I came up with this issue whilst practicing assignment in C. When I try to initialize any variable with it's name(identifier), which doesn't even exist, is not throwing any kind of error.

int x = x;

As far as I know associativity of assignment operator is right to left. So the following code should throw an error whilst i'm initializing a variable with an rvalue which doesn't even exist. Rather, it's assigning some kind of garbage value to it. Why is this happening?

CodePudding user response：

Setting value to be itself is undefined behavior in c standard. After appending compiler option -Winit-self and warning occurs.

CodePudding user response：

GCC 11.2 does diagnose this if you use -Wuninitialized -Winit-self.

I suspect int x = x; may have been used as an idiom¹ for “I know what I am doing; do not warn me about x being uninitialized,” and so it was excluded from -Wuninitialized, but a separate warning was provided with -Winit-self.

Note that while the behavior of int x = x; is not defined by the C standard (if it is inside a function and the address of x is not taken), neither does it violate any constraints of the C standard. This means a compiler is not required to diagnose the problem. Choosing to issue a warning message is a matter of choices and quality of the C implementation rather than rules of the C standard.

Apple Clang 11.0 does not warn for int x = x; even with -Wuninitialized -Winit-self. I suspect this is a bug (a deviation from what the authors would have wanted, if not from the rules of the C standard), but perhaps there was some reason for it.

Consider code such as:

int FirstIteration = 1;
int x;
for (int i = 0; i < N;   i)
{
    if (FirstIteration)
        x = 4;
    x = foo(x);
    FirstIteration = 0;
}

A compiler might observe that x = 4; is inside an if, and therefore reason that x might not be initialized in foo(x). The compiler might be designed to issue a warning message in such cases and be unable to reason that the use of FirstIteration guarantees that x is always initialized before being used in foo(x). Taking int x = x; as an assertion from the author that they have deliberately designed the code this way gives them a way to suppress the spurious warning.

Footnote

¹ There are several idioms which are used to tell a compiler the author is deliberately using some construction which is often an error. For example, if (x = 3) … is completely defined C code that sets x to 3 and always evaluates as true for the if, but it is usually a mistake as x == 3 was intended. The code if ((x = 3)) … is identical in C semantics, but the presence of the extra parentheses is an idiom used to say “I used assignment deliberately,” and so a compiler may warn for if (x = 3) … but not for if ((x = 3)) ….