Re: using "!!" in "c"

"pemo" <usenetmeister@xxxxxxxxx> writes:
[snip]
> !! is a good way of turning a scalar value into 1 or 0.

So is (value, 0) (i.e., a comma operator). It turns a scalar value
into 1 or 0. It just happens to be 0.

!! maps 0 to 0, and any non-zero value to 1, for any scalar operand.

> For example, this
>
> printf("%p\n", (void *)"boo");
>
> will output some value that's not 0, and very unlikely to be 1, e.g.,
> 0040300

It will output some implementation-specific sequence of printing
characters. The sequence may or may not look like a number.

> However, this
>
> printf("%p\n", (void *)!!"boo");
>
> will output 000001, e.g., '1'

That invokes undefined behavior.

"boo" yields a char* value. The "!" operator yields 1 if its operand
compares equal to 0, 0 otherwise. In this case, it compares the char*
value to a null pointer value. Since "boo" cannot be a null pointer,
!"boo" is guaranteed to yield 0. Applying "!" again yields the int
value 1.

So the above is equivalent to

printf("%p\n", (void*)1);

The conversion of the int value 1 to void* may yield a trap
representation; passing this to printf() (or doing anything else with
it) invokes undefined behavior.

> I've seen/used !! in functions that allocate memory, and return 1 if the
> memory was allocated ok, else 0.
>
> A routine like this would look something like ...
>
> void * p;
>
> int allocStuff(size_t n)
> {
> p = malloc(n);
>
> return !!p;
> }

That's fine, since !!p is an int value and the function returns an int.

The thing to remember is that !!foo doesn't just normalize foo to 0 or
1; it yields a value of type int, regardless of the type of foo.

--
Keith Thompson (The_Other_Keith) kst-u@xxxxxxx <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.
.

Relevant Pages

  • Re: Memory address and Pointer
    ... int main ... The 'x' format specifier expects an unsigned int argument. ... The expression '*p' yields a value of type int. ... bar = foo + 10; ...
    (comp.lang.c)
  • Re: [C] strcat() question (ongoing)
    ... > At the machine level, yes, basically (depending on your definition ... We can only "PUT" data in / or assign data to variables such as an int or ... foo is an object? ... > house and look on the microwave. ...
    (alt.comp.lang.learn.c-cpp)
  • Re: call of variadic function
    ... arguments that should be passed to this function are of type int. ... You call foo with more arguments than are ... which is the standard way to access arguments of a variadic function ... Here the else clause of my sentence specifies one of the numerous non- ...
    (comp.lang.c)
  • Re: RISC OS modules with stock gcc?
    ... int bar; ... DCD &ff000004 ... IMPORT bar ... EXPORT foo ...
    (comp.sys.acorn.programmer)
  • Re: call of variadic function
    ... arguments that should be passed to this function are of type int. ... call and the definition of function foo itself does NOT produce any ... which is the standard way to access arguments of a variadic function ... Here the else clause of my sentence specifies one of the numerous non- ...
    (comp.lang.c)