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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: bugprone-switch-missing-default-case
Date: Wed, 22 Oct 2025 12:41:49 -0700
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David Brown <david.brown@hesbynett.no> writes:
> 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.

No, pozz meant "without any defined behavior", which is a direct quote
from the cited document.

That document is poorly written.  The phrase "without any defined
behavior" strongly implies that the behavior is undefined, which is
simply wrong.

It says:

    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.

It would be more accurate to say:

    When a switch statement lacks a default case, if a value is
    encountered that does not match any of the specified cases, the
    switch statement will do nothing and the program will continue
    execution without handling the value.

A warning might be warranted, but the behavior is well defined.

Note that the documentation is for the add-on tool clang-tidy, not for
the clang compiler.

I've submitted a bug report :

https://github.com/llvm/llvm-project/issues/164699

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */