clang -O2  -Wall -Wextra -Werror=implicit-function-declaration unistd/getresgid.c -o unistd/getresgid -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
unistd/getresgid.c:11:7: error: implicit declaration of function 'getresgid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( getresgid(&rgid, &egid, &sgid) < 0 )
             ^
unistd/getresgid.c:11:7: note: did you mean 'getegid'?
/usr/include/unistd.h:116:8: note: 'getegid' declared here
gid_t    getegid(void);
         ^
1 error generated.