Re: Quick check on signed promotion of bytes

Eric Sosman:

1) x is promoted to signed int.
2) The complement is take of this signed int.
3) This signed int is then converted to an unsigned char

That's right, under your assumptions. On "exotic" machines
where UCHAR_MAX > INT_MAX (for example, on hardware where all
of char, short, and int are 16 bits wide), then x promotes to
an unsigned int instead of to an int in step 1.


So just to confirm if I'm thinking right. We start off with:

char unsigned x, y;

x = 0xf0;

y = ~x;

What we _want_ this code to do is to give us the value 0x0f for y (i.e.
the complement of x). However what _could_ happen is:

1) x is promoted to signed int.

=> Now we have a signed int with the value 0xf0

2) The complement is taken of this signed int.

=> Now we have some other number that is negative. The exact number
depends on the number system in use (e.g. sign-magnitude), and
also the amount of bits in an int.

3) The signed int is converted to an unsigned char.

=> This process is well-defined, but we don't know what signed
value we have.

Therefore, it is my assumption that will not give the desired behaviour
on every implementation of the Standard -- we could get something
totally different from 0x0f as an answer (even on an 8-bit-byte system).

That sound right?

--
Tomás Ó hÉilidhe
.

Relevant Pages

  • Re: Printf question.
    ... >> Correction: since a signed int can hold any value of type unsigned ... the value in the unsigned char 'a' is promoted to type int. ... The whole language does begin to break down when we push the limits of ...
    (comp.lang.c)
  • Re: ones complement of unsigned char
    ... Does it mean that since the operand is unsigned, the promoted type is ... x is promoted to unsigned int before applying operator ~. ... 'x' is promoted to signed int, ... into on this platform can hold all values of unsigned char. ...
    (comp.lang.c)
  • Re: Quick check on signed promotion of bytes
    ... Tomás Ó hÉilidhe wrote: ... The complement is take of this signed int. ... What we _want_ this code to do is to give us the value 0x0f for y (i.e. the complement of x). ... The signed int is converted to an unsigned char. ...
    (comp.lang.c)
  • Re: Printf question.
    ... >> In the case of integer types of lesser rank than int, ... >> promotions" occur. ... Since a signed int can hold the value 200, ... the value in the unsigned char 'a' is promoted to type int. ...
    (comp.lang.c)
  • Re: Quick check on signed promotion of bytes
    ... The complement is take of this signed int. ... This signed int is then converted to an unsigned char ...
    (comp.lang.c)