clang -O2  -Wall -Wextra -Werror=implicit-function-declaration unistd/fdatasync.c -o unistd/fdatasync -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
unistd/fdatasync.c:14:7: error: implicit declaration of function 'fdatasync' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( fdatasync(fileno(fp)) < 0 )
             ^
1 error generated.