How can I tell gcc to warn (or fail) on switch/case statements without a break?

Question

I have a complicated switch statement, and I forgot to put a break at the end of one of the cases. This is quite legal, and as a result I had a fall-through to the next case.

Is there any way to have gcc warn (or even better, fail) if I neglect to put a break statement?

I realize that there are many valid use cases (and I use them often in my code), as exemplified in this question, so obviously such a warning (or failure) would need a simple waiver so that I could easily say, "I do want to fall-through here."

Is there any way to tell gcc to do this?

Honestly, if that's what you want, I would say just don't use switch/case. The only advantage of switch/case that still remains is that it is easy to get fall through where you want it. — David Schwartz, Oct 09 '11 at 12:06
Hehehe, C# forces you to put the break statement and won't compile if you don't — BlackBear, Oct 09 '11 at 12:18
Just curious, can we see the code? Maybe there's an even better solution that doesn't require a switch. — fredoverflow, Oct 09 '11 at 12:41
@DavidSchwartz: The other advantage is that it's easier on the eye and easier to add new cases. — Nathan Fellman, Oct 09 '11 at 14:01
@FredOverflow: I don't think corporate security would like that idea :-) — Nathan Fellman, Oct 09 '11 at 14:02
I'm going to try writing a checker for this using the ROSE framework: http://rosecompiler.org/ — Jason Dagit, Oct 28 '11 at 01:13
Now although `gcc` support for this feature is still a work in progress, [clang has -Wimplicit-fallthrough](http://stackoverflow.com/a/27965827/1708801) — Shafik Yaghmour, Jan 15 '15 at 15:36

score 6 · Answer 1 · answered Oct 09 '11 at 12:14

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There's a discussion about such a feature (-Wswitch-break) at http://gcc.gnu.org/bugzilla/show_bug.cgi?id=7652. But it doesn't seem to be implemented yet

answered Oct 09 '11 at 12:14

Vsevolod Dyomkin

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Looks like it got in in gcc 7 – smichak Jun 02 '17 at 13:45

score 4 · Answer 2 · answered Jan 25 '12 at 23:36

This check is available in Cppcheck, a free static analyser for C and C++ code. The check is currently marked "experimental", so you will need to use the --experimental command line switch to turn it on.

This check warns against a nonempty case clause that falls through to the next case without a control flow statement such as break, continue, return, etc, unless there is a comment with wording such as // fall through immediately preceding the next case.

You can get an idea for the kinds of constructs this handles by having a look at the switchFallThroughCase test cases in the source code.

I can no longer find this kind of check in cppcheck-1.83 and am not able to enable it with --experimental — Andy, May 09 '18 at 07:33

score 3 · Answer 3 · answered Oct 18 '17 at 08:56

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GCC 7 has a warning enabled with -Wextra or -Wimplicit-fallthrough(=[1-5])?: https://developers.redhat.com/blog/2017/03/10/wimplicit-fallthrough-in-gcc-7/

answered Oct 18 '17 at 08:56

Dan Berindei

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score 2 · Answer 4 · answered Oct 09 '11 at 12:15

I just went through gcc options, and there is none that will at least give you a notice. There are -Wswitch, -Wswitch-default and -Wswitch-enum ( http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#Warning-Options ), but none of them will work for you.

my best bet would be to use 'else if' statements

score 1 · Answer 5 · answered Oct 09 '11 at 12:17

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You could construct a regexp for grep/perl/emacs/etc to find all places where there's no break before case.

answered Oct 09 '11 at 12:17

Alexey Frunze

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score 0 · Answer 6 · answered Oct 09 '11 at 12:23

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Short answer is no, there is no such flag in gcc to do that. Switch case is used for the fall through more often so that is why it does not make sense to have such a flag in gcc.

answered Oct 09 '11 at 12:23

Harman

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I think it makes sense, it just doesn't need to default to on. Seems like it may be appropriate for -Wextra or -pedantic. – Jason Dagit Oct 28 '11 at 01:13

How can I tell gcc to warn (or fail) on switch/case statements without a break?

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