Деление int на size_t Опции

[sonycman]

Возникла непонятка.
Использую деление переменной типа int на значение, возвращаемое функцией sizeof():

int t;
...
t = t / sizeof(buf);
if (t < 0) ...
else ...

Так вот - знак t может быть различный, но после деления на sizeof() компилятор считает, что знак результата становится положительный и отбрасывает ветку кода для обработки отрицательного значения. Даже функция деления используется беззнаковая.
Если вместо sizeof() подставить константу, то всё начинает работать как положено.

Что я делаю не так, подскажите?

[Genadi Zawidowsk...]

Ну написано же в стандарте явно про беззнаковость size_t...

3.3.3.4 The sizeof operator

Constraints

The sizeof operator shall not be applied to an expression that has
function type or an incomplete type, to the parenthesized name of such
a type, or to an lvalue that designates a bit-field object.

Semantics

The sizeof operator yields the size (in bytes) of its operand,
which may be an expression or the parenthesized name of a type. The
size is determined from the type of the operand, which is not itself
evaluated. The result is an integer constant.

When applied to an operand that has type char , unsigned char , or
signed char , (or a qualified version thereof) the result is 1. When
applied to an operand that has array type, the result is the total
number of bytes in the array./35/ When applied to an operand that has
structure or union type, the result is the total number of bytes in
such an object, including internal and trailing padding.

The value of the result is implementation-defined, and its type (an
unsigned integral type) is size_t defined in the <stddef.h> header.

И про преобразование перед делением:

3.2.1.5 Usual arithmetic conversions

Many binary operators that expect operands of arithmetic type cause
conversions and yield result types in a similar way. The purpose is
to yield a common type, which is also the type of the result. This
pattern is called the usual arithmetic conversions: First, if either
operand has type long double, the other operand is converted to long
double . Otherwise, if either operand has type double, the other
operand is converted to double. Otherwise, if either operand has
type float, the other operand is converted to float. Otherwise, the
integral promotions are performed on both operands. Then the
following rules are applied: If either operand has type unsigned long
int, the other operand is converted to unsigned long int.
Otherwise, if one operand has type long int and the other has type
unsigned int, if a long int can represent all values of an unsigned
int, the operand of type unsigned int is converted to long int ; if a
long int cannot represent all the values of an unsigned int, both
operands are converted to unsigned long int. Otherwise, if either
operand has type long int, the other operand is converted to long int.
Otherwise, if either operand has type unsigned int, the other
operand is converted to unsigned int. Otherwise, both operands have
type int.

The values of operands and of the results of expressions may be
represented in greater precision and range than that required by the
type; the types are not changed thereby.

Сообщение отредактировал Genadi Zawidowski - Jan 5 2014, 00:21