cc -pthread  -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 -lpthread -lrt -lintl -liconv
unistd/fdatasync.c: In function ‘main’:
unistd/fdatasync.c:14:14: error: implicit declaration of function ‘fdatasync’ [-Wimplicit-function-declaration]
   14 |         if ( fdatasync(fileno(fp)) < 0 )
      |              ^~~~~~~~~