Comment by adrian_b

20 hours ago

The C language does not have any data type that has the property "can't be negative".

Signed integers can be negative. The so-called "unsigned" integers of C are integer residues modulo 2^N, which are neither positive nor negative, i.e. these concepts are not applicable to "unsigned" integers.

An alternative view is that any C "unsigned" is both positive and negative. For example the unsigned short "1" is the same number as "65537" and as "-65535".

So any sizeof value in C is negative (while also being positive).

In contradiction with what you say, the change described in TFA, by making sizes 64-bit signed integers, is the only method to guarantee that the sizes are non-negative in a language that does not have dedicated non-negative integers.

Other programming languages have non-negative integers, but C and C++ and many languages derived from them do not have such integers.

The arithmetic operations with non-negative integers differ from the arithmetic operations of C. On overflows and underflows, they either generate exceptions or have saturating behavior.

4 comments

adrian_b

alberto-m 20 hours ago

> An alternative view is that any C "unsigned" is both positive and negative. For example the unsigned short "1" is the same number as "65537" and as "-65535".

This can be disproven by the fact that dividing by `unsigned e = 1U` is well defined and always yields the starting number. If the unsigned numbers were really modular numbers as you suggest, division could not be defined.

adrian_b 19 hours ago

This does not demonstrate anything. It is just additional evidence that the C standard contains contradictory rules about "unsigned" integers.
The oldest parts of the C language are all consistent with "unsigned" numbers being non-negative integers. The implicit conversions between different sizes of "unsigned", the sizeof operator, the relational operators and division are consistent with non-negative integers.
However the first C standard, instead of defining the correct behavior has left undefined many corner cases of the arithmetic operations, allowing the implementation of "unsigned" as either non-negative integers or integer residues.
Eventually, the undefined behaviors for addition, subtraction and multiplication have been defined to be those of integer residues, not those of non-negative integers.
These contradictory properties are the cause of many confusions and bugs.
In extensible languages, like C++, it is possible to define proper non-negative integers and integer residues and bit strings and to always use those types instead of the built-in "unsigned".
In C, it is better to always use signed numbers and avoid unsigned, by casting unsigned to bigger sizes of signed before using such a value.

marshray 19 hours ago

Are you claiming that the following program could possibly print "-1" ?

    #include <stdio.h>
    int main() {
        unsigned short a = 1;
        long b = a;
        printf("%ld\n", b);
    }

If not, why?

cperciva 20 hours ago

Leaving aside the fact that, yes, unsigned integer types are definitely not negative -- my point wasn't about types at all. Objects cannot take up a negative number of bytes of memory!