Undefined/Unspecified/Implementation-defined behaviour warnings?

Question

Can't a compiler warn (even better if it throws errors) when it notices a statement with undefined/unspecified/implementation-defined behaviour?

Probably to flag a statement as error, the standard should say so, but it can warn the coder at least. Is there any technical difficulties in implementing such an option? Or is it merely impossible?

Reason I got this question is, in statements like a[i] = ++i; won't it be knowing that the code is trying to reference a variable and modifying it in the same statement, before a sequence point is reached.

Answer 1

It all boils down to

• Quality of Implementation: the more accurate and useful the warnings are, the better it is. A compiler that always printed: "This program may or may not invoke undefined behavior" for every program, and then compiled it, is pretty useless, but is standards-compliant. Thankfully, no one writes compilers such as these :-).

• Ease of determination: a compiler may not be easily able to determine undefined behavior, unspecified behavior, or implementation-defined behavior. Let's say you have a call stack that's 5 levels deep, with a const char * argument being passed from the top-level, to the last function in the chain, and the last function calls printf() with that const char * as the first argument. Do you want the compiler to check that const char * to make sure it is correct? (Assuming that the first function uses a literal string for that value.) How about when the const char * is read from a file, but you know that the file will always contain valid format specifier for the values being printed?

• Success rate: A compiler may be able to detect many constructs that may or may not be undefined, unspecified, etc.; but with a very low "success rate". In that case, the user doesn't want to see a lot of "may be undefined" messages—too many spurious warning messages may hide real warning messages, or prompt a user to compile at "low-warning" setting. That is bad.

For your particular example, gcc gives a warning about "may be undefined". It even warns for printf() format mismatch.

But if your hope is for a compiler that issues a diagnostic for all undefined/unspecified cases, it is not clear if that should/can work.

Let's say you have the following:

#include <stdio.h>
void add_to(int *a, int *b)
{
    *a = ++*b;
}

int main(void)
{
    int i = 42;
    add_to(&i, &i); /* bad */
    printf("%d\n", i);
    return 0;
}

Should the compiler warn you about *a = ++*b; line?

As gf says in the comments, a compiler cannot check across translation units for undefined behavior. Classic example is declaring a variable as a pointer in one file, and defining it as an array in another, see comp.lang.c FAQ 6.1.