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Groups > comp.lang.c > #394652
| From | David Brown <david.brown@hesbynett.no> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: bugprone-switch-missing-default-case |
| Date | 2025-10-22 15:41 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <10damv3$ku01$1@dont-email.me> (permalink) |
| References | <10da67g$3q59f$1@dont-email.me> <10da8bo$hker$1@dont-email.me> <10dag3d$jkju$1@dont-email.me> |
On 22/10/2025 13:44, Richard Harnden wrote:
> On 22/10/2025 10:32, Janis Papanagnou wrote:
>> On 22.10.2025 10:56, pozz wrote:
>>>
>>>> Switch statements without a default case can lead to unexpected
>>>> behavior and incomplete handling of all possible cases. When a switch
>>>> statement lacks a default case, if a value is encountered that does
>>>> not match any of the specified cases, the program will continue
>>>> execution without any defined behavior or handling.
>>>
>>> Maybe I misunderstood that sentence caused by my bad English. I knew
>>> that in case the switch value is not present in any case inside the
>>> switch, the program continues without doing anything (in the switch) and
>>> without any problem.
>>>
>>> int x = 3;
>>> switch(x) {
>>> case 1: printf("Hello");break;
>>> case 2: printf("World");break;
>>> }
>>>
>>> Will the program execution continue without any defined behaviour?
Presumably you meant "without any undefined behaviour" ? The code is
fine - if no cases match and there is no default case, execution
continues from the end of the switch statement. Like most warnings,
this is about a possible bug in the code - not a definite one.
>>
>> Your program fragment is well defined.
>>
>> What the poster certainly tried to express was that in case you
>> haven't implemented a complete list of all possible cases and
>> also not provided a 'default' to catch all non-specified cases,
>> then you might get in troubles with your program, probably by
>> possible oversights, future extensions, new data, and whatnot.
>>
>> Personally I have the habit to always define a default branch,
>> and even if that default is impossible to reach you'll find an
>> error message (like "internal error with unexpected value...")
>> generated at that place.
I don't think it is normally appropriate to add a default case unless
you actually need it - code that exists but can never be reached is
untestable and can be confusing to people reading the code. But
sometimes it can be useful to add a "default : printf("Internal
error...");" for debugging, however.
In some code, I will have "default : __builtin_unreachable();" to
improve code efficiency.
>>
> Use an enum, and the compiler will warn you ...
>
> $ cat x.c
> #include <stdio.h>
>
> enum x {A, B, C};
>
> int main(void)
> {
> enum x x = C;
>
> switch (x)
> {
> case A:
> printf("A\n");
> break;
>
> case B:
> printf("B\n");
> break;
> }
>
> return 0;
> }
>
> $ gcc -Wall x.c
> x.c: In function ‘main’:
> x.c:9:9: warning: enumeration value ‘C’ not handled in switch [-Wswitch]
> 9 | switch (x)
> | ^~~~~~
>
>
gcc has a number of warning options regarding switches :
-Wimplicit-fallthrough=n
Warns when you have fall-through between cases, unless you add a
[[fallthrough]] attribute, "// fallthrough" comment, or the like. It is
enabled by -Wextra.
-Wswitch
Warns when you have a switch on an enumerated type, and are missing an
enumeration value and don't have a default case. This is in -Wall.
-Wswitch-default
Warns when a switch does not have a default case.
-Wswitch-enum
Warns when you have a switch on an enumerated type, and are missing an
enumeration value, even if you have a default case.
I think it is generally a good idea to use enumerated types when you are
representing a number of possible options, rather than a plain "int"
with "magic numbers" or individual #define constants. Then "-Wswitch"
(usually from "-Wall") or "-Wswitch-enum" is typically a good idea.
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bugprone-switch-missing-default-case pozz <pozzugno@gmail.com> - 2025-10-22 10:56 +0200
Re: bugprone-switch-missing-default-case Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-22 11:32 +0200
Re: bugprone-switch-missing-default-case Richard Harnden <richard.nospam@gmail.invalid> - 2025-10-22 12:44 +0100
Re: bugprone-switch-missing-default-case Thiago Adams <thiago.adams@gmail.com> - 2025-10-22 10:05 -0300
Re: bugprone-switch-missing-default-case Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-22 18:22 +0000
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 09:12 +0200
Re: bugprone-switch-missing-default-case Thiago Adams <thiago.adams@gmail.com> - 2025-10-23 08:03 -0300
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 17:06 +0200
Re: bugprone-switch-missing-default-case Thiago Adams <thiago.adams@gmail.com> - 2025-10-23 13:15 -0300
Re: bugprone-switch-missing-default-case bart <bc@freeuk.com> - 2025-10-23 17:46 +0100
Re: bugprone-switch-missing-default-case Thiago Adams <thiago.adams@gmail.com> - 2025-10-23 14:12 -0300
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 22:40 +0200
Re: bugprone-switch-missing-default-case scott@slp53.sl.home (Scott Lurndal) - 2025-10-23 20:51 +0000
Re: bugprone-switch-missing-default-case Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-23 16:21 -0700
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-24 08:59 +0200
Re: bugprone-switch-missing-default-case scott@slp53.sl.home (Scott Lurndal) - 2025-10-24 15:51 +0000
Re: bugprone-switch-missing-default-case tTh <tth@none.invalid> - 2025-10-23 23:57 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-24 09:22 +0200
Re: bugprone-switch-missing-default-case antispam@fricas.org (Waldek Hebisch) - 2025-10-23 23:23 +0000
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-24 09:53 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-22 15:41 +0200
Re: bugprone-switch-missing-default-case Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-22 16:05 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-22 17:23 +0200
Re: bugprone-switch-missing-default-case Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-23 04:54 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 09:30 +0200
Re: bugprone-switch-missing-default-case antispam@fricas.org (Waldek Hebisch) - 2025-10-23 22:40 +0000
Re: bugprone-switch-missing-default-case Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-22 12:41 -0700
Re: bugprone-switch-missing-default-case pozz <pozzugno@gmail.com> - 2025-10-23 08:47 +0200
Re: bugprone-switch-missing-default-case Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-23 16:17 -0700
Re: bugprone-switch-missing-default-case Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-23 19:13 -0700
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 10:44 +0200
Re: bugprone-switch-missing-default-case Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-22 15:56 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-22 17:25 +0200
Re: bugprone-switch-missing-default-case Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2025-10-23 04:39 +0200
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-23 09:08 +0200
Re: bugprone-switch-missing-default-case Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-10-23 16:31 -0700
Re: bugprone-switch-missing-default-case David Brown <david.brown@hesbynett.no> - 2025-10-24 10:09 +0200
Re: bugprone-switch-missing-default-case Kaz Kylheku <643-408-1753@kylheku.com> - 2025-10-22 18:13 +0000
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