C 中的指针和转换

Question

I've started learning pointers this time. Im trying to read bytes from this array. Task is almost done but CLang keep warns me with warning I don't understand. Here's my code. Warning says : " Function call argument is an uninitialized value"

int main(void)
{
    int tab[] = {67305985,134678021,202050057};
    int *pp=0;
    pp=tab;

    char *wsk=(char*)pp;

    for (int i = 0; i < 12; i++)
    {
        if((wsk+i)!=(void*)NULL)
            printf("%d ",*(wsk+i));    // warning on this line
        else
            return 0;
    }

}

Answer 1

These warnings are from Clang Static Analyzer (or whatever they call it these days).

It looks like a false positive to me, if we assume that int is at least 4 bytes and the real code has #include <stdio.h> and no other changes.

If you're using the latest version of the analyzer, you could file a clang bug report. Well -- you could if they allowed people who don't already have accounts to file bug reports. Maybe someone else reading this thread can do it.

Note: it would help the question to post exactly which version of the analyzer you are running (this may be different to the compiler you're using to build -- some IDEs use different compilers for building than for these inline messages).

Answer 2

This if statement

if((wsk+i)!=(void*)NULL)

does not make sense. The macro NULL is already a null pointer constant. So there is no sense to cast it to void * .

And the pointer wsk+i can not be equal to NULL in this loop because initially it points to an object.

Just remove the if statement and output each character in the loop.

And it is a bad idea to use magic numbers like 12 used in the loop.

You could write for example

const size_t N = sizeof( tab ) / sizeof( *tab );

and then in the loop

for ( size_t i = 0; i < N * sizeof( int ); i++ )
//...

As for the warning then it is irrelative to the presented code provided that you included the header <stdio.h> .

Answer 3

wsk will never be null, since you've set it to be the same as tab (arrays, in C/C++ are just pointers to a block of data. The warning is telling you that no matter how much you add to wsk it will always be non-null. What you need to do is limit your iterations in the loop to the number of valid items in the array, since C/C++ arrays have no terminators of any kind.

The printf might just be confusing CLang. Try printf("%d ", wsk[i]);