cc -pthread  -Wall -Wextra -Werror -Wno-error=deprecated -Wno-error=deprecated-declarations unistd/fdatasync.c -o unistd/fdatasync -D_POSIX_C_SOURCE=202405L  -lm -lpthread  -liconv
unistd/fdatasync.c:6:19: error: use of undeclared identifier 'fdatasync'
    6 | int (*foo)(int) = fdatasync;
      |                   ^
1 error generated.