(void)fwrite triggers -Wunused-result again

[BillyDonahue]

Regression of #1098 introduced here 211d312

https://github.com/fmtlib/fmt/blob/master/include/fmt/format-inl.h#L158

Unfortunately the (void) cast doesn't actually convince GCC that we've paid attention to the std::fwrite result.

Maybe report_error can return a bool indicating whether the error was reported successfullly, calculated from the fwrite result. We don't have to actually look at that bool, but GCC may shut up if we look like we care.

It's all kind of silly. The std::fputc on the next line has no such annotation but it's just as capable of failing.

 [2021/03/17 13:37:16.683] /opt/mongodbtoolchain/v3/bin/g++ -o build/cached/third_party/fmt/dist/src/format.o -c -std=c++17 -Wall  -Werror  src/third_party/fmt/dist/src/format.cc

 [2021/03/17 13:37:18.244] In file included from src/third_party/fmt/dist/src/format.cc:8:
 [2021/03/17 13:37:18.244] src/third_party/fmt/dist/include/fmt/format-inl.h: In function 'void fmt::v7::detail::report_error(fmt::v7::detail::format_func, int, fmt::v7::string_view)':
 [2021/03/17 13:37:18.244] src/third_party/fmt/dist/include/fmt/format-inl.h:158:20: error: ignoring return value of 'size_t fwrite(const void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Werror=unused-result]
 [2021/03/17 13:37:18.244]    (void)std::fwrite(full_message.data(), full_message.size(), 1, stderr);
 [2021/03/17 13:37:18.244]          ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[vitaut]

What gcc version are you using? I wasn't able to repro this on gcc 9.1: https://godbolt.org/z/5EPcbY.

[BillyDonahue]

It's GCC 8.3, but I think the more important thing is the system's glibc version, as the __wur marking was removed from fwrite etc in glibc 2.16 (Bugzilla#11959). This is a Red Hat Enterprise Linux 6.2 machine, still using glibc 2.12. Godbolt can't select glibc versions, unfortunately.

$ rpm -q glibc
glibc-2.12-1.212.el6_10.3.x86_64

$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.2 (Santiago)

$ uname -a
Linux ip-... 2.6.32-220.el6.x86_64 #1 SMP Wed Nov 9 08:03:13 EST 2011 x86_64 x86_64 x86_64 GNU/Linux

$ /opt/mongodbtoolchain/v3/bin/g++ --version
g++ (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat fwrite_test.cpp
#include <cstdio>
int main() {
    const char msg[] = "Hello, world!\n";
    (void)fwrite(msg, 1, sizeof(msg)-1, stdout);
    return 0;
}

$ /opt/mongodbtoolchain/v3/bin/g++ -O2 -D_FORTIFY_SOURCE=1 fwrite_test.cpp -o fwrite_test
fwrite_test.cpp: In function 'int main()':
fwrite_test.cpp:6:17: warning: ignoring return value of 'size_t fwrite(const void*, size_t, size_t, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
     (void)fwrite(msg, 1, sizeof(msg)-1, stdout);
           ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

least-annoying fix for this that I've seen is to use an if condition to uselessly examine the result.
I've verified that it works.

if (fwrite(msg, 1, sizeof(msg)-1, stdout)){}

[vitaut]

Fixed in 308510e.