cc -pthread  -Wall -Wextra -Werror=implicit-function-declaration unistd/setresgid.c -o unistd/setresgid -D_GNU_SOURCE -D_BSD_SOURCE -D_ALL_SOURCE -D_DEFAULT_SOURCE  -lm -lpthread  -liconv
unistd/setresgid.c:11:7: error: call to undeclared function 'getresgid'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   11 |         if ( getresgid(&rgid, &egid, &sgid) < 0 )
      |              ^
unistd/setresgid.c:13:7: error: call to undeclared function 'setresgid'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
   13 |         if ( setresgid(rgid, egid, sgid) < 0 )
      |              ^
2 errors generated.