GCC 7, -Wimplicit-fallthrough warnings, and portable way to clear them?

We are catching warnings from GCC 7 for implicit fall through in a switch statement. Previously, we cleared them under Clang (that's the reason for the comment seen below):

g++ -DNDEBUG -g2 -O3 -std=c++17 -Wall -Wextra -fPIC -c authenc.cpp
asn.cpp: In member function ‘void EncodedObjectFilter::Put(const byte*, size_t)’:
asn.cpp:359:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
    m_state = BODY;  // fall through
                  ^
asn.cpp:361:3: note: here
   case BODY:
   ^~~~

The GCC manual states to use __attribute__ ((fallthrough)), but its not portable. The manual also states "... it is also possible to add a fallthrough comment to silence the warning", but it only offer FALLTHRU (is this really the only choice?):

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    …
  }

Is there a portable way to clear the fall through warning for both Clang and GCC? If so, then what is it?

GCC expects the marker comment on its own line, like this:

  m_state = BODY;
  // fall through
case BODY:

The marker also has to come right before the case label; there cannot be an intervening closing brace }.

fall through is among the markers recognized by GCC. It's not just FALLTHRU. For a full list, see the documentation of the -Wimplicit-fallthrough option. Also see this posting on the Red Hat Developer blog.

C++17 adds a [[fallthrough]] attribute that can be used to suppress such warnings. Note the trailing semicolon:

  m_state = BODY;
  [[fallthrough]];
case BODY:

Clang supports -Wimplicit-fallthrough warnings, but does not enable them as part of -Wall or -Wextra. Clang does not recognize comment markers, so the attribute-based suppression has to be used for it (which currently means the non-standard __attribute__((fallthrough)) construct for the C front end).

Note that suppressing the warning with marker comments only works if the compiler actually sees the comment. If the preprocessor runs separately, it needs to be instructed to preserve comments, as with the -C option of GCC. For example, to avoid spurious warnings with ccache, you need to specify the -C flag when compiling, or, with recent versions of ccache, use the keep_comments_cpp option.

C++17 [[fallthrough]]

Example:

int main(int argc, char **argv) {
    switch (argc) {
        case 0:
            argc = 1;
            [[fallthrough]];
        case 1:
            argc = 2;
    };
}

Compile with:

g++ -std=c++17 -Wimplicit-fallthrough main.cpp

If you remove the [[fallthrough]];, GCC warns:

main.cpp: In function ‘int main()’:
main.cpp:5:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
             argc = 1;
             ~~^~~
main.cpp:6:9: note: here
         case 1:
         ^~~~

Also note from the example that the warning only happens if you fall beacross two cases: the last case statement (case 1 here) generates no warnings even though it has no break.

The following constructs don't generate the warning either:

#include <cstdlib>

[[noreturn]] void my_noreturn_func() {
    exit(1);
}

int main(int argc, char **argv) {
    // Erm, an actual break
    switch (argc) {
        case 0:
            argc = 1;
            break;
        case 1:
            argc = 2;
    }

    // Return also works.
    switch (argc) {
        case 0:
            argc = 1;
            return 0;
        case 1:
            argc = 2;
    }

    // noreturn functions are also work.
    // https://stackoverflow.com/questions/10538291/what-is-the-point-of-noreturn/47444782#47444782
    switch (argc) {
        case 0:
            argc = 1;
            my_noreturn_func();
        case 1:
            argc = 2;
    }

    // Empty case synonyms are fine.
    switch (argc) {
        case 0:
        case 1:
            argc = 2;
    }

    // Magic comment mentioned at:
    // https://stackoverflow.com/a/45137452/895245
    switch (argc) {
        case 0:
            argc = 1;
            // fall through
        case 1:
            argc = 2;
    }

    switch (argc) {
        // GCC extension for pre C++17.
        case 0:
            argc = 1;
            __attribute__ ((fallthrough));
        case 1:
            argc = 2;
    }

    switch (argc) {
        // GCC examines all braches.
        case 0:
            if (argv[0][0] == 'm') {
                [[fallthrough]];
            } else {
                return 0;
            }
        case 1:
            argc = 2;
    }
}

We can see from the last one that GCC examines all possible branches, and warns if any of them don't have [[fallthrough]]; or break or return.

You might also want to check for feature availability with macros as in this GEM5 inspired snippet:

#if defined __has_cpp_attribute
    #if __has_cpp_attribute(fallthrough)
        #define MY_FALLTHROUGH [[fallthrough]]
    #else
        #define MY_FALLTHROUGH
    #endif
#else
    #define MY_FALLTHROUGH
#endif

See also: https://en.cppreference.com/w/cpp/language/attributes/fallthrough

Tested on GCC 7.4.0, Ubuntu 18.04.

See also

C version of this question: https://stackoverflow.com/questions/44511436/how-to-do-an-explicit-fall-through-in-c

Another example: The Linux kernel provides a fallthrough pseudo-keyword macro. Which can be used as:

switch (cond) {
case 1:
    foo();
    fallthrough;
case 2:
    bar();
    break;
default:
    baz();
}

In kernel v5.10 its implemented like this:

#if __has_attribute(__fallthrough__)
# define fallthrough                    __attribute__((__fallthrough__))
#else
# define fallthrough                    do {} while (0)  /* fallthrough */
#endif

Clean C solution:

int r(int a) {
    switch(a) {
    case 0:
        a += 3;
    case 1:
        a += 2;
    default:
        a += a;
    }
    return a;
}

becomes:

int h(int a) {
    switch(a) {
    case 0:
        a += 3;
        goto one;
    case 1:
    one:
        a += 2;
        goto others;
    default:
    others:
        a += a;
    }
    return a;
}

EDIT: Moved the labels after case statements, as suggested by Stéphane Gourichon in comments, to see the fallthrough more easily.

Nobody mentioned disabling the warning altogether and this might not be the answer the OP was looking for but I think it should be included for completeness since it also works for both compilers:

-Wno-implicit-fallthrough

If for some reason you are not able to change the source code, this keeps the compile output clean allowing for a clear view on other problems (but of course one has to be aware of what one loses).