How to do an explicit fall-through in C

Question

The newer versions of gcc offer the Wimplicit-fallthrough, which is great to have for most switch statements. However, I have one switch statement where I want to allow fall throughs from all case-statements.

Is there a way to do an explicit fall through? I'd prefer to avoid having to compile with Wno-implicit-fallthrough for this file.

EDIT: I'm looking for a way to make the fall through explicit (if it's possible), not to turn off the warning via a compiler switch or pragma.

Perhaps [this](https://stackoverflow.com/a/3394268/1553090) is what you want. — paddy, Jun 13 '17 at 02:29
I found the same one. https://stackoverflow.com/questions/3378560/how-to-disable-gcc-warnings-for-a-few-lines-of-code — Retired Ninja, Jun 13 '17 at 02:31
Check out the description at [`-Wimplicit-fallthrough`](https://gcc.gnu.org/onlinedocs/gcc-7.1.0/gcc/Warning-Options.html#index-Wimplicit-fallthrough) in the GCC documentation? There won't be any non-extension mechanism — the whole issue is all extension. — Jonathan Leffler, Jun 13 '17 at 04:54

Sergey Kalinichenko · Accepted Answer · 2021-04-22T20:13:22.170

22

Use __attribute__ ((fallthrough))

switch (condition) {
    case 1:
        printf("1 ");
        __attribute__ ((fallthrough));
    case 2:
        printf("2 ");
        __attribute__ ((fallthrough));
    case 3:
        printf("3\n");
        break;
}

edited Apr 22 '21 at 20:13

answered Jun 13 '17 at 02:32

Sergey Kalinichenko

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Also note that the above is gcc-specific. For C++ there is also a standardized way to write it since C++17: [[fallthrough]]; – lwho Jul 10 '18 at 09:01

Ciro Santilli Путлер Капут 六四事 · Answer 2 · 2020-05-07T12:08:25.063

7

GCC fallghrough magic comments

You should not use this if you can help it, it is insane, but good to know about:

int main(int argc, char **argv) {
    (void)argv;
    switch (argc) {
        case 0:
            argc = 1;
            // fall through
        case 1:
            argc = 2;
    };
}

prevents the warning on GCC 7.4.0 with:

gcc -Wall -Wextra main.c

man gcc describes how different comments may or not be recognized depending on the value of:

-Wimplicit-fallthrough=n

C++17 [[fallthrough]] attribute

C++17 got a standardized syntax for this: GCC 7, -Wimplicit-fallthrough warnings, and portable way to clear them?

edited May 07 '20 at 12:08

answered Aug 05 '19 at 12:46

Ciro Santilli Путлер Капут 六四事

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Thanks for the magic comments. It actually worked.My code needs to be backward compatible with c++03 also and \_\_attribute__ ((fallthrough)) didnt work. The comments worked like magic :) – Rahul Sreeram May 07 '20 at 12:01

score 3 · Answer 3 · answered Jun 13 '17 at 02:28

3

You should be able to use GCC diagnostic pragmas to disable that particular warning for your source file or some portion of a source file. Try putting this at the top of your file:

#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"

answered Jun 13 '17 at 02:28

David Grayson

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How to do an explicit fall-through in C

3 Answers3

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