cc -O2 -pipe  -pthread  -Wall -Wextra -Werror=implicit-function-declaration unistd/posix_close.c -o unistd/posix_close -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread 
unistd/posix_close.c:9:7: error: call to undeclared function 'posix_close'; ISO C99 and later do not support implicit function declarations [-Werror,-Wimplicit-function-declaration]
        if ( posix_close(0, POSIX_CLOSE_RESTART) < 0 )
             ^
unistd/posix_close.c:9:22: error: use of undeclared identifier 'POSIX_CLOSE_RESTART'
        if ( posix_close(0, POSIX_CLOSE_RESTART) < 0 )
                            ^
2 errors generated.