Incorrect passing of uint64_t to va_list

Question

I am writing a custom printf function and a uint64_t seems to be passed incorrectly to va_list:

Problematic point:

printf("Number is %C, and the next one is %C", 0xff00ffb7, 0xffffffff);

Part of my printf implementation that produces the incorrect result:

format++;
uint64_t num = va_arg(parameters, uint64_t);

When debugging with gdb the value of num becomes 0xffffffffff00ffb7 rather than 0xff00ffb7 which I expect, and for the next %C num becomes 0. Is this some standard behavior I'm missing or am I doing something wrong?

`0xff00ffb7` is an `int` literal. If you want to pass it as an `uint64_t` then cast it to an `uint64_t`. — dxiv, Jan 04 '21 at 19:24
@dxiv is there a specific reason it won't get upgraded to a `uint64_t`? — frogstair, Jan 04 '21 at 19:29
@frogstair That works the other way around. It has no specific reason to be taken as anything else than an `int` literal, as you wrote it. — dxiv, Jan 04 '21 at 19:31
Possibly relevant is: [better understanding type promotion of variadic parameters in c](https://stackoverflow.com/questions/44421125/better-understanding-type-promotion-of-variadic-parameters-in-c) — Weather Vane, Jan 04 '21 at 19:34
@dxiv `0xff00ffb7` is an `unsigned` constant when `unsigned` is 32-bit. C does not define `int` literal. — chux - Reinstate Monica, Jan 04 '21 at 19:36

chux - Reinstate Monica · Accepted Answer · 2021-01-04T20:36:58.823

If 0xff00ffb7 is meant to be a uint64_t for a variadic function, cast it.

printf("Number is %C", (uint64_t) 0xff00ffb7);

0xff00ffb7, as an integer constant, has the type of int, unsigned, long, unsigned long, long long or , unsigned long long: the first one it "fits". With 32-bit int/unsigned, 0xff00ffb7 would be unsigned and likely the source of OP's trouble.

Appending u to a constant is a good idea to insure it is some unsigned type.

Appending L or LL to a constant is not a good idea to insure a constant is uint64_t. Either might match (LL often will), but these suffixes do not guarantee that type.

Using UINT64_C(0xff00ffb7) forms a constant of type uint_least64_t which is certainly same as uint64_t on machines that have uint64_t.

Incorrect passing of uint64_t to va_list

1 Answers1