did it seem to work? That's a rather complicated question, that it probably won't be possible to completely answer here.

Your program has two, completely unrelated problems. The first problem, which isn't too serious, is the line

return a, b, c;

at the end of function sum. In English, this line basically says, "Take the value a and throw it away, then take the value b and throw it away, then take the value c and have it be the actual return value."

And then the second, much more serious problem is the line

printf("%d %d %d", x);

in main. Your compiler should have warned you about this problem; mine says warning: more '%' conversions than data arguments. You're telling printf, "I want you to print three int values". And then you're giving it one int value, x, to print. So printf is blindly reaching for the other two values, and getting... what?

By a reasonably incredible coincidence, when printf reaches out to fetch the values you didn't pass, it is somehow grabbing... one of the values that got thrown away by the poorly-written return statement back in function sum. I'm not even sure how this happened, although when I tried your program on my computer, I got a similar result.

This may be too obvious to mention, but: this result is not guaranteed, it's not something to depend on, and it's not something to learn from. If you want to return multiple values from a function, there are other ways to, as this question's other answers have suggested.

CodePudding user response：

Short answer: no, you cannot return multiple values (at least, not as you did). Good news, you do have some ways to return multiple values from a function:

Method 1: use a struct

typedef struct result {
  int a;
  int b;
  int c;
} result_t;

result_t foo(int a, int b, int c) {
  return (result_t) { a   1, b   1, c   1 };
}

int main(void) {
  result_t res = foo(1, 2, 3);
  printf("res.a is %d\n", res.a); // OUTPUT: res.a is 2
  return 0;
}

Method 2: use pointers as function arguments

void some_calc(int input, int* output1, int* output2) {
  *output1 = input * 2;
  *output2 = input / 2;
}

int main(void) {
  int res1, res2;
  some_calc(50, &res1, &res2);
  printf("50 * 2 = %d and 50 / 2 = %d\n", res1, res2);
  return 0;
}

CodePudding user response：

This statement

printf("%d %d %d" ,x);

invokes undefined behavior because the number of arguments is less than the number of conversion specifiers.

In the return statement of the function

int sum(int a ,int b , int c ){
    return a , b ,c;
}

there is used the comma operator. The value of an expression with the comma operator is the value of the right-most operand. So in fact you have

int sum(int a ,int b , int c ){
    return c;
}

From the C Standard (6.5.17 Comma operator)

2 The left operand of a comma operator is evaluated as a void expression; there is a sequence point between its evaluation and that of the right operand. Then the right operand is evaluated; the result has its type and value.

The function should look like

int sum(int a ,int b , int c ){
    return a   b   c;
}

of if to avoid a possible overflow then

long long int sum(int a ,int b , int c ){
    return ( long long int )a   b   c;
}

and in main you have tp write

printf("%lld\n", x );

If you want to return several objects from a function then a simplest way is declare a structure that contain the number of data members equal to the number of objects that you are going to return

For example

struct tuple
{
    int a;
    int b;
    int c;
};

struct tuple f( int a, int b, int c );

CodePudding user response：

One way to achieve with structs,

#include <stdio.h>

    struct Values
    {
        int a;
        int b;
        int c;
    } values;
    
    
    struct Values sum(int a ,int b , int c ){
        return (struct Values){ a, b, c};
    }
    
    int main() {
        int a=10, b=100 , c=1000 ;
        struct Values x = sum(a , b ,c);
        printf("%d %d %d\n" ,x.a, x.b, x.c);
        return 0;
    }