普通的旧C：访问返回结构的函数的成员

Question

Please could you help me with a question of "legitimacy".

Assuming that foo() is a function returning a structure, is it officially acceptable to assign a member of the structure directly to a variable, for example

x = foo().member

Both the GNU-C complier and an embedded C compiler (Keil) accept this without any grumbles but is this actually legitimate according to the official C standard or is it just a relaxed attitude of these particular compilers? If it's legit, has it always been legit or is it a recent development?

Here's an example that compiles and runs OK:

typedef struct
{
    int a;
    int b;
} footype;

footype testfoo(void)
{
    footype n;

    n.a = 1;
    n.b = 2;
    return n;
}

int main()
{
    printf("\nTest = %d\n", testfoo().a);
    return 0;
}

Answer 1

是的，这是标准的C语法。

Answer 2

The testfoo function returns a temporary instance of the structure. This instance can be used at once as show in the example, and the temporary instance is valid until the full expression (ie the printf call in your case) is done.

It's equivalent to

{
    footype temp = testfoo();
    printf("\nTest = %d\n", temp.a);
}

Answer 3

It's a standard case of expression evaluation result.
The flow:

testfoo().a

is a standard C expression where:

1. testfoo() result, what the function returns, is in our case a struct footype .
2. Then compiler applies the operator . to the this struct.
3. The dot operator access the field a and creates a new result.

4. The final expression result is used in the printf() . This is always valid.
   Ie:

   int *bar(void); //bar is afunnction that returns an int pointer ... i = bar()[10]; //We access the 10th element pointed by bar()

Answer 4

Out of interest I compiled your example and looked at the assembly generated. First, every compiler can solve this in its own way. I use VC2008.

In your example, the compiler returned the struct in its registers.

I changed the struct to include a large string, so the compiler would run out of registers. It now changed the signature of the function to receive a pointer, and had allocated an instance of the struct on the stack (caller's stackframe), then called the function with a pointer to this struct. The function copied the result to the struct on the stack using this pointer.