clang -O2  -Wall -Wextra -Werror=implicit-function-declaration sys_resource/getpriority.c -o sys_resource/getpriority -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm  -lintl
sys_resource/getpriority.c:16:30: error: implicit declaration of function 'getpgid' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        if ( getpriority(PRIO_PGRP, getpgid(0)) == -1 && errno )
                                    ^
1 error generated.