Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #394600

Re: signed vs unsigned and gcc -Wsign-conversion

Show all headers | View raw

On 2025-10-20, David Brown <david.brown@hesbynett.no> wrote:
> On 20/10/2025 17:03, pozz wrote:
>> After many years programming in C language, I'm always unsure if it is 
>> safer to use signed int or unsigned int.
>> 
>> Of course there are situations where signed or unsigned is clearly 
>> better. For example, if the values could assume negative values, signed 
>> int is the only solution. If you are manipulating single bits (&, |, ^, 
>> <<, >>), unsigned ints are your friends.
>> 
>> What about other situations? For example, what do you use for the "i" 
>> loop variable?
>> 
>> I recently activated gcc -Wsign-conversion option on a codebase and 
>> received a lot of warnings. I started to fix them, usually expliciting 
>> casting. Is it the way or is it better to avoid the warning from the 
>> beginning, choosing the right signed or unsigned type?
>> 
>> 
>
> Signed and unsigned types are equally safe.  If you are sure you are 
> within the ranges you know will work for the types you use, your code is 
> safe.  If you are not sure, you are unsafe. 

Safe generally means that the language somehow protects from harm, not
that you protect yourself.

Correct code operating on correct inputs, using unsafe constructs,
is still called unsafe code.

However using unsigned types due to them being safe is often poorly
considered because if something goes wrong contrary to the programmer's
intent, there likely will be undefined behavior somewhere.

E.g. an array underflow using an unsigned index will not produce
integer overlow undefined behavior, but the access will go out of
bounds, which is undefined behavior.

There are bugs which play out without any undefined behavior:
the program calculates something contrary to its requirements,
but stays within the confines of the defined language.

The odds that by using unsigned numbers you will get only that type of
bug are low, and even if so, it is not a big comfort.

Signed numbers behave more like mathematical integers, in cases
when there is no overflow.

If a, b and c are small, non-negative quantities, you might be tempted
to make them unsigned. But if you do so, then you can no longer make
this derivation of inequalities:

  a + b > c
      
      c > a - b

Under the unsigned types, we cannot add -b to both sides of the
inequality, preserving its truth value, even if all the operands
are tiny numbers that fit into a single decimal digit!

If b happens to be greater than a, we get a huge value on the right
side that is now larger than c, not smaller.

Gratuitous use of unsigned types impairs our ability to
algebra to simplify code, due to the "cliff" at zero.

This is a nuanced topic where there isn't a one-type-fits-all answer,
but I gravitate toward signed; use of unsigned has to be justified in
some way.

When sizes are being calculated and they come from functions or
operators that produce size_t, then that tends to dictate unsigned.

If the quantities are large and can possibly overflow, there are
situations in which unsigned makes that simpler.

For instance if a and b are unsigned such that a + b can semantically
overflow (i.e. the result of the natural addition of a + b  doesn't 
fit into the type). It is simpler to detect: you can just do the
addition, and then test:

  c = a + b;

when there is no overflow, it must be that (c >= a && c >= b)
so if either (c < a) or (c < b) is true, it overflowed.

This is significantly less verbose than a correct overflow test
for signed addition, which has to avoid doing the actual addition,
and has to be split into three cases: a and b have opposite
sign (always okay), a and b are both positive, and a and b are
both negative.

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar

Thread

signed vs unsigned and gcc -Wsign-conversion pozz <pozzugno@gmail.com> - 2025-10-20 17:03 +0200
  Re: signed vs unsigned and gcc -Wsign-conversion Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-20 17:38 +0200
  Re: signed vs unsigned and gcc -Wsign-conversion Michael S <already5chosen@yahoo.com> - 2025-10-20 19:43 +0300
    Re: signed vs unsigned and gcc -Wsign-conversion Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-20 19:07 +0200
    Re: signed vs unsigned and gcc -Wsign-conversion scott@slp53.sl.home (Scott Lurndal) - 2025-10-20 18:01 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-21 04:27 +0200
      Re: signed vs unsigned and gcc -Wsign-conversion David Brown <david.brown@hesbynett.no> - 2025-10-21 09:13 +0200
    Re: signed vs unsigned and gcc -Wsign-conversion BGB <cr88192@gmail.com> - 2025-10-20 17:44 -0500
    Re: signed vs unsigned and gcc -Wsign-conversion Lawrence D’Oliveiro <ldo@nz.invalid> - 2025-10-20 23:36 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-20 23:52 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-20 16:58 -0700
  Re: signed vs unsigned and gcc -Wsign-conversion David Brown <david.brown@hesbynett.no> - 2025-10-20 20:03 +0200
    Re: signed vs unsigned and gcc -Wsign-conversion Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-20 20:09 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion rbowman <bowman@montana.com> - 2025-10-21 01:43 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion David Brown <david.brown@hesbynett.no> - 2025-10-21 12:42 +0200
        Re: signed vs unsigned and gcc -Wsign-conversion James Kuyper <jameskuyper@alumni.caltech.edu> - 2025-10-21 14:44 -0400
          Re: signed vs unsigned and gcc -Wsign-conversion David Brown <david.brown@hesbynett.no> - 2025-10-21 22:56 +0200
  Re: signed vs unsigned and gcc -Wsign-conversion Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-20 14:48 -0700
    Re: signed vs unsigned and gcc -Wsign-conversion Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-20 17:13 -0700
      Re: signed vs unsigned and gcc -Wsign-conversion Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-21 01:45 +0000
        Re: signed vs unsigned and gcc -Wsign-conversion antispam@fricas.org (Waldek Hebisch) - 2025-10-21 03:52 +0000
  Re: signed vs unsigned and gcc -Wsign-conversion Lawrence D’Oliveiro <ldo@nz.invalid> - 2025-10-20 23:35 +0000
    Re: signed vs unsigned and gcc -Wsign-conversion Lawrence D’Oliveiro <ldo@nz.invalid> - 2025-10-20 23:38 +0000
      Re: signed vs unsigned and gcc -Wsign-conversion Bonita Montero <Bonita.Montero@gmail.com> - 2025-10-21 09:57 +0200
        Re: signed vs unsigned and gcc -Wsign-conversion Lawrence D’Oliveiro <ldo@nz.invalid> - 2025-10-21 19:45 +0000
  Re: signed vs unsigned and gcc -Wsign-conversion antispam@fricas.org (Waldek Hebisch) - 2025-10-21 04:42 +0000
  Re: signed vs unsigned and gcc -Wsign-conversion Tim Rentsch <tr.17687@z991.linuxsc.com> - 2025-12-15 00:25 -0800

csiph-web